MOST Science Results
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2008, Monthly Notices of the Royal Astronomical Society, in press
"MOST satellite photometry of stars in the M67 field: Eclipsing binaries, blue stragglers and delta Scuti variables."
Authors:
Theodor Pribulla, Slavek Rucinski, Jaymie M. Matthews, Rainer Kuschnig, Jason F. Rowe, David B. Guenther, Anthony F. J. Moffat, Dimitar Sasselov, Gordan A. H. Walker, Werner W. Weiss
Abstract:
We present two series of MOST (Microvariability & Oscillations of STars) spacebased
photometry, covering nearly continuously 10 days in 2004 and 30 days in 2007, of
selected variable stars in the upper Main Sequence of the old open cluster M67. New
high-precision light curves were obtained for the blue-straggler binary/triple systems
AH Cnc, ES Cnc and EV Cnc. The light curve of ES Cnc is modelled in detail,
assuming two dark photospheric spots and Roche geometry. An analysis of the light
curve of AH Cnc indicates a low mass ratio (q ~ 0:13) and a high inclination angle for
this system. Two new long-period eclipsing binaries, GSC 814{323 and HD 75638 (non-
members of M67) were discovered. We also present ground-based DDO spectroscopy of
ES Cnc and of the newly found eclipsing binaries. Especially interesting is HD 75638,
a member of a visual binary, which must itself be a triple or a higher-multiplicity
system. New light curves of two delta Scuti pulsators, EX Cnc and EW Cnc, have been
analyzed leading to detection of 26 and 8 pulsation frequencies of a high temporal
stability.
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2008, The Astrophysical Journal, in Press
"The nature of p-modes and granulation in Procyon: New MOST photometry and new Yale convection models"
Authors:
D.B. Guenther, T. Kallinger, M. Gruberbauer, D. Huber, W.W. Weiss, R. Kuschnig, P. Demarque, F. Robinson,
J.M. Matthews, A.F.J. Moffat, S.M. Rucinski, D. Sasselov, G.A.H. Walker
Abstract:
We present new photometry of Procyon obtained by MOST during a 38-day run in 2007 and frequency analyses of those data. The long time coverage and low point-to-point scatter of the light curve yield an average noise amplitude of about 1.5 – 2.0 ppm in the frequency range 500 – 1500 µHz. This is half the noise level obtained from each of the previous two Procyon campaigns by MOST in 2004 and 2005.
The 2007 MOST amplitude spectrum shows some evidence for p-mode signal: excess power centred near 1000 µHz and an autocorrelation signal near 55 µHz (suggestive of a mode spacing around that frequency), both consistent with p-mode model predictions. However, we do not see regularly spaced frequencies aligned in common l-valued ridges in echelle diagrams of the most significant peaks in the spectrum unless we select modes from the spectrum using a-priori assumptions. The most significant peaks in the spectrum are scattered by more than ±5 µHz about the predicted l-valued ridges, a value that is consistent with the scatter among individually identified frequencies obtained from groundbased radial velocity (RV) observations. We argue that the observed scatter is intrinsic to the star, due to short lifetimes of the modes and the dynamic structure of Procyon's thin convection zone.
We compare the MOST Procyon amplitude and power density spectra with preliminary results of 3D numerical models of convection by the Yale group. These models show that, unlike in the Sun, Procyon’s granulation signal in luminosity has a peak coinciding with the expected frequency region for p-modes near 1000 µHz.
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2008, The Astrophysical Journal, in Press
"MOST detects SPBe pulsations in HD 127756 and HD 217543: Asteroseismic rotation rates independent of vsini"
Authors:
C. Cameron, H. Saio, R. Kuschnig, G. A. H. Walker, J. M. Matthews, D. B.
Guenther, A. F. J. Moffat, S. M. Rucinski, D. Sasselov, W. W. Weiss
Abstract:
The MOST (Microvariability and Oscillations of Stars) satellite has discov-
ered SPBe (Slowly Pulsating Be) oscillations in the stars HD 127756 (B1/B2
Vne) and HD 217543 (B3 Vpe). For HD 127756, 30 significant frequencies are
identified from 31 days of nearly continuous photometry; for HD 217543, up to
40 significant frequencies from 26 days of data. In both cases, the oscillations
fall into three distinct frequency ranges, consistent with models of the stars. The
variations are caused by nonradial g-modes (and possibly r-modes) distorted by
rapid rotation and excited near the Fe ionization zone by the opacity mechanism.
A comparison of pulsation models and observed frequency groups yields a rota-
tion frequency for each star, independently of vsini. The rotation rates of these
stars, as well as those of the SPBe stars previously discovered by MOST, HD
163868 and beta CMi, are all close to their critical values.
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2008, The Astrophysical Journal Letters, 679, L45-L48
"MOST finds no coherent oscillations in the hot carbon-rich Wolf-Rayet star HD 165763"
Authors:
A. F. J. Moffat, S. V. Marchenko, B. E. Zhilyaev, J. F. Rowe, V. Muntean,
A.-N. Chene, J. M. Matthews, R. Kuschnig, D. B. Guenther, S. M. Rucinski,
D. Sasselov, G. A. H. Walker, W. W. Weiss
Abstract:
We have photometrically monitored the V=8 mag Galactic Population I WC5 star WR 111 for 3 weeks
nonstop using the MOST microsatellite. Each of the ~27,000 data points has a precision of ~3 mmag. We find
no coherent Fourier components above the 50 part per million level over the whole interval for frequencies
f > 10 cd^-1 (periods P < 2.4 hr). This limit is nearly 2 orders of magnitude below recent predictions for early-
type WR stars based on strange-mode pulsation simulations, with expected periods in the range 10–30 minutes.
Simultaneous spectroscopic observations of WR 111 reveal a normal level of stochastic clumps propagating in
the wind, which possibly manifest themselves in the slow 1/f rise in the MOST power spectrum below f~ 10
cd^-1 . Time-frequency analysis of the MOST data shows no obvious short-lived frequencies above the 1 mmag level, in stark contrast to the highly variable cool WR stars WR 123 (WN8) and WR 103 (WC9d), monitored
previously by MOST. Radiation pressure therefore appears to be the main, if not sole, driver of WR 111’s strong
wind.
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2008, Astronomy & Astrophysics, 482, 691-697
"MOST detects an active spot on tau Bootis induced by its planetary companion"
Authors:
Gordon A.H. Walker, Bryce Croll, Jaymie M. Matthews, Rainer Kuschnig,
Daniel Huber, Werner W. Weiss, Evgenya Shkolnik, Slavek M. Rucinski, David
B. Guenther, Anthony F.J. Moffat, Dimitar Sasselov
Abstract:
Context. There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems.
