128 research outputs found
NICMOS Imaging of the Nuclei of Arp 220
We report high resolution imaging of the ultraluminous infrared galaxy Arp
220 at 1.1, 1.6, and 2.22 microns with NICMOS on the HST. The
diffraction-limited images at 0.1--0.2 arcsecond resolution clearly resolve
both nuclei of the merging galaxy system and reveal for the first time a number
of luminous star clusters in the circumnuclear envelope. The morphologies of
both nuclei are strongly affected by dust obscuration, even at 2.2 microns :
the primary nucleus (west) presents a crescent shape, concave to the south and
the secondary (eastern) nucleus is bifurcated by a dust lane with the southern
component being very reddened. In the western nucleus, the morphology of the
2.2 micron emission is most likely the result of obscuration by an opaque disk
embedded within the nuclear star cluster. The morphology of the central
starburst-cluster in the western nucleus is consistent with either a
circumnuclear ring of star formation or a spherical cluster with the bottom
half obscured by the embedded dust disk. Comparison of cm-wave radio continuum
maps with the near-infrared images suggests that the radio nuclei lie in the
dust disk on the west and near the highly reddened southern component of the
eastern complex. The radio nuclei are separated by 0.98 arcseconds
(corresponding to 364 pc at 77 Mpc) and the half-widths of the infrared nuclei
are approximately 0.2-0.5 arcseconds. At least 8, unresolved infrared sources
-- probably globular clusters -- are also seen in the circumnuclear envelope at
radii 2-7 arcseconds . Their near-infrared colors do not significantly
constrain their ages.Comment: LaTex, 15 pages with 1 gif figure and 5 postscript figures. ApJL
accepte
The Edinburgh-Cape Blue Object Survey - III. Zone 2; galactic latitudes -30? > b > -40?
The Edinburgh–Cape Blue Object Survey seeks to identify point sources with an ultraviolet
excess. Results for zone 2 of the survey are presented here, covering that part of the South
Galactic Cap between 30◦ and 40◦ from the Galactic plane and south of about −12. ◦ 3 of
declination. Edinburgh–Cape zone 2 comprises 66 UK Schmidt Telescope fields covering
about 1730 deg2, in which we find some 892 blue objects, including 423 hot subdwarfs
(∼47 per cent); 128 white dwarfs (∼14 per cent); 25 cataclysmic variables (∼3 per cent); 119
binaries (∼13 per cent), mostly composed of a hot subdwarf and a main-sequence F or G star;
66 horizontal branch stars (∼7 per cent) and 48 ‘star-like’ extragalactic objects (∼5 per cent).
A further 362 stars observed in the survey, mainly low-metallicity F- and G-type stars, are also
listed. Both low-dispersion spectroscopic classification and UBV photometry are presented for
almost all of the hot objects and either spectroscopy or photometry (or both) for the cooler
ones.Department of HE and Training approved lis
High Mass Triple Systems: The Classical Cepheid Y Car
We have obtained an HST STIS ultraviolet high dispersion Echelle mode
spectrum the binary companion of the double mode classical Cepheid Y Car. The
velocity measured for the hot companion from this spectrum is very different
from reasonable predictions for binary motion, implying that the companion is
itself a short period binary. The measured velocity changed by 7 km/ s during
the 4 days between two segments of the observation confirming this
interpretation. We summarize "binary" Cepheids which are in fact members of
triple system and find at least 44% are triples. The summary of information on
Cepheids with orbits makes it likely that the fraction is under-estimated.Comment: accepted by A
Multiperiodicity, modulations and flip-flops in variable star light curves I. Carrier fit method
The light curves of variable stars are commonly described using simple
trigonometric models, that make use of the assumption that the model parameters
are constant in time. This assumption, however, is often violated, and
consequently, time series models with components that vary slowly in time are
of great interest. In this paper we introduce a class of data analysis and
visualization methods which can be applied in many different contexts of
variable star research, for example spotted stars, variables showing the
Blazhko effect, and the spin-down of rapid rotators. The methods proposed are
of explorative type, and can be of significant aid when performing a more
thorough data analysis and interpretation with a more conventional method.Our
methods are based on a straightforward decomposition of the input time series
into a fast "clocking" periodicity and smooth modulating curves. The fast
frequency, referred to as the carrier frequency, can be obtained from earlier
observations (for instance in the case of photometric data the period can be
obtained from independently measured radial velocities), postulated using some
simple physical principles (Keplerian rotation laws in accretion disks), or
estimated from the data as a certain mean frequency. The smooth modulating
curves are described by trigonometric polynomials or splines. The data
approximation procedures are based on standard computational packages
implementing simple or constrained least-squares fit-type algorithms.Comment: 14 pages, 23 figures, submitted to Astronomy and Astrophysic
A Spectroscopic Survey of a Sample of Active M Dwarfs
A moderate resolution spectroscopic survey of Fleming's sample of 54 X-ray
selected M dwarfs with photometric distances less than 25 pc is presented.
Radial and rotation velocities have been measured by fits to the H-alpha
profiles. Radial velocities have been measured by cross correlation. Artificial
broadening of an observed spectrum has produced a relationship between H-alpha
FWHM and rotation speed, which we use to infer rotation speeds for the entire
sample by measurement of the H-alpha emission line. We find 3 ultra-fast
rotators (UFRs, vsini > 100km/s), and 8 stars with 30 < vsini < 100 km/s. The
UFRs have variable emission. Cross-correlation velocities measured for
ultra-fast rotators (UFRs) are shown to depend on rotation speed and the
filtering used. The radial velocity dispersion of the sample is 17 km/s. A new
double emission line spectroscopic binary with a period of 3.55 days has been
discovered, and another known one is in the sample. Three other objects are
suspected spectroscopic binaries, and at least six are visual doubles. The only
star in the sample observed to have significant lithium is a known TW Hya
Association member, TWA 8A. These results show that there are a number of young
(< 10^8 yr) and very young (< 10^7 yr) low mass stars in the immediate solar
neighbourhood. The H-alpha activity strength does not depend on rotation speed.
Our fast rotators are less luminous than similarly fast rotators in the
Pleiades. They are either younger than the Pleiades, or gained angular momentum
in a different way.Comment: 38 pages incl. 14 figures and 4 tables, plus 12 pages of table for
electronic journal only; LaTeX, aastex.cls. Accepted 07/18/02 for publication
in The Astronomical Journa
Is there a mass discrepancy in the Cepheid binary OGLE-LMC-CEP0227?
Context. The Cepheid mass discrepancy, the difference between masses
predicted from stellar evolution and stellar pulsation calculations, is a
challenge for the understanding of stellar astrophysics. Recent models of the
eclipsing binary Cepheid OGLE-LMC-CEP-0227 have suggested that the discrepancy
may be resolved. Aims. We explore for what physical parameters do stellar
evolution models agree with the measured properties of OGLE-LMC-CEP0227 and
compare to canonical stellar evolution models assuming no convective core
overshooting. Methods. We construct state-of-the-art stellar evolution models
for varying mass, metallicity, and convective core overshooting and compare the
stellar evolution predictions with the observed properties. Results. The
observed mass, effective temperature, and radius of the two stars in the binary
system are well fit by numerous combinations of physical parameters, suggesting
a Cepheid mass discrepancy of 10-20% relative to canonical stellar evolution
models. Conclusions. The properties of the observed binary Cepheid suggest that
the Cepheid mass discrepancy is still a challenge and requires more specific
observations, such as the rate of period change, to better constrain and
understand the necessary physics for stellar evolution models to resolve the
discrepancy.Comment: 5 pages, 3 figures, A&A accepte
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
The DA+dMe eclipsing binary EC13471-1258: its cup runneth over...just
EC13471-1258 is a detached eclipsing binary with Porb = 3h37m, comprising a
DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf lasting 14
min, and a large amplitude ellipsoidal variation are seen in the light curve.
Flares from the dMe star occur regularly. Each star contributes roughly equal
amounts of light at 5500 Ang.
HST STIS spectra show strong Ly alpha with weak metal lines, and yield Teff =
14220 K, log g = 8.34, Z = 1/30th solar, K = 138 km/s and V sin i = 400 km/s
for the white dwarf. Optical spectra yield the spectral type (M3.5-4.0), Teff =
3100 K, Z = solar, K = 266 km/s and V sin i = 140 km/s for the dMe star. The H
alpha emission line comprises 2 or more components and implies that very weak
mass transfer is occurring. The dynamical solution also implies that the dMe
star just fills its Roche lobe. Accurate masses and radii for each star were
derived: the dMe values favour the Clemens et al. (1998) mass-radius relation.
The large rotational velocity of the white dwarf (400 km/s) suggests that the
system has transferred mass in the past so that it is presently a hibernating
cataclysmic variable. The metallicity contrast between the component stars
provides an opportunity for tests of diffusion theory.Comment: 25 pages, 18 figures, accepted for publication in MNRA
The Cepheid mass discrepancy and pulsation-driven mass loss
Context. A longstanding challenge for understanding classical Cepheids is the
Cepheid mass discrepancy, where theoretical mass estimates using stellar
evolution and stellar pulsation calculations have been found to differ by
approximately 10 - 20%. Aims. We study the role of pulsation-driven mass loss
during the Cepheid stage of evolution as a possible solution to this mass
discrepancy. Methods. We computed stellar evolution models with a Cepheid
mass-loss prescription and various amounts of convective core overshooting. The
contribution of mass loss towards the mass discrepancy is determined using
these models, Results. Pulsation-driven mass loss is found to trap Cepheid
evolution on the instability strip, allowing them to lose about 5 - 10% of
their total mass when moderate convective core overshooting, an amount
consistent with observations of other stars, is included in the stellar models.
Conclusions. We find that the combination of moderate convective core
overshooting and pulsation-driven mass loss can solve the Cepheid mass
discrepancy.Comment: 4 pages, 2 figures and 2 tables. Accepted for publication A&A Letter
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