56 research outputs found
Spitzer 24 micron Survey of Debris Disks in the Pleiades
We performed a 24 micron 2 Deg X 1 Deg survey of the Pleiades cluster, using
the MIPS instrument on Spitzer. Fifty four members ranging in spectral type
from B8 to K6 show 24 micron fluxes consistent with bare photospheres. All Be
stars show excesses attributed to free-free emission in their gaseous
envelopes. Five early-type stars and four solar-type stars show excesses
indicative of debris disks. We find a debris disk fraction of 25 % for B-A
members and 10 % for F-K3 ones. These fractions appear intermediate between
those for younger clusters and for the older field stars. They indicate a decay
with age of the frequency of the dust-production events inside the planetary
zone, with similar time scales for solar-mass stars as have been found
previously for A-stars.Comment: accepted to Ap
The epsilon Chamaeleontis young stellar group and the characterization of sparse stellar clusters
We present the outcomes of a Chandra X-ray Observatory snapshot study of five
nearby Herbig Ae/Be (HAeBe) stars which are kinematically linked with the
Oph-Sco-Cen Association (OSCA). Optical photometric and spectroscopic followup
was conducted for the HD 104237 field. The principal result is the discovery of
a compact group of pre-main sequence (PMS) stars associated with HD 104237 and
its codistant, comoving B9 neighbor epsilon Chamaeleontis AB. We name the group
after the most massive member. The group has five confirmed stellar systems
ranging from spectral type B9-M5, including a remarkably high degree of
multiplicity for HD 104237 itself. The HD 104237 system is at least a quintet
with four low mass PMS companions in nonhierarchical orbits within a projected
separation of 1500 AU of the HAeBe primary. Two of the low-mass members of the
group are actively accreting classical T Tauri stars. The Chandra observations
also increase the census of companions for two of the other four HAeBe stars,
HD 141569 and HD 150193, and identify several additional new members of the
OSCA.
We discuss this work in light of several theoretical issues: the origin of
X-rays from HAeBe stars; the uneventful dynamical history of the
high-multiplicity HD 104237 system; and the origin of the epsilon Cha group and
other OSCA outlying groups in the context of turbulent giant molecular clouds.
Together with the similar eta Cha cluster, we paint a portrait of sparse
stellar clusters dominated by intermediate-mass stars 5-10 Myr after their
formation.Comment: Accepted for publication in the Astrophysical Journal. 32 pages and 7
figure
Debris disks in main sequence binary systems
We observed 69 A3-F8 main sequence binary star systems using the Multiband
Imaging Photometer for Spitzer onboard the Spitzer Space Telescope. We find
emission significantly in excess of predicted photospheric flux levels for
9(+4/-3)% and 40(+7/-6)% of these systems at 24 and 70 microns, respectively.
Twenty two systems total have excess emission, including four systems that show
excess emission at both wavelengths. A very large fraction (nearly 60%) of
observed binary systems with small (<3 AU) separations have excess thermal
mission. We interpret the observed infrared excesses as thermal emission from
dust produced by collisions in planetesimal belts. The incidence of debris
disks around main sequence A3-F8 binaries is marginally higher than that for
single old AFGK stars. Whatever combination of nature (birth conditions of
binary systems) and nurture (interactions between the two stars) drives the
evolution of debris disks in binary systems, it is clear that planetesimal
formation is not inhibited to any great degree. We model these dust disks
through fitting the spectral energy distributions and derive typical dust
temperatures in the range 100--200 K and typical fractional luminosities around
10^-5, with both parameters similar to other Spitzer-discovered debris disks.
Our calculated dust temperatures suggest that about half the excesses we
observe are derived from circumbinary planetesimal belts and around one third
of the excesses clearly suggest circumstellar material. Three systems with
excesses have dust in dynamically unstable regions, and we discuss possible
scenarios for the origin of this short-lived dust.Comment: ApJ, in press. 57 pages, including 7 figures (one of which is in
color
Local effects in astrometric binary orbits: perspective transformation and light-travel time
In order to get astrometric parameters achieving the precision permitted by
the the forthcoming generation of astrometri cmeasurements, it will be
necessary to take into account effects that were neglected until the present
time. Two effects concerning the orbital elements of binary stars are
considered hereafter: the former is the local perspective (LP) effect, which is
due to the variation of the distance and of the orientation of the orbital
plane during the observation time span. The latter effect is the light--travel
time (LTT), which is also related to the orientation of the orbital plane.
Taking these effects into account would allow to find the ascending nodes of
the orbits, and lead to orbital elements more accurate than when they are
ignored. It is derived from simulations that, at a distance of 5 pc, and
assuming velocities typical of Pop.I stars, the position of the right ascending
node could be derived for a few simulated unresolved binaries when the
astrometric measurements have errors around 1 microas. For the resolved brown
dwarf binary 2MASS J07464256 +2000321, it appears that ignoring the LP effect
would result in underestimating the masses of the components by 14 per cent of
the errors as soon as the astrometric errors are around 20 microas for each
measurement. However, a `degenerate LP solution', taking into account the
variation of the semi-major axis when the distance is varying, should provide
reliable masses when the measurement errors are larger than 1 or 2 microas. A
few binaries in the Gaia program could deserve a degenerate LP solution,
whereas a the complete LP+LTT solution could be justified for resolved binaries
observed with SIM.Comment: 11 pages, 9 figures, 3 tables Accepted by Monthly Notices of the
Royal Astronomical Societ
Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Late-B to Early-F Type Binaries
We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of
the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region,
including 18 of 23 Pleiades members. Five candidate Pleiades members have also
been detected, confirming their cluster membership. Fifty-seven sources have no
optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5.