Aims. We shall demonstrate from MOST satellite photometry and Ca II K line emission that there has been a persistent, variable region on the surface of tau Boo A which tracked its giant planetary companion for some 440 planetary revolutions and lies ~68 degrees (phi = 0.8) in advance of the sub-planetary point.
Methods. The light curves are folded on a range of periods centered on the planetary orbital period and phase dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Ca II K line reversals.
Results. The region varies in brightness on the time scale of a rotation by ~ 1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. The 2004 light curve gives a spot rotation period of 3.5 +/- 0.7 d compared to the known planetary orbital period of 3.3125 d. The amplitude spectrum of the 2005 light curve shows no marked peak at the orbital period but the mean absolute deviation (MAD) of the light curve has a well defined maximum (half width ~0.15 d) centered on the orbital frequency. Over the 123 planetary orbits spanned by the photometry the variable region detected in 2004 and in 2005 are synchronised to the planetary orbital period within 0.0015 d. The Ca II K line in 2001, 2002 and 2003 also shows enhanced K-line
variability centered on phi = 0.8, extending coverage to some 440 planetary revolutions.
Conclusions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at phi =0.3 and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Ca II K line reversal results make a good case for an active region induced magnetically on the surface of tau Boo A by its planetary companion.
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2008, Astronomy & Astrophysics,483, 239-248
"MOST photometry of the roAp star 10 Aquilae"
Authors:
Huber, D.; Saio, H.; Gruberbauer, M.; Weiss; W.W.; Rowe, J.F.; Hareter,
M.; Kallinger, T.; Reegen, D.B.; Guenther, D.B.; Kuschnig, R.; Matthews,
J.M.; Moffat, A.F.J.; Rucinski, S.M.; Sasselov, D.; Walker, G.A.H.
Abstract:
Context. We present 31.2 days of nearly continuous MOST photometry of the rapidly oscillating Ap star 10Aql.
Aims. The goal was to provide an unambiguous frequency identification for this little studied star, as well as to discuss the detected frequencies in the context of magnetic models and analyze the influence of the magnetic field on the pulsation. Methods. Using traditional Fourier analysis techniques on three independent data reductions, intrinsic frequencies for the star are identified. Theoretical non-adiabatic axisymmetric modes influenced by a magnetic field having polar field strengths BP = 0–5 kG were computed to compare the observations to theory.
Results. The high-precision data allow us to identify three definite intrinsic pulsation frequencies and two other candidate frequencies with low S/N. Considering the observed spacings, only one (?? = 50.95 microHz) is consistent with the main sequence nature of roAp stars. The comparison with theoretical models yields a best fit for a 1.95 Msun model having solar metallicity, suppressed envelope convection, and homogenous helium abundance. Furthermore, our analysis confirms the suspected slow rotation of the star and sets new lower limits to the rotation period (Prot = 1 month) and inclination (i > 30 ± 10 degrees).
Conclusions. The observed frequency spectrum is not rich enough to unambiguously identify a model. On the other hand, the models hardly represent roAp stars in detail due to the approximations needed to describe the interactions of the magnetic field with stellar structure and pulsation. Consequently, errors in the model frequencies needed for the fitting procedure can only be estimated. Nevertheless, it is encouraging that models which suppress convection and include solar metallicity, in agreement with current concepts of roAp stars, fit the observations best.
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2008, The Astrophysical Journal, in press
"MOST spacebased photometry of the exoplanet system HD 189733:
Precise timing measurements of transits across an active star"
Authors:
Miller-Ricci, E.; Rowe, J.F.; Sasselov, D.; Matthews, J.M.; Kuschnig, R.;
Croll, B.; Guenther, D.B.; Moffat, A.F.J.; Rucinski, S.M.; Walker, G.A.H.;
Weiss, W.W.
Abstract:
We have measured transit times for HD189733b passing in front of its bright
(V= 7.67) chromospherically actie and spotted parent star. Nearly continuous
broadband optical photometry of this system was obtained with the MOST
(Microvariability & Oscillations of STars) space telescope during 21 days in
August 2006, monitoring 10 consecutive transits. We have used these data to
search for deviations from a constant orbital period which can indicate the
presence of additional planets in the system that are as yet undetected by Doppler
searches. There are no transit timing variations above the level of +/- 30 s,
ruling out super-Earths (of masses 1-4 Msun) in the 1:2 and 2:3 inner resonances
and planets of 20 Msun in the 2:1 outer resonance of the known planet. We also
discuss complications in measuring transit times for a planet that transits an
active star with large star spots, and how the transits help constrain and test
spot models. This has implications for the large number of such systems expected
to be discovered by the CoRoT and Kepler missions.
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2008, Canadian Undergraduate Physics Journal, VI, Issue 2, 7-10
"Searching for asteroids around another star"
Authors:
Moldovan, R. & Matthews, J.M.
Abstract:
We present an ongoing search for Trojan asteroids in the transmitting exoplanetary system HD 209458 based
on photometry with Canada's MOST (Microvariability & Oscillations of STars) satellite. MOST monitered
this exoplanetary system for 14 and 44 days in 2004 and 2005, respectively, with nearly continuous
time coverage. Any Trojan asteroid swarms will orbit in roughly the same plane as the known giant planet
in this system and with the same period. OUr preliminary analysis orf the MOST photometry of HD 209458 to
detect a Trojan signal is sensitive to dips in the starlight due to transits of about 10^-4 relative to the
mean brightness of the host star. This corresponds to a Trojan swarm whose total mass would be of the order
10^-3 Earth masses, compared to the previous upper limit of about 13 Earth masses.
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2008, Communications in Asteroseismology, 153, 84-103
"MOST found solar-type oscillations in the K2 giant star HD 20884"
Authors:
Kallinger, T.; Guenther, D.B.; Weiss, W.W.; Hareter, M.; Reegen, P.;
Matthews, J.M.; Kuschnig, R.; Walker, G.A.H.; Rucinski, S.M.; Moffat,
A.F.J.; Sasselov, D.
Abstract:
We found evidence for radial p-modes and nonradial mixed modes in the oscillation
spectrum of the K giant HD20884 based on 20.6 days of nearly continuous
high-precision photometry obtained by the Canadian microsatellite MOST. Oscillation
frequencies range from 5-31 microHz (periods of about 2.3 d - 9 hr) with
luminosity amplitudes between about 300 and 950 ppm and mode lifetimes exceeding
10 days are indicated. The mode identifications are based on searches
of a large grid of models for a best fit to the frequencies and temperature of
HD20884. The latter is better constrained now by spectroscopy obtained at
the David Dunlap Observatory as part of this work.
