225 research outputs found
An Eccentric Eclipsing Binary: CGAur
In this study, we present CG Aur's photometric observations obtained in the
observing seasons 2011 and 2012, the first available multi-colour light curves.
Their shape indicates that the system is an Algol binary. The light curve
analyses reveal that CG Aur is a detached binary system with an effective
temperature difference between the components, approximately 1000 K. The first
estimate of the absolute dimensions of the components indicated that the system
locates on the main sequence in the HR diagram. The primary component is
slightly evolved from the ZAMS.Comment: 18 pages, 6 figures, 3 table
Modeling the near-UV band of GK stars, Paper II: NLTE models
We present a grid of atmospheric models and synthetic spectral energy
distributions (SEDs) for late-type dwarfs and giants of solar and 1/3 solar
metallicity with many opacity sources computed in self-consistent Non-Local
Thermodynamic Equilibrium (NLTE), and compare them to the LTE grid of Short &
Hauschildt (2010) (Paper I). We describe, for the first time, how the NLTE
treatment affects the thermal equilibrium of the atmospheric structure (T(tau)
relation) and the SED as a finely sampled function of Teff, log g, and [A/H]
among solar metallicity and mildly metal poor red giants. We compare the
computed SEDs to the library of observed spectrophotometry described in Paper I
across the entire visible band, and in the blue and red regions of the spectrum
separately. We find that for the giants of both metallicities, the NLTE models
yield best fit Teff values that are ~30 to 90 K lower than those provided by
LTE models, while providing greater consistency between \log g values, and, for
Arcturus, Teff values, fitted separately to the blue and red spectral regions.
There is marginal evidence that NLTE models give more consistent best fit Teff
values between the red and blue bands for earlier spectral classes among the
solar metallicity GK giants than they do for the later classes, but no model
fits the blue band spectrum well for any class. For the two dwarf spectral
classes that we are able to study, the effect of NLTE on derived parameters is
less significant.Comment: Submitted to The Astrophysical Journal. Observed spectrophotometric
library, and grids of NLTE and LTE) synthetic spectra for GK stars available
at http://www.ap.smu.ca/~ishort/PHOENI
The Discovery of a Strong Magnetic Field and Co-rotating Magnetosphere in the Helium-weak Star HD 176582
We report the detection of a strong, reversing magnetic field and variable
H-alpha emission in the bright helium-weak star HD 176582 (HR 7185). Spectrum,
magnetic and photometric variability of the star are all consistent with a
precisely determined period of 1.5819840 +/- 0.0000030 days which we assume to
be the rotation period of the star. From the magnetic field curve, and assuming
a simple dipolar field geometry, we derive a polar field strength of
approximately 7 kG and a lower limit of 52 degrees for the inclination of the
rotation axis. However, based on the behaviour of the H-alpha emission we adopt
a large inclination angle of 85 degrees and this leads to a large magnetic
obliquity of 77 degrees. The H-alpha emission arises from two distinct regions
located at the intersections of the magnetic and rotation equators and which
corotate with the star at a distance of about 3.5 R* above its surface. We
estimate that the emitting regions have radial and meridional sizes on the
order of 2 R* and azimuthal extents (perpendicular to the magnetic equator) of
less than approximately 0.6 R*. HD 176582 therefore appears to show many of the
cool magnetospheric phenomena as that displayed by other magnetic helium-weak
and helium-strong stars such as the prototypical helium-strong star sigma Ori
E. The observations are consistent with current models of magnetically confined
winds and rigidly-rotating magnetospheres for magnetic Bp stars.Comment: 16 pages, 6 figure
Projected Rotational Velocities and Stellar Characterization of 350 B Stars in the Nearby Galactic Disk
Projected rotational velocities (vsini) are presented for a sample of 350
early B-type main sequence stars in the nearby Galactic disk. The stars are
located within ~1.5 kpc from the Sun, and the great majority within 700 pc. The
analysis is based on high-resolution spectra obtained with the MIKE
spectrograph on the Magellan Clay 6.5-m telescope at the Las Campanas
Observatory in Chile.Spectral types were estimated based on relative
intensities of some key line absorption ratios and comparisons to synthetic
spectra. Effective temperatures were estimated from the reddening-free Q index,
and projected rotational velocities were then determined via interpolation on a
published grid that correlates the synthetic full width at half maximum of the
He I lines at 4026, 4388 and 4471 A with vsini. As the sample has been selected
solely on the basis of spectral types it contains an selection of B stars in
the field, in clusters, and in OB associations. The vsini distribution obtained
for the entire sample is found to be essentially flat for vsini values between
0-150 km/s, with only a modest peak at low projected rotational velocities.
Considering subsamples of stars, there appears to be a gradation in the vsini
distribution with the field stars presenting a larger fraction of the slow
rotators and the cluster stars distribution showing an excess of stars with
vsini between 70 and 130 km/s. Furthermore, for a subsample of potential
runaway stars we find that the vsini distribution resembles the distribution
seen in denser environments, which could suggest that these runaway stars have
been subject to dynamical ejection mechanisms.Comment: 38 pages, 11 figures. Complete sample table. AJ accepte
Tycho 2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infrared
emission due to the presence of circumstellar material. To identify such stars,
we have positionally correlated the infrared MSX point source catalogue and the
Tycho 2 optical catalogue. A near/mid infrared colour criteria has been
developed to select infrared excess stars. The search yielded 1938 excess
stars, over half (979) have never previously been detected by IRAS. The excess
stars were found to be young objects such as Herbig Ae/Be and Be stars, and
evolved objects such as OH/IR and carbon stars. A number of B type excess stars
were also discovered whose infrared colours could not be readily explained by
known catalogued objects.Comment: Added Comment: 16 pages, 9 figures, accepted for publication in MNRA
Cosmic Optical Background: the View from Pioneer 10/11
We present the new constraints on the cosmic optical background (COB)
obtained from an analysis of the Pioneer 10/11 Imaging Photopolarimeter (IPP)
data. After careful examination of data quality, the usable measurements free
from the zodiacal light are integrated into sky maps at the blue (~0.44 um) and
red (~0.64 um) bands. Accurate starlight subtraction is achieved by referring
to all-sky star catalogs and a Galactic stellar population synthesis model down
to 32.0 mag. We find that the residual light is separated into two components:
one component shows a clear correlation with thermal 100 um brightness, while
another betrays a constant level in the lowest 100 um brightness region.
Presence of the second component is significant after all the uncertainties and
possible residual light in the Galaxy are taken into account, thus it most
likely has the extragalactic origin (i.e., the COB). The derived COB brightness
is (1.8 +/- 0.9) x 10^(-9) and (1.2 +/- 0.9) x 10^(-9) erg/s/cm2/sr/A at the
blue and red band, respectively, or 7.9 +/- 4.0 and 7.7 +/- 5.8 nW/m2/sr. Based
on a comparison with the integrated brightness of galaxies, we conclude that
the bulk of the COB is comprised of normal galaxies which have already been
resolved by the current deepest observations. There seems to be little room for
contributions of other populations including "first stