60 research outputs found
Single and Composite Hot Subdwarf Stars in the Light of 2MASS Photometry
Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data
Release Catalog, we have retrieved near-IR magnitudes for several hundred hot
subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically
Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample
size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS
photometry alone or in combination with visual photometry (Johnson BV or
Stromgren uvby) available in the literature, we show that it is possible to
identify hot subdwarf stars that exhibit atypically red IR colors that can be
attributed to the presence of an unresolved late type companion. Utilizing this
large sample, we attempt for the first time to define an approximately volume
limited sample of hot subdwarfs. We discuss the considerations, biases, and
difficulties in defining such a sample.
We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a
magnitude limited sample have colors that are consistent with the presence of
an unresolved late type companion. Binary stars are over-represented in a
magnitude limited sample. In an approximately volume limited sample the
fraction of composite-color binaries is about 30%.Comment: to appear in Sept 2003 AJ, 41 pages total, 12 figures, 2 tables are
truncated (full tables to appear in electronic journal or available by
request
A Search for Jovian Planets around Hot White Dwarfs
Current searches for extrasolar planets have concentrated on observing the
reflex Doppler shift of solar-type stars. Little is known, however, about
planetary systems around non-solar-type stars. We suggest a new method to
extend planetary searches to hot white dwarfs. Near a hot white dwarf, the
atmosphere of a Jovian planet will be photoionized and emit hydrogen
recombination lines, which may be detected by high- dispersion spectroscopic
observations. Multi-epoch monitoring can be used to distinguish between non-LTE
stellar emission and planetary emission, and to establish the orbital
parameters of the detected planets. In the future, high-precision astrometric
measurements of the hot white dwarf will allow the masses of the detected
planets to be determined. Searches for Jovian planets around hot white dwarfs
will provide invaluable new insight on the development of planetary systems
around stars more massive than the Sun and on how stellar evolution affects
these systems. We present high-dispersion spectroscopic observations of the
white dwarf Feige 34 to demonstrate the complexity and feasibility of the
search method.Comment: 10 pages, 2 figures, accepted for publication in the ApJ Letter
Stellar evolution with mass loss - comparison of numerical and semi-analytical computations.
We present here results of stellar evolution calculations that include the
latest advances in radiative opacities and neutrino cooling, and discuss on the
basis of these models how the internal stellar structure responds to mass-loss
from the stellar surface. This problem has particular importance for the
development of semi-analytical algorithms for effi cient calculation of
synthetic stellar populations with realistic (and hence complex) mass-loss
scenarios. We therefore compare our numerical results with test calculations
based on a semi-analytical stellar evolution method developed by us. Although
small, but important, differences between results from the two methods are
revealed, the evolutionary tracks in the HR-diagram predicted by the two
approaches are almost identical.Comment: uuencoded Postscript, 4 pages
Seasonal characteristics of the relationship between daily precipitation intensity and surface temperature
Past studies have argued that the intensity of extreme precipitation events should increase exponentially with temperature. This argument is based on the principle that the atmospheric moisture holding capacity increases according to the Clausius-Clapeyron equation and on the expectation that precipitation formation should follow accordingly. We test the latter assumption by investigating to what extent a relation with temperature can be observed intraseasonally in present-day climate. For this purpose, we use observed and simulated daily surface temperature and precipitation over Europe. In winter a general increase in precipitation intensity is indeed observed, while in summer we find a decrease in precipitation intensity with increasing temperature. We interpret these findings by making use of model results where we can distinguish separate precipitation types and investigate the moisture content in the atmosphere. In winter, the Clausius-Clapeyron relationship sets a limit to the increase in the large-scale precipitation with increasing temperature. Conversely, in summer the availability of moisture, and not the atmosphere's capacity to hold this moisture, is the dominant factor at the daily timescale. For convective precipitation, we find a peak like structure which is similar for all subregions, independent of the mean temperature, contrary to large-scale precipitation which has a more monotonic dependence on temperature
A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars
We present new results of a survey for weak magnetic fields among DA white
dwarfs with inclusion of some brighter hot subdwarf stars. We have detected
variable circular polarization in the Halpha line of the hot subdwarf star
Feige 34 (SP: sdO). From these data, we estimate that the longitudinal magnetic
field of this star varies from -1.1 +/- 3.2 kG to +9.6 +/- 2.6 kG, with a mean
of about +5 kG and a period longer than 2 h. In this study, we also confirm the
magnetic nature of white dwarf WD1105-048 and present upper limits of kilogauss
longitudinal magnetic fields of 5 brightest DA white dwarfs. Our data support
recent finding that 25% of white dwarfs have kilogauss magnetic fields. This
frequency also confirms results of early estimates obtained using the magnetic
field function of white dwarfs.Comment: accepted for publication in the Astrphysical Journa
Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk
We perform a detailed photometric study of the stellar populations in a
Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We
present the first U,B,V,I photometry of the old open cluster Auner1 and
determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the
background of the cluster, at more than 9 kpc, we detect a young population
most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect
the signature of an older population and identify its Turn Off and Red Giant
Branch. This population is found to have a mean age of 7 Gyrs and a mean
metallicity of Z = 0.006 . We reconstruct the geometry of the stellar
distribution and argue that this older population - often associated to the
Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk
component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in
the Astronomical Journa
Direct Observations of the Ionizing Star in the UC HII Region G29.96-0.02: A Strong Constraint on the Stellar Birth Line for Massive Stars
We have observed the ultracompact HII region G29.96-0.02 in the near infrared
J, H, and K bands and in the Br-gamma line. By comparison with radio
observations, we determine that the extinction to the nebula is AK = 2.14 with
a 3 sigma uncertainty of 0.25. We identify the ionizing star and determine its
intrinsic K magnitude. The star does not have an infrared excess and so appears
to be no longer accreting. The K magnitude and the bolometric luminosity allow
us to place limits on the location of the ionizing star in the HR diagram. The
3 sigma upper limit on the effective temperature of the ionizing star is 42500
K. We favor a luminosity appropriate for star with a mass in excess of about 60
solar masses. The limit on the temperature and luminosity exclude stars on the
ZAMS and stars within 10^6 yr of the ZAMS. Since the age of the UC HII region
is estimated to be only about 10^5 yr, we suggest that this is direct evidence
that the stellar birth line for massive stars at twice solar metallicity must
be significantly redder than the ZAMS.Comment: 42 pages; LaTex; 11 Postscript figures; accepted for publication in
Ap
Optical Multicolor Photometry of Spectrophotometric Standard Stars
Photoelectric data on the Johnson-Kron-Cousins UBVRI broadband photometric
system are provided for a set of stars which have been used as
spectrophotometric standard stars at the Hubble Space Telescope.Comment: 76 pages, 48 figures - published version available here:
http://www.journals.uchicago.edu/AJ/journal/issues/v133n3/205588/205588.html
. Also see ERRATUM at:
http://www.journals.uchicago.edu/AJ/journal/issues/v133n5/205838/205838.htm
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