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2008, Astronomy & Astrophysics, 480, 223-232
"MOST photometry and modeling of the rapidly oscillating (roAp) star gamma Equ"
Authors:
Gruberbauer, M.; Saio, H.; Huber, D.; Kallinger, T.; Weiss, W. W.;
Guenther, D. B.; Kuschnig, R.; Matthews, J. M.; Moffat, A. F. J.;
Rucinski, S.; Sasselov, D.; Walker, G. A. H.
Abstract:
Aims. Despite photometry and spectroscopy of its oscillations obtained over the past 25 years, the pulsation frequency spectrum of the rapidly oscillating Ap (roAp) star gamma Equ has remained poorly understood. Better time-series photometry, combined with recent advances to incorporate interior magnetic field geometry into pulsational models, enable us to perform improved asteroseismology of this roAp star.
Methods. We obtained 19 days of continuous high-precision photometry of gamma Equ with the Most (Microvariability & Oscillations of STars) satellite. The data were reduced with two different reduction techniques and significant frequencies were identified. Those frequencies were fitted by interpolating a grid of pulsation models that include dipole magnetic fields of various polar strengths.
Results. We identify 7 frequencies in gamma Equ that we associate with 5 high-overtone p-modes and 1st and 2nd harmonics of the dominant p-mode. One of the modes and both harmonics are new discoveries for this star. Our best model solution (1.8 Msun, log Teff ~ 3.882; polar field strength ~8.1 kG) leads to unique mode identifications for these frequencies (l = 0, 1, 2 and 4). This is the first purely asteroseismic fit to a grid of magnetic models. We measure amplitude and phase modulation of the primary frequency due to beating with a closely spaced frequency that had never been resolved. This casts doubts on theories that such modulation – unrelated to the rotation of the star – is due to a stochastic excitation mechanism.
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2008, Astronomy & Astrophysics, 480, 223-232
"g-modes in the late-type Be star beta CMi detected by the MOST satellite"
Authors:
Saio, H.; Cameron, C.; Kuschnig, R.; Walker, G. A. H.; Matthews, J. M.;
Rowe, J. F.; Lee, U.; Huber, D.; Weiss, W. W.; Guenther, D. B.; Moffat, A.
F. J.; Rucinski, S. M.; Sasselov, D.
Abstract:
The Microvariability & Oscillations of STars (MOST) satellite has detected low-amplitude light variations
(delta m ~ 1 mmag) in the late-type Be star beta CMi (B8Ve). The photometric variations have periods of
~ 0.3 days. This is consistent with high-order, prograde (m = -1) g-modes of a nearly critically rotating
3.5 Msun model.
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2007, The Astrophysical Journal, in press
"MOST Spacebased Photometry of the Transiting Exoplanet System HD209458: Transit Timing To Search For Additional Planets"
Authors:
Miller-Ricci, E., Rowe, J., Sasselov, D., Matthews, J.M, Kuschnig, R.,
Guenther, D.B., Moffat, A.F.J., Rucinski, S.M., Walker, G.A.H., Weiss,
W.W.
Abstract:
We report on the measurement of transit times for the HD 209458 planetary
system from photometry obtained with the MOST (Microvariability & Oscillations
of STars) space telescope. Deviations from a constant orbital period can
indicate the presence of additional planets in the system that are yet undetected,
potentially with masses approaching an Earth mass. The MOST data sets of HD
209458 from 2004 and 2005 represent unprecedented time coverage with nearly
continuous observations spanning 14 and 43 days and monitoring 3 transits and
12 consecutive transits, respectively. The transit times we obtain show no variations
on three scales: (a) no long-term change in P since before 2004 at 25 ms
level, (b) no trend in transit timings during the 2005 run, and (c) no individual
transit timing deviations above 80 sec level. Together with previously published
transit times from Agol & Steffen (2007), this allows us to place limits on the
presence of additional close-in planets in the system, in some cases down to below
an Earth mass. This result, along with previous radial velocity work, now
eliminates the possibility that a perturbing planet could be responsible for the
additional heat source needed to explain HD 209458b’s anomalous low density.
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2007, The Astrophysical Journal, in press
"Spot modulation of the transit exoplanet
system HD 189733: MOST detects moderate spin-orbit misalignment"
Authors:
Croll, B.; Matthews, J.M.; Walker, G.A.H.; Rowe, J.F.; Miller-Ricci, E.;
Kuschnig, R.; Sasselov, D.; Rucinski, S.M.; Walker, A.; Guenther, D.B.;
Moffat, A.F.J.; Weiss, W.W.
Abstract:
We present a simple starspot model to reproduce the variability in the
exoplanet-hosting star HD 189733 observed in 21 days of nearly continuous pho-
tometry returned by the MOST space mission. MOST detected significant ro-
tational modulation of the star’s optical light curve, at a level of ~3% of the
stellar signal. We model this observed rotational modulation with two large dark
starspots at different latitudes, rotating non-differentially. Our spot modeling
efforts indicate that the stellar inclination angle is measurably different than
edge-on, suggesting moderate spin-orbit misalignment between the stellar spin
axis and orbit normal of the hot Jupiter exoplanet. Our result favours migration
theories that tend to randomize spin-orbit alignment. However, more complicated
spot geometries, the evolution of the spots with time, or the effects of differen-
tial rotation could all affect our results, and mask the true rotation period and
stellar inclination angle of HD 189733. Continued photometric monitoring of HD
189733, especially with the time coverage and precision possible from space, will
be needed to address these issues.
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2007, Astronomy & Astrophysics, 478, 497-505
"Nonradial p-modes in the red giant epsilon Ophiuchi? Pulsation model fits to MOST
photometry"
Authors:
Kallinger, T., Guenther, D.B., Matthews, J.M., Weiss, W.W., Huber, D.,
Kuschnig, R., Moffat, A.F.J.; Sasselov, D.; Walker, G.A.H.
Abstract:
The G9.5 giant epsilon Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10–20 d. The best fit model itself, constrained only by the observed frequencies, lies within +/- 1 sigma of epsilon Oph’s position in the HR-diagram and the interferometrically
determined radius.
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2007, The Astrophysical Journal, 671, 2129-2138
"Looking for super-Earths in the HD 189733 system: A search for transits in MOST spacebased photometry"
Authors:
Croll, Bryce; Matthews, Jaymie M.; Rowe, Jason F.; Gladman, Brett;
Miller-Ricci, Eliza; Sasselov, Dimitar; Walker, Gordon A. H.; Kuschnig,
Rainer; Lin, Douglas N. C.; Guenther, David B.; Moffat, Anthony F. J.;
Rucinski, Slavek M.; Weiss, Werner W.
