4,542 research outputs found
Recent MOST space photometry
The Microvariability and Oscillations of STars (MOST) photometric satellite
has already undertaken more than 64 primary campaigns which include some
clusters and has obtained observations of >850 secondary stars of which ~180
are variable. More than half of the variables pulsate, with the majority being
of B-type. Since 2006 January, MOST has operated with only a single CCD for
both guiding and science. The resulting increase in read-out cadence has
improved precision for the brightest stars. The 2007 light curve for Procyon
confirms the lack of predicted p-modes with photometric amplitudes exceeding 8
ppm as we found in 2004 and 2005. p-modes have been detected in other
solar-type stars as well as pre-main sequence objects, roAp and delta Scuti
variables. g-modes have been detected in a range of slowly pulsating B stars,
Be stars and beta Cephei variables. Differential rotation has been defined for
several spotted solar-type stars and limits set to the albedo of certain
transiting planets and the presence of other perturbing planets. The mission is
expected to continue as long as the experiment operates.Comment: 9 pages, 7 figures, from HELAS-II meetin
Modulation of Ca II H & K Emission by Short-Period Planets
We have detected modulation of the Ca II H&K reversal structure in four out
of five 51 Peg-type stars whose planets have orbital periods between 3 and 4
days. We observe two effects in the K-core: (1) a broad 3-A variation at ~1%
level and (2) changes on a scale of 0.5 A (~1-3%) in each of the three reversal
features. The nightly variations are coherent in both H and K. From
differential radial velocities measured to better than 20 m/s, up-to-date
phases were extracted. The enhancements in the reversals tend to be greatest at
the sub-planetary points which may imply that there is a magnetic interaction
between the star's outer layers and the magnetosphere of the planet. These
high-S/N (500 per pixel in the continuum) and high-resolution (R = 110,000)
data are too few to confirm orbital synchronization.Comment: 4 pages, 2 figures, in newpasp.sty format. To be published by PASP
(ed. Drake Deming) in the proceedings of the conference entitled "Scientific
Frontiers in Research on Extrasolar Planets," held at the Carnegie
Institution of Washington, DC, June 18-21, 200
Evidence for Planet-induced Chromospheric Activity on HD 179949
We have detected the synchronous enhancement of Ca II H & K emission with the
short-period planetary orbit in HD 179949. High-resolution spectra taken on
three observing runs extending more than a year show the enhancement coincides
with phi ~ 0 (the sub-planetary point) of the 3.093-day orbit with the effect
persisting for more than 100 orbits. The synchronous enhancement is consistent
with planet-induced chromospheric heating by magnetic rather than tidal
interaction. Something which can only be confirmed by further observations.
Independent observations are needed to determine whether the stellar rotation
is sychronous with the planet's orbit. Of the five 51 Peg-type systems
monitored, HD 179949 shows the greatest chromospheric H & K activity. Three
others show significant nightly variations but the lack of any phase coherence
prevents us saying whether the activity is induced by the planet. Our two
standards, tau Ceti and the Sun, show no such nightly variations.Comment: 10 pages, 6 figures. Submitted to Ap
The Radial Velocity Precision of Fiber-fed Spectrographs
We have measured the radial velocities of five 51 Peg-type stars and one star
with constant velocity. Our measurements, on 20 \AA centered at 3947 \AA, were
conventional using Th/Ar comparison spectra taken every 20 or 40 minutes
between the stellar exposures. Existing IRAF routines were used for the
reduction. We find 20 m s, provided 4 measurements
(out of 72) with residuals are neglected. The observations were
made with the CFHT Gecko spectrograph, fiber-fed with the CAFE system
(R110,000). 10 m s seems possible with
additional care. This study was incidental to the main program and so not
exhaustive but the small value of implies that the fiber
feed/image slicer system on Gecko + CAFE, essentially eliminates the long
standing problem of guiding errors in radial velocity measurements. We are not
suggesting this conventional approach for serious Doppler planet searches
(especially with Gecko which has such a small multiplex gain), but the
precision is valuable for observations made in spectral regions remote from
telluric lines or captive-gas fiducials. Instrument builders might consider the
advantages of the CAFE optics which incorporate agitation and invert the object
and pupil for slit and grating illumination in future spectrograph designs.Comment: 12 pages, 3 figure
