101 research outputs found
Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalogue
We present refined coordinates and proper motion data for the high proper
motion (HPM) stars in the Luyten Half-Second (LHS) catalogue. The positional
uncertainty in the original Luyten catalogue is typically >10" and is often
>30". We have used the digital scans of the Palomar Observatory Sky Survey
(POSS) I and POSS II plates to derive more accurate positions and proper
motions of the objects. Out of the 4470 candidates in the LHS catalogue, 4323
objects were manually re-identified in the POSS I and POSS II scans. A small
fraction of the stars were not found due to the lack of finder charts and
digitized POSS II scans. The uncertainties in the revised positions are
typically ~2", but can be as high as ~8" in a few cases; this is a large
improvement over the original data. Cross-correlation with the Tycho-2 and
Hipparcos catalogues yielded 819 candidates (with m_R < 12). For these brighter
sources, the position and proper motion data have been replaced with the more
accurate Tycho/Hipparcos data. In total, we have revised proper motion
measurements and coordinates for 4040 stars and revised coordinates for 4330
stars, which are presented here.Comment: 108 pages. Accepted for Publication in ApJ Suppl. Some errors caused
by the transcription errors in the original LHS catalogue have been corrected
in this resubmission. The most current version of the catalogue is also
available online at http://www.stsci.edu/~ksahu/lh
Lucky Imaging survey for southern M dwarf binaries
While M dwarfs are the most abundant stars in the Milky Way, there is still
large uncertainty about their basic physical properties (mass, luminosity,
radius, etc.) as well as their formation environment. Precise knowledge of
multiplicity characteristics and how they change in this transitional mass
region, between Sun-like stars on the one side and very low mass stars and
brown dwarfs on the other, provide constraints on low mass star and brown dwarf
formation. In the largest M dwarf binary survey to date, we search for
companions to active, and thus preferentially young, M dwarfs in the solar
neighbourhood. We study their binary/multiple properties, such as the
multiplicity frequency and distributions of mass ratio and separation, and
identify short period visual binaries, for which orbital parameters and hence
dynamical mass estimates can be derived in the near future. The observations
are carried out in the SDSS i' and z' band using the Lucky Imaging camera
AstraLux Sur at the ESO 3.5 m New Technology Telescope. In the first part of
the survey, we observed 124 M dwarfs of integrated spectral types M0-M6 and
identified 34 new and 17 previously known companions to 44 stars. We derived
relative astrometry and component photometry for these systems. More than half
of the binaries have separations smaller than 1 arcsec and would have been
missed in a simply seeing-limited survey. Correcting our sample for selection
effects yields a multiplicity fraction of 32+/-6% for 108 M dwarfs within 52 pc
and with angular separations of 0.1-6.0 arcsec, corresponding to projected
separation 3-180 AU at median distance 30 pc. Compared to early-type M dwarfs
(M>0.3M_Sun), later type (and hence lower mass) M dwarf binaries appear to have
closer separations, and more similar masses.Comment: 18 pages, 9 figures. Minor corrections and changes. Revised to match
accepted A&A versio
Detection of a transit by the planetary companion of HD 80606
We report the detection of a transit egress by the ~ 3.9-Jupiter-mass planet
HD 80606b, an object in a highly-eccentric orbit (e ~ 0.93) about its parent
star of approximately solar type. The astrophysical reality of the signal of
variability in HD 80606 is confirmed by observation with two independent
telescope systems, and checks against several reference stars in the field.
Differential photometry with respect to the nearby comparison star HD 80607
provides a precise light curve. Modelling of the light curve with a full
eccentric-orbit model indicates a planet/star-radius ratio of 0.1057 +/-
0.0018, corresponding to a planet radius of 1.029 R_J for a solar-radius parent
star; and a precise orbital inclination of 89.285 +/- 0.023 degrees, giving a
total transit duration of 12.1 +/- 0.4 hours. The planet hence joins HD 17156b
in a class of highly eccentric transiting planets, in which HD 80606b has both
the longest period and most eccentric orbit. The recently reported discovery of
a secondary eclipse of HD 80606b by the Spitzer Space Observatory permits a
combined analysis with the mid-time of primary transit in which the orbital
parameters of the system can be tightly constrained. We derive a transit
ephemeris of T_tr = HJD (2454876.344 +/- 0.011) + (111.4277 +/- 0.0032) E.Comment: Accepted for publication in MNRAS Letter
A New Method to Identify Nearby, Young, Low-mass Stars
We describe a new method to identify young, late-type stars within ~150 pc of
the Earth that employs visual or near-infrared data and the GALEX GR4/5
database. For spectral types later than K5, we demonstrate that the ratio of
GALEX near-ultraviolet (NUV) to visual and near-IR emission is larger for stars
with ages between 10 and 100 Myr than for older, main sequence stars. A search
in regions of the sky encompassing the TW Hya and Scorpius-Centaurus
Associations has returned 54 high-quality candidates for followup.
Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 angstrom
absorption in at least 17 systems. Because GALEX surveys have covered a
significant fraction of the sky, this methodology should prove valuable for
future young star studies.Comment: 27 pages, 8 figures, accepted for publication in ApJ. Some
significant changes were made in proof, we recommend readers use the ApJ
versio
Orbit Determination of Close Binary Systems using Lucky Imaging
We present relative positions of visual binaries observed during 2009 with
the FastCam "lucky-imaging" camera at the 1.5-m Carlos Sanchez Telescope (TCS)
at the Observatorio del Teide. We obtained 424 CCD observations (averaged in
198 mean relative positions) of 157 binaries with angular separations in the
range 0.14-15.40", with a median separation of 0.51". For a given system, each
CCD image represents the sum of the best 10-25% images from 1000-5000
short-exposure frames. Derived internal errors were 7 mas in r and 1.2^{\circ}
(9 mas) in q. When comparing to systems with very well-known orbits, we find
that the rms deviation in r residuals is 23 mas, while the rms deviation in q
residuals is 0.73 deg/r. We confirmed 18 Hipparcos binaries and we report new
companions to BVD 36 A and J 621 B. For binaries with preliminary orbital
parameters, the relative radial velocity was estimated as well. We also present
four new revised orbits computed for LDS 873, BU 627 A-BC, BU 628 and HO 197
AB. This work is the first results on visual binaries using the FastCam
lucky-imaging camera.Comment: 23 pages, 10 figures, 14 tables, accepted August 18th, 2011, to be
published in MNRA
Chemical composition of A and F dwarfs members of the Pleiades open cluster
Abundances of 18 chemical elements have been derived for 16 A (normal and
chemically peculiar CP) and 5 F dwarfs members of the Pleiades open cluster in
order to set constraints on evolutionary models. The abundances, rotational
velocities and microturbulent velocities were derived by iteratively adjusting
synthetic spectra to observations at high resolution (R~42000 and R~75000) and
high signal-to-noise (S/N) ratios. The abundances obtained do not exhibit any
clear correlation with the effective temperature nor the projected rotational
velocity. Interestingly, A stars exhibit larger star-to-star variations in C,
Sc, Ti, V, Cr, Mn, Sr, Y, Zr and Ba than F stars. F stars exhibit solar
abundances for almost all the elements. In A stars, the abundances of Si, Ti
and Cr are found to be correlated with that of Fe, the [X/Fe] ratios being
solar for these three elements. The derived abundances have been compared to
the predictions of published evolutionary models at the age of Pleiades (100
Myr). For the F stars, the predicted slight underabundances of light elements
and overabundances of Cr, Fe and Ni are indeed confirmed by our findings. For A
stars, the predicted overabundances in iron peak elements are confirmed in a
few stars only. The large scatter of the abundances in A stars, already found
in the Hyades, Coma Berenices and the UMa group and in field stars appears to
be a characteristic property of dwarf A stars. The occurence of hydrodynamical
processes competing with radiative diffusion in the radiative zones of the A
dwarfs might account for the found scatter in abundances.Comment: 7 pages, 3 figures, accepted in A&
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
XHIP-II: Clusters and associations
Context. In the absence of complete kinematic data it has not previously been
possible to furnish accurate lists of member stars for all moving groups. There
has been an unresolved dispute concerning the apparent inconsistency of the
Hipparcos parallax distance to the Pleiades.
Aims. To find improved candidate lists for clusters and associations
represented among Hipparcos stars, to establish distances, and to cast light on
the Pleiades distance anomaly.
Methods. We use a six dimensional fitting procedure to identify candidates,
and plot CMDs for 20 of the nearest groups. We calculate the mean parallax
distance for all groups.
Results. We identify lists of candidates and calculated parallax distances
for 42 clusters and 45 associations represented within the Hipparcos catalogue.
We find agreement between parallax distance and photometric distances for the
most important clusters. For single stars in the Pleiades we find mean parallax
distance 125.6 \pm 4.2 pc and photometric distance 132 \pm 3 pc calibrated to
nearby groups of similar in age and composition. This gives no reason to doubt
either the Hipparcos database or stellar evolutionary theory.Comment: Accepted for publication in Astronomy Letters, 10 pages, 2 fig
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
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
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