Abstract:
We have made a comprehensive transit search for exoplanets down to ~1.5-2 Earth radii in the HD 189733 system,
based on 21 days of nearly uninterrupted broadband optical photometry obtainedwith the MOST (Microvariability
and Oscillations of STars) satellite in 2006. We have searched these data for realistic limb-darkened transits from exoplanets other than the known hot Jupiter, HD 189733b, with periods ranging from about 0.4 days to 1 week. Monte Carlo statistical tests of the data with synthetic transits inserted into the data set allow us to rule out additional close-in exoplanets with sizes ranging from about 0.15 - 0.31 RJ (Jupiter radii), or 1.7- 3.5 Rearth (Earth radii), on orbits whose planes are near that of HD 189733b. These null results constrain theories that invoke lower mass hot super-Earth and hot Neptune planets in orbits similar to HD 189733b, due to the inward migration of this hot Jupiter. This work also illustrates the feasibility of discovering smaller transiting planets around chromospherically active stars.
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August 15th, 2007
"Searching for p-modes in eta Bootis and Procyon using MOST"
Authors:
D.B. Guenther, T. Kallinger, P. Reegen, W. W. Weiss, J. M. Matthews,
R. Kuschnig, A.F.J. Moffat, S.M. Rucinski, D. Sasselov, G.A.H. Walker
Abstract:
We present frequency analyses of new photometry obtained in 2005 by
the MOST (Microvariability & Oscillations of STars) satellite of two
solar-type stars, eta Bootis and Procyon, and reanalyses of MOST data
of these stars obtained in 2004. With improved strategies to identify
and correct stray light artifacts in the MOST Fabry Imaging data, we
produce amplitude spectra from the reduced data and compare them to
p-mode oscillation spectra computed from stellar models. We confirm
the null result from the 2004 MOST observations of Procyon. We find
no evidence for spectral power with regular spaced frequencies
characteristic of radial p-modes, nor do we find excess power in the
expected p-mode frequency range. For eta Boo, the frequencies present
in the 2005 data do not match within the resolution those identified
in the 2004 data after excluding all frequencies we suspect to be due
to stray light contamination. However, there is clear evidence for
excess power within the p-mode range predicted by models. We discuss
the implications of these results for mode lifetimes in these stars
and the sensitivity of high-precision photometry to solar-type
oscillations in the presence of granulation.
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August 10th, 2007
"MOST photometry and DDO spectroscopy of the eclipsing (white dwarf +
red dwarf) binary V471 Tau"
Authors:
Krzysztof Z. Kaminski, Slavek M. Rucinski, Jaymie M. Matthews, Rainer
Kuschnig, Jason F. Rowe, David B. Guenther, Anthony F. J. Moffat,
Dimitar Sasselov, Gordon A. H. Walker, Werner W. Weiss
Abstract:
The Hyades K2 V + WD system 471 Tau is a prototype postYcommon envelope
system and a likely cataclysmic binary progenitor. We present 10 days of
nearly continuous optical photometry by the MOST (Microvariability and
Oscillations of Stars) satellite and partly simultaneous optical
spectroscopy from DDO (David Dunlap Observatory) of the binary. The
photometric data indicate that the spot coverage of the K dwarf component
was less than observed in the past, suggesting that we monitored the star
close to a minimum in its activity cycle. Despite the low spot activity,
we still detected seven flarelike events whose estimated energies are
among the highest ever observed in V471 Tau and whose times of occurrence
do not correlate with the binary orbital phase. A detailed O - C analysis
of the times of eclipse over the last 35 years reveals timing variations
which could be explained in several ways, including perturbations by an
as-yet-undetected third body in the system or by a small orbital
eccentricity inducing slow apsidal motion. The DDO spectra result in
improved determinations of the K dwarf projected rotation velocity, v sin
i = 92 km/s, and the orbital amplitude, K = 150.5 km/s. The spectra also
allow us to measure changes in H alpha emission strength and radial
velocity variations. We measure a larger H alpha velocity amplitude than
found previously, suggesting that the source of the emission in V471 Tau
was less concentrated around the subYwhite dwarf point on the K star than
had been observed in previous studies.
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August 10th, 2007
"Discovery of the strongly eccentric, short-period binary nature of
the B-type system HD 313926 by the MOST satellite"
Authors:
Slavek Rucinski, Rainer Kuschnig, Jaymie M. Matthews, Wojtek Dimitrov,
Theodor Pribulla, David B. Guenther, Anthony F. J. Moffat, Dimitar
Sasselov, Gordon A. H. Walker, Werner W. Weiss
Abstract:
The MOST photometric space mission discovered an eclipsing binary
among its guide stars in June 2006 which combines a relatively large
eccentricity e = 0.20 with an orbital period of only 2.27 days. HD
313926 appears to consist of two early-type stars of spectral type B3
– B7. It has a largest eccentricity among known early-type binaries
with periods less than 3.5 d. Despite the large components indicated
by its spectral type and light curve model, and its short period, the
orbit of HD 313926 has not yet circularised so it is probably very
young, even compared to other young B stars.
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May 30th, 2007
"MOST photometry of the RRd Lyrae variable AQ Leo:
Two radial modes, 32 combination frequencies, and beyond"
Authors:
Michael Gruberbauer, Katrien Kolenberg, Jason F. Rowe, Daniel Huber,
Jaymie M. Matthews, Piet Reegen, Rainer Kuschnig, Chris Cameron,
Thomas Kallinger, Werner W. Weiss, David B. Guenther, Anthony F. J., Moffat,
Slavek Rucinski, Dimitar Sasselov, Gordon A. H. Walker
Abstract:
Highly precise and nearly uninterrupted optical photometry of the RR Lyrae star AQ
Leo was obtained with the MOST (Microvariability & Oscillations of STars) satellite
over 34.4 days in February-March 2005. AQ Leo was the first known double-mode
RR Lyrae pulsator (RRd star). Three decades after its discovery, MOST observa-
tions have revealed that AQ Leo oscillates with at least 42 frequencies, of which 32
are linear combinations (up to the sixth order) of the radial fundamental mode and
its first overtone. Evidence for period changes of these modes is found in the data.
The other intrinsic frequencies may represent an additional nonradial pulsation mode
and its harmonics (plus linear combinations) which warrant theoretical modeling. The
unprecedented number of frequencies detected with amplitudes down to millimag pre-
cision also presents an opportunity to test nonlinear theories of mode growth and
saturation in RR Lyrae pulsators.
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April 6th, 2007
"Detection of solar-like oscillations in the red giant star epsilon Ophiuchi by MOST spacebased photometry"
Authors:
C. Barban, J. M. Matthews, J. De Ridder, F. Baudin, R. Kuschnig,
A. Mazumdar, R. Samadi, D. B. Guenther, A. F. J. Mofat, S. M.
Rucinski, D. Sasselov, G. A. H. Walker, and W. W. Weiss
Abstract:
Context. Solar-like oscillations have been discovered in a few red
giants, including eps Oph, through spectroscopy. Acoustic modes
around 60 microHz were clearly seen in this star, but daily aliasing
of the groundbased data made it impossible to isolate unambiguously
the p-mode frequencies in the eigenspectrum, and hence the correct
value of the large spacing, to asteroseismically constrain the mass
of this pulsating star.
Aims. We obtained about 28 days of contiuous high-precision
photometry of eps Oph in May - June 2005 with the MOST
(Microvariability & Oscillations of STars) satellite. The thorough
time sampling removes the ambiguity of the frequency identifcations
based on the groundbased discovery data.
Methods. We identify equidistant peaks in the Fourier spectrum of
the MOST photometry in the range where the p-modes were seen
spectroscopically. Those peaks are searched by autocorrelation of
the power spectrum to estimate the value of the large separation in
the p-mode eigenspectrum. Having isolated the oscillation modes, we
determine their mode parameters (frequency, amplitude and line
width) by fitting the distribution of peaks to Lorentzian profiles.
Results. The clear series of equidistant peaks in the power spectrum,
with amplitudes from about 30 to 130 ppm, are consistent with radial
modes spaced by a mean value of 5.3 +\- 0.1 microHz. This large
separation matches one of the two possibilities allowed by the
groundbased observations thus constraining the stellar models to a
much greater extent than previously possible. The line widths and
Lorentzian fits indicate a rather short average mode lifetime: 2.7
(+ 0.6, - 0.8) days.
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January 29th, 2007
"The Differential Rotation of kappa 1 Ceti as observed by MOST - II"
Authors:
Gordon A.H. Walker, Bryce Croll, Rainer Kuschnig, Andrew Walker, Slavek M.
Rucinski, Jaymie M. Matthews, David B. Guenther, Anthony F.J. Moffat, Dimitar
Sasselov, Werner W. Weiss
Abstract:
We first reported evidence for differential rotation of kappa 1 Ceti in Paper I. In
this paper we demonstrate that the differential rotation pattern closely matches
that for the sun. This result is based on additional MOST (Microvariability &
Oscillations of STars) observations in 2004 and 2005, as well as those from 2003.
Using StarSpotz, a program developed specifically to analyse MOST photometry,
we have solved for k, the differential rotation coefficient, and PEQ, the equatorial
rotation period using the light curves from all three years. The absolute range
in spot latitudes is 10 deg to 75 deg and k = 0.090+0.006/-0.005 – less than the solar value
but consistent with the younger age of the star. k is also well constrained by
the independent spectroscopic estimate of vsini. We demonstrate independently
that the pattern of differential rotation with latitude is indeed solar.
Details are given of the parallel tempering formalism used in finding the most
robust solution which gives PEQ = 8.77+0.03/-0.04 days – smaller than that usually
adopted, implying an age < 750 My. Our values of PEQ and k can explain the
range of rotation periods others have found by spots or activity at a variety of latitudes.
Historically, Ca II activity seems to occur consistently between latitudes
50 deg and 60 deg which might indicate a permanent magnetic feature. Knowledge of
k and PEQ are key to understanding the dynamo mechanism and rotation structure
in the convective zone as well assessing age for solar-type stars. We recently
published values of k and PEQ for eps Eri based on MOST photometry and expect
to analyse MOST light curves for several more spotted, solar-type stars.
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January 7th, 2007
"Looking for giant Earths in the HD 209458 system: A search for transits in MOST space-based photometry"
Authors:
Bryce Croll, Jaymie M. Matthews, Jason F. Rowe, Rainer Kuschnig, Andrew Walker,
Brett Gladman, Dimitar Sasselov, Chris Cameron, Gordon A.H. Walker, Douglas N.C.
Lin, David B. Guenther, Anthony F.J. Moffat, Slavek M. Rucinski, Werner W. Weiss
Abstract:
We have made a comprehensive transit search for exoplanets down to about 2
Earth radii in the HD 209458 system, based on nearly uninterrupted broadband
optical photometry obtained with the MOST (Microvariability & Oscillations of
STars) satellite, spanning 14 days in 2004 and 44 days in 2005. We have searched
these data for limb-darkened transits at periods other than that of the known
giant planet, from about 0.5 days to 2 weeks. Monte Carlo statistical tests of
the data with synthetic transits inserted allow us to rule out additional close-in
exoplanets with sizes ranging from about 0.20 - 0.36 RJ (Jupiter radii), or 2.2 -
4.0 R_earth (Earth radii) on orbits whose planes are near that of HD 209458b. These
null results constrain theories that invoke lower-mass planets in orbits similar to
HD 209458b to explain its anomalously large radius, and those that predict “hot
Earths” due to the inward migration of HD 209458b.
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October 31st, 2006
"MOST Photometry of the roAp star HD 134214"
Authors:
C.Cameron, J.M. Matthews, J.F. Rowe,
R.Kuschnig, D.B. Guenther, A.F.J. Moffat, S.M. Rucinski,
D. Sasselov, G.A.H. Walker, W.W. Weiss
Abstract:
We present 10.27 hrs of photometry of the roAp star HD 134214 obtained by
the MOST satellite. The star is shown to be monoperiodic and oscillating at a
frequency of 2948.97 +/- 0.55 microHz. This is consistent with earlier ground based
photometric campaigns (e.g. Kreidl et al. 1994). We do not detect any of the
additional frequencies identifed in the recent spectroscopic study by Kurtz et
al. (2006) down to an amplitude limit of 0.36 mmag (2 sigma significance limit).
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October 23rd, 2006
"HD 114839 - An Am star showing both delta Scuti and gamma Dor pulsations discovered through MOST photometry"
Authors:
H. King, J.M. Matthews, J.F. Rowe,
C.Cameron, R.Kuschnig, D.B. Guenther, A.F.J. Moffat, S.M. Rucinski,
D. Sasselov, G.A.H. Walker, W.W. Weiss
Abstract:
Using MOST (Microvariability and Oscillations of STars) satellite guide star
photometry, we have discovered a metallic A star showing hybrid p- and g-
mode pulsations. HD 114839 was observed nearly continuously for 10 days in
March, 2005. We identify frequencies in three groups: the first centered near 2
cycles/day, in the gamma Dor pulsation range, and two others near 8 and 20, both in
the delta Scuti range. This is only the fourth known such hybrid pulsator, including
another MOST discovery (Rowe et al. 2006, this issue).