The On/Off Nature of Star-Planet Interactions
Evidence suggesting an observable magnetic interaction between a star and its
hot Jupiter appears as a cyclic variation of stellar activity synchronized to
the planet's orbit. In this study, we monitored the chromospheric activity of 7
stars with hot Jupiters using new high-resolution echelle spectra collected
with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT. We searched for
variability in several stellar activity indicators (Ca II H, K, the Ca II
infrared triplet, Halpha, and He I). HD 179949 has been observed almost every
year since 2001. Synchronicity of the Ca II H & K emission with the orbit is
clearly seen in four out of six epochs, while rotational modulation with
P_rot=7 days is apparent in the other two seasons. We observe a similar
phenomenon on upsilon And, which displays rotational modulation (P_rot=12 days)
in September 2005, in 2002 and 2003 variations appear to correlate with the
planet's orbital period. This on/off nature of star-planet interaction (SPI) in
the two systems is likely a function of the changing stellar magnetic field
structure throughout its activity cycle. Variability in the transiting system
HD 189733 is likely associated with an active region rotating with the star,
however, the flaring in excess of the rotational modulation may be associated
with its hot Jupiter. As for HD 179949, the peak variability as measured by the
mean absolute deviation for both HD 189733 and tau Boo leads the sub-planetary
longitude by 70 degrees. The tentative correlation between this activity and
the ratio of Mpsini to the planet's rotation period, a quantity proportional to
the hot Jupiter's magnetic moment, first presented in Shkolnik et al. 2005
remains viable. This work furthers the characterization of SPI, improving its
potential as a probe of extrasolar planetary magnetic fields.Comment: Accepted for publication in the Astrophysical Journa
Limits to Interstellar C_4 and C_5 Towards zeta Ophiuchi
We have made a sensitive search for the origin bands in the known electronic
transitions of the linear carbon chains C_4 and C_5 at 3789 and 5109 A towards
zeta Oph (A_V <= 1). The incentive was a recent detection of C_3 in this
interstellar cloud with a column density of 1.6 x 10^12 cm^-2 plus the
availability of laboratory gas phase spectra of C_4 and C_5. Further, some
models of diffuse interstellar clouds predict that the abundance of these
latter species should be within an order of magnitude of C_3. Despite achieving
S/N of 2300 to 2600 per pixel at a resolution of ~110,000, the searches were
negative, leading to 3 sigma upper limits to the column density of N(C_5) = 2 x
10^11 cm^-2 and N(C_4) = 4 x 10^12-13 cm^-2 where these values rely on
theoretically calculated oscillator strengths. The implication of these limits
are discussed on the choice of molecules for study in future attempts to
identify the carriers of the stronger diffuse interstellar bands.Comment: 12 pages, 3 figure
Investigating Ca II emission in the RS CVn binary ER Vulpeculae using the Broadening Function Formalism
The synchronously rotating G stars in the detached, short-period (0.7 d),
partially eclipsing binary, ER Vul, are the most chromospherically active
solar-type stars known. We have monitored activity in the Ca II H & K reversals
for almost an entire orbit. Rucinski's Broadening Function Formalism allows the
photospheric contribution to be objectively subtracted from the highly blended
spectra. The power of the BF technique is also demonstrated by the good
agreement of radial velocities with those measured by others from less crowded
spectral regions. In addition to strong Ca II emission from the primary and
secondary, there appears to be a high-velocity stream flowing onto the
secondary where it stimulates a large active region on the surface 30 - 40
degrees in advance of the sub-binary longitude. A model light curve with a spot
centered on the same longitude also gives the best fit to the observed light
curve. A flare with approximately 13% more power than at other phases was
detected in one spectrum. We suggest ER Vul may offer a magnified view of the
more subtle chromospheric effects synchronized to planetary revolution seen in
certain `51 Peg'-type systems.Comment: Accepted to AJ; 17 pages and 16 figure
An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite
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 . 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 x 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 x
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.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical
Journal (July 2006, v645n1
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