The unidentified X-ray sources are probably background AGN and not stars. The
Chandra field of view contains seven intermediate mass cluster members. Five of
these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 +
F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have
high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties
comparable to non-flaring K-type stars. Since all five stars are visual or
spectroscopic binaries with X-ray properties similar to F-G stars, the
late-type binary companions are probably producing the observed coronal X-ray
emission. Strengthening this conclusion is the nondetection by Chandra of two A
stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray
luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7
stars with cooler companions. Despite the low number statistics, the Chandra
data appear to confirm the expectation that late-B and A stars are not strong
intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual
increase in coronal temperature with decreasing mass from F4 to K. M stars
appear to have somewhat cooler coronae than active K stars.Comment: 22 pages, 4 figures, 7 tables, to appear in Ap
The N2K Consortium. IV. New temperatures and metallicities for 100,000+ FGK dwarfs
We have created a framework to facilitate the construction of specialized
target lists for radial velocity surveys that are biased toward stars that (1)
possess planets and (2) are easiest to observe with current detection
techniques. We use a procedure that uniformly estimates fundamental stellar
properties of Tycho 2 stars, with errors, using spline functions of broadband
photometry and proper motion found in Hipparcos/Tycho 2 and 2MASS. We provide
estimates of temperature and distance for 2.4 million Tycho 2 stars that lack
trigonometric distances. For stars that appear to be FGK dwarfs according to
estimated temperature and absolute magnitude, we also derive [Fe/H] and
identify unresolved binary systems with mass ratios between 1.25 and 3. Our
spline function models are trained on the unique Valenti & Fischer (2005) set,
composed of 1000 dwarfs with precise stellar parameters estimated from HIRES
spectroscopy. For FGK dwarfs with V photometric error less than 0.05
magnitudes, or V < 9, our temperature model gives a one-sigma error of
+58.7/-65.9 K and our metallicity model gives a one-sigma error of +0.13/-0.14
dex. Our estimates of distance and spectral type enable us to isolate 354,822
Tycho 2 dwarfs, 321,996 of which are absent from Hipparcos, with giant and
subgiant contamination at 2.6% and 7.2%, respectively. 2,500 of these FGK
dwarfs are bright (V 0.2). Our metallicity
estimates have been used to identify targets for N2K (Fischer et al. 2005), a
large-scale radial velocity search for Hot Jupiters, which has published the
detection of 4 Hot Jupiters with one transit. The broadband filtering outlined
here is the first screening tier for N2K; the second tier is a low-resolution
spectroscopy program headed by S.E. Robinson (astro-ph/0510150).Comment: 32 pages, 8 figures, accepted by ApJS in October 2005. Data files
temporarily stored at http://www.ucolick.org/~ammons/tycho_parameter
The Very Slow Wind From the Pulsating Semiregular Red Giant L2 Pup
We have obtained 11.7 and 17.9 micron images at the Keck I telescope of the
circumstellar dust emission from L2 Pup, one of the nearest (D = 61 pc)
mass-losing, pulsating, red giants that has a substantial infrared excess. We
propose that the wind may be driven by the stellar pulsations with radiation
pressure on dust being relatively unimportant, as described in some recent
calculations. L2 Pup may serve as the prototype of this phase of stellar
evolution where it could lose about 15% of its initial main sequence mass.Comment: ApJ, in pres
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1
Atmospheric parameters and pulsational properties for a sample of \,Sct, \,Dor, and hybrid {\it Kepler} targets
We report spectroscopic observations for 19 \,Sct candidates observed
by the {\it Kepler} satellite both in long and short cadence mode. For all
these stars, by using spectral synthesis, we derive the effective temperature,
the surface gravity and the projected rotational velocity. An equivalent
spectral type classification has been also performed for all stars in the
sample. These determinations are fundamental for modelling the frequency
spectra that will be extracted from the {\it Kepler} data for asteroseismic
inference. For all the 19 stars, we present also periodograms obtained from
{\it Kepler} data. We find that all stars show peaks in both low-
(\,Dor; g mode) and high-frequency (\,Sct; p mode) regions.
Using the amplitudes and considering 5\,c/d as a boundary frequency, we
classified 3 stars as pure \,Dor, 4 as \,Dor\,-\,\
hybrid, Sct, 5 as \,Sct\,-\,\,Dor hybrid, and 6 as pure
\,Sct. The only exception is the star KIC\,05296877 which we suggest
could be a binary.Comment: 11 pages, 6 figures, MNRAS main journa
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