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October 23rd, 2006
"Discovery of hybrid gamma Dor and delta Sct pulsations in BD+184914 through MOST spacebased photometry"
Authors:
J.F. Rowe, J.M. Matthews, C. Cameron, D.A. Bohlender,
H. King, R. Kuschnig, D.B. Guenther, A.F.J. Moffat, S.M. Rucinski,
D. Sasselov, G.A.H. Walker, W.W. Weiss
Abstract:
We present a total of 57 days of continuous, high-cadence photometry (14 days
in 2004 and 43 in 2005) of the star BD+18 4914 obtained with the MOST
satellite. We detect 16 frequencies down to a signal-to-noise of 3.6 (amplitude
~ 0.5 mmag). Six of these are less than 3 cycles/day, and the other ten are
between 7 and 16 cycles/day. We intrepret the low frequencies as g-mode
gamma Doradus-type pulsations and the others as delta Scuti-type p-modes, making
BD+18 4914 one of the few known hybrid pulsators of its class. If the g-mode
pulsations are high-overtone non-radial modes with identical low degree l, we
can assign a unique mode classification of n={12, 20, 21, 22, 31, 38} based
on the frequency ratio method.
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September 14th, 2006
"MOST DETECTS g-MODES IN THE LATE-TYPE Be STAR beta CMi (B8Ve)"
Authors:
H. Saio, C. Cameron, R. Kuschnig, G.A.H. Walker, J.M. Matthews,
J.F. Rowe, U. Lee, D. Huber, W.W. Weiss, D.B. Guenther, A.F.J.
Moffat, S.M. Rucinski, and D. Sasselov
Abstract:
The Microvariability and Oscillations of stars (MOST) satellite
has detected low-amplitude light variations (about 1 mmag) in
the Be star beta CMi (B8Ve). The observations lasted 41 days
and the variations have typical periods of about 0.3 days. We
demonstrate that the dominant frequencies are consistent with
prograde high-order g-modes of m = -1 excited by the Fe-bump
of opacity in an intermediate-mass (approximately 3.5 solar
masses) star with a nearly critical rotation period of 0.38 days.
This is the first detection of nonradial g-mode pulsations in a
Be star later than B6 leading to the possibility that pulsations
are excited in all classical Be stars.
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July 28th, 2006
Errata for the Aerts et al. paper on delta Ceti
"delta Ceti Is Not Monoperiodic:Seismic Modeling of a beta Cephei Star from MOST Space-based Photometry"
Authors: C.Aerts, S.V. Marchenko et al.
Abstract:
The beta Cephei star delta Ceti was considered one of the
few monoperiodic variables in its class. Despite (or perhaps
because of) its apparently simple oscillation spectrum,
it has been challenging and controversial to identify
this star's pulsation mode and constrain its physical
parameters seismically. Broadband time-resolved photometry
of delta Ceti spanning 18.7 days with a duty cycle of about
65% obtained by the Microvariability and Oscillations of
Stars (MOST) satellite - the first scientific observations
ever obtained by MOST - reveals that the star is actually
multiperiodic. Besides the well-known dominant frequency
of f1=6.205886 day-1, we have discovered in the MOST data
its first harmonic 2f1 and three other frequencies
(f2=3.737, f3=3.673, and f4=0.318 day-1), all detected
with a signal-to-noise ratio (S/N)>4. In retrospect, f2
was also present in archival spectral line-profile data
but at lower S/N. We present seismic models whose modes
match exactly the frequencies f1 and f2. Only one model
falls within the common part of the error boxes of the
star's observed surface gravity and effective temperature
from photometry and spectroscopy. In this model, f1 is the
radial (l=0) first overtone, and f2 is the g2 (l=2, m=0)
mode. This model has a mass of 10.2+/-0.2 Msolar and an
age of 17.9+/-0.3 Myr, making delta Ceti an evolved beta
Cephei star. If f2 and f3 are rotationally split components
of the same g2 mode, then the star's equatorial rotation
velocity is either 27.6 km s-1 or half this value. Given
its vsini of about 1 km s-1, this implies that we are
seeing delta Ceti nearly pole-on.
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July 5th, 2006
"G- and P-Modes in the B Supergiant HD163899 (B2Ib/II)"
Authors: H.Saio, R. Kuschnig et al.
Abstract:
The Microvariability and Oscillations of Stars (MOST) satellite
observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a
guide star and detected 48 frequencies <= 2.8 c/d with
amplitudes of a few milli-magnitudes (mmag) and less. The frequency
range embraces g- and p-mode pulsations. It was generally thought
that no g-modes are excited in less luminous B supergiants because
strong radiative damping is expected in the core. Our theoretical
models, however, show that such g-modes are excited in massive
post-main-sequence stars, in accordance with these observations.
The nonradial pulsations excited in models between 20Msun at
log Teff ~ 4.41 and 15Msun at log Teff ~ 4.36 are roughly consistent
with the observed frequency range. Excitation by the Fe-bump in opacity
is possible because g-modes can be partially reflected at a convective
zone associated with the hydrogen-burning shell, which significantly
reduces radiative damping in the core. The MOST light curve of HD 163899
shows that such a reflection of g-modes actually occurs, and reveals
the existence of a previously unrecognized type of variable, slowly
pulsating B supergiants (SPBsg) distinct from alpha Cyg variables.
Such g-modes have great potential for asteroseismology.
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May 30th, 2006
"Differential Rotation of epsilon Eridani Deteted by MOST"
Authors: Bryce Croll, Gordon A.H. Walker et al.
Abstract:
The Microvariability and Oscillations of STars (MOST) photometric satellite
observed three rotations of eps Eri continuously in late 2005. We detected two
spots (?m ~ 0.01) at different latitudes (20.0 deg, 31.5 deg) revolving with different
periods (11.35, 11.55 d) from which we derive a differential rotation coefficient,k = 0.11(+.03/
-.02) in agreement with the prediction by Brown et al. (2004) for a young
sun-like star having roughly twice the solar angular velocity. The light curve
was analysed with the program StarSpotz, a modification of SPOTMODEL by
Rib´arik et al. (2003). The best fitting value for the inclination angle i = (30 ±3)deg is
compatible with inclinations already estimated for the disc (~ 25 deg) and planetary
orbit (26.2 deg). The inclination also leads to an equatorial rotation speed of 3.42
km/s and the photometric value of vsini = 1.7 km/s. When compared with
spectroscopically determined values, the photometric vsini allows, in principle,
an independent estimate of the macroturbulent velocity. Both spots would have
distorted the radial velocity curve ~ ±10 m/s by the Rossiter-McLaughlin
effect which is similar to the stellar radial velocity ’noise’ detected by others.
Details are given of the StarSpotz model and of the uniqueness tests which we
applied to arrive at a best solution and realistic estimates of errors in the derived
parameters.
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May 4th, 2006
"An Upper Limit on the Albedo of HD209458b: Direct Imaging Photometry with the MOST Satellite"
Authors: J.F. Rowe & J.M. Matthews et al.
Abstract:
We present space-based photometry of the transiting exoplanetary system
HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars)
satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses. The
HD 209458 photometry was obtained in MOST’s lower-precision Direct Imaging
mode, which is used for targets in the brightness range 6.5 = V = 13. We describe
the photometric reduction techniques for this mode of observing, in particular
the corrections for stray Earthshine. We do not detect the secondary eclipse in
the MOST data, to a limit in depth of 0.053 mmag (1 sigma). We set a 1 sigma upper limit
on the planet-star flux ratio of 4.88 × 10^-5 corresponding to a geometric albedo
upper limit in the MOST bandpass (400 to 700 nm) of 0.25. The corresponding
numbers at the 3 sigma level are 1.34×10^-4 and 0.68 respectively. HD 209458b is half
as bright as Jupiter in the MOST bandpass. This low geometric albedo value is
an important constraint for theoretical models of the HD209458b atmosphere, in
particular ruling out the presence of reflective clouds. A second MOST campaign
on HD 209458 is expected to be sensitive to an exoplanet albedo as low as 0.13
(1 sigma), if the star does not become more intrinsically variable in the meantime.
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May 4th, 2006
"Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data"
Authors: P.Reegan et al.
Abstract:
The MOST (Microvariability and Oscillations of Stars) satellite obtains ultraprecise photometry
from space with high sampling rates and duty cycles. Astronomical photometry or
imaging missions in low Earth orbits, like MOST, are especially sensitive to scattered light
from Earthshine, and all these missions have a common need to extract target information from
voluminous data cubes. They consist of upwards of hundreds of thousands of two-dimensional
CCD frames (or subrasters) containing from hundreds to millions of pixels each, where the
target information, superposed on background and instrumental effects, is contained only in a
subset of pixels (Fabry Images, defocused images, mini-spectra).We describe a novel reduction
technique for such data cubes: resolving linear correlations of target and background pixel intensities.
This step-wise multiple linear regression removes only those target variations which
are also detected in the background. The advantage of regression analysis versus background
subtraction is the appropriate scaling, taking into account that the amount of contamination
may differ from pixel to pixel. The multivariate solution for all pairs of target/background
pixels is minimally invasive of the raw photometry while being very effective in reducing
contamination due to, e.g. stray light. The technique is tested and demonstrated with both
simulated oscillation signals and real MOST photometry.
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March 31st, 2006
"Discovery of the new slowly pulsating B star HD 163830 (B5II/III) from MOST spacebased photometry"
Authors: C. Aerts et al.
Abstract:
We report the discovery of a new slowly pulsating B (SPB) star, with the
largest number of detected frequencies to date by more than a factor of three,
based on 37 days of MOST (Microvariability & Oscillations of STars) satellite
guide star photometry. The star HD 163830 (V = 9.3, B5II/III) varies in twenty
detected frequencies in the range 0:035-1.06 d^-1 (0.4-12.3 uHz) with amplitudes
from 0.7 to 7.6 mmag (S=N from 4 to 43). Eighteen of these frequencies are
consistent with low-degree, high-order nonradial g-modes of seismic models of
an evolved 4.5 Msun star. We are unable to identify one unique model due to lack
of mode identi cations. The lowest two frequencies may be associated with the
rotation of HD 163830, but rm proof of this must await future spectroscopic
data.
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December 17th, 2005
"MOST Detects g-Modes in the Be Star HD 163868"
Authors: G.A.H Walker, R.Kuschnig, J.M Matthews et al.
Abstract:
We have extracted a 37 day light curve with a precision of 0.0012 mag per point for the Microvariability and Oscillations of Stars (MOST) guide star, HD 163868 (B5 Ve). Its rich frequency spectrum resembles that of a slowly pulsating B (SPB) star but, being a rapid rotator, we designate it SPBe. The 60 most significant periods lie in three distinct groups centered on 8 days and 14 and 7 hr. We demonstrate that the 14 and 7 hr periods can be modeled by two swarms of high-order, prograde sectorial g-modes (m =-1,-2), which are destabilized by the iron opacity bump. Our model also predicts a group of r-modes with periods near 2.3 days, which correspond to frequencies observed in the tail of the 8 day group. The remaining periodicities, between 7 and 11 days, cannot be explained by unstable modes in our model.
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October 13th, 2005
"Oscillations in the Massive Wolf-Rayet Star WR123 with the
MOST Satellite."
Authors: L.Lefevre, S.V. Marchenko et al.
Abstract:
We present the results of intensive visual-broadband photometric
monitoring of the highly variable WN8 Wolf-Rayet star WR123, obtained
by the MOST (Microvariability and Oscillations of STars) satellite, in a way
a kind of precursor to COROT to be launched in 2006. This rst Canadian
astronomical space telescope observed WR123 for 38 days non-stop during
June/July 2004. Fourier analysis shows that no periodic signal is stable for
more than several days in the low-frequency domain (f < 1d^-1), where most
of the power is contained. Also, no signi cant variability is seen at all in
the high-frequency domain (10d^-1 < f < 1400d^-1) down to the level of 0.2
mmag, an order of magnitude lower than theoretical predictions for strangemode
pulsations. On the other hand, there seems to be a relatively stable 9.8
h periodic signal present throughout the whole run. This period is probably
too short to represent the axial rotation of the star, unless related to multiple
sub-structures equidistantly spread along the stellar equator. It is also too
short to be orbital in nature; it is more likely to be related to pulsational instablilities
(although with a much longer period than expected), thus nally
revealing a possible fundamental driver behind the highly variable wind of
this object, and others of similar type.
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October 11th, 2005
"Detection of Long-Period Variations in the Subdwarf B star PG0101+039 on the Basis of Photometry from the MOST Satellite."
Authors: Suzanne Randall, J.M Matthews et al.
Abstract:
We report the detection of three discrete pulsation frequencies in the long-period variable subdwarf B star PG0101+039 on the basis of ~400 hoursof MOST wide-band photometry. The periodicities uncovered lie at 7235s,5227S, and 2650s respectively and are associated with amplitudes between 0.03% and 0.06% of the mean brightness, lower than those measured in any other variables of this kind. We also find evidence for luminosity variations consistent with an ellipsoidal deformation of the subdwarf in the rotationally locked short-period binary system predicted from radial velocity measurements and evolutionary models. Our atmospheric modelling of two independant time-averaged optical spectra of PG0101+039 yields Teff ~ 28,300 K and log(g) ~ 5.52, making it one of the hottest long-period variable subdwarf B stars known. The fact that we nevertheless detect brightness variations in the data is in conflict with predictions from current models, which place the theoretical blue ed!
ge for observable long-period instablities at a temperature around 4000 K cooler than that of PG0101+039.
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September 21, 2005
"Stellar Model Analysis of the Oscillation Spectrum of eta Bootis Obtained from MOST"
Authors: D.B Guenther,T.Kallinger et al.
Abstract:
Eight consecutive low-frequency radial p-modes are identified in the G0 IV star eta Bootis based on 27 days of ultraprecise rapid photometry obtained by the MOST (Microvariability & Oscillations of Stars) satellite. The MOST data extend smoothly to lower overtones the sequence of radial p-modes reported in earlier groundbased spectroscopy by other groups. The sampling is nearly continuous, hence, the ambiguities in p-mode identifications due to aliases, such as the cycle/day alias found in ground observations, are not an issue. The lower-overtone modes from the MOST data constrain the interior structure of the model of eta Boo, giving a best fit on a grid of ~300,000 stellar models for a composition of (X, Z) = (0.71, 0.04), a mass of M = 1.71±0.05 Msun, and an age of t = 2.40±0.03
Gyr. The surface temperature and luminosity of this model, which was constrained only by using the oscillation modes, is close (one sigma) to current best estimates of eta Boo’s surface temperature and luminosity. With the interior fit anchored by the lower-overtone modes seen by MOST, standard models are not able to fit the higher overtone modes with the same level of accuracy. The discrepancy, model minus observed frequency, increases from 0.5 µHz at 250 µHz to 5 µHz at 1000 µHz, and is similar to the discrepancy that exists between the Sun’s observed p-mode frequencies and the p-mode frequencies of the standard solar model. This discrepancy promises to be a powerful constraint on models of 3D convection.
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March 24, 2005
"Pulsations of the Oe star ζ Ophiuchi from MOST satellite photometry and ground-based spectroscopy"
Authors: G.A.H. Walker, R. Kuschnig et al.
Abstract:
Twenty-four days of highly precise Microvariability and Oscillations of STars (MOST) satellite photometry obtained in mid-2004 of the rapidly rotating O9.5 V star ζ Ophiuchi have yielded at least a dozen significant oscillation frequencies between 1 and 10 cycles day-1, clearly indicating its relationship to β Cephei variables. Eight periods were found in the He I λ4922 and Hβ line profile variations (LPV) of which six coincide with those from the MOST photometry. This unique photometric and spectroscopic detection of radial and nonradial pulsations leads to a plausible model in which high l-modes are excited when their frequencies in the corotating frame are similar to those of low-order radial modes. We propose that the dominant photometric 4.6 hr period (f1) corresponds to a radial first overtone excited by the κ-mechanism associated with the Fe opacity bump. No unambiguous rotational period can be identified in!
either the light curve or the LPV.
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October 25, 2004
"Differential rotation of the active G5V star kappa 1 Ceti:
Photometry from the MOST satellite"
Authors: Slavek M. Rucinski, Gordon A.H. Walker et al.
Abstract:
About 30.5 days of nearly uninterrupted broadband photometry of the solar-type star kappa 1 Ceti,
obtained with the MOST (Microvariability & Oscillations of STars) satellite, shows evidence for
two large starspots with different rotation periods of 8.9 and approximately 9.3 days.
Ground based measurements in 2002 and 2003 of Ca II H & K emission reveal variations in chromospheric
activity with a period of about 9.3 days. The data were obtained during the MOST commissioning phase.
When the data are combined with historical observations, they indicate that the 9.3-day spot has been
stable in its period for over 30 years. The photometry, with a sampling rate of approximately once per
minute, was also used to search for acoustic (p-mode) oscillations in the star. We detect no clear
evidence for p-modes in the kappa 1 Ceti photometry, with a noise level around 7 - 9 µmag at frequencies
in the range 0.5 - 4 mHz (3-sigma detection limit of 21 - 27 µmag). There were no flares or planetary
transits during the 30.5 days of MOST monitoring with light amplitudes greater than 2 mmag (durations
greater than 200 minutes) and 3 mmag (2 ? 200 min durations). While this rules out any close-in planets
of >= 0.5 Jupiter diameters with an orbital inclination close to 90 degrees, the scatter in differential radial
velocities permit a close giant planet in a more highly inclined orbit.
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July 1, 2004
"No stellar p-mode oscillations in space-based photometry of Procyon"
Refereed paper presenting results of 32-day observation run of Procyon - PDF
Authors: Jaymie Matthews, Rainer Kuschnig et al.
Appeared in the July 1, 2004 issue of Nature (Vol.430, pp 51-53).
Abstract: Pressure-driven (p-mode) oscillations
at the surface of the Sun, resulting from sound waves travelling through the
solar interior, are a powerful proble of solar structure, just as seismology
can reveal details about the interior of the Earth. Astronomers have hoped
to exploit p-mode asteroseismology in Sun-like stars to test detailed models
of stellar structure and evolution, but the observations are extremely
difficult. The bright star Procyon has been considered one of the best
candidates for asterseismology, on the basis of models and previous reports
of p-modes detected in ground-based spectroscopy. Here we present a search
for p-modes in 32 days of nearly continuous photometric satellite-based
observations of Procyon. If there are p-modes in Procyon, they must have
lifetimes less than 2-3 days and/or peak amplitudes <15 parts per million,
which defy expectations from the Sun's oscillations and previus theoretical
predictions. Target selection for future planned asterseismology space
missions may need to be reconsidered, as will the theory of stellar
oscillations.
Read the UBC press release
View the supplementary materials
Figure 1
Figure 2
Figure 3
Figure captions
View hi-res versions of the figures from the paper
Figure 1
Figure 2
Figure 3
Figure 4
Download the paper and all figures
Nature_Procyon.zip
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June 15, 2004
MOST gives us a unique look at a stellar "pre-teen", kappa 1 Ceti, which is much more active
than the Sun and whose "face hasn't cleared up."
Image/animation credit: H.R. MacMillan Space Centre and UBC
Click here to view the animation (4.47 MB)
You may need to download the DivX codec to view the animation.
read more...
Official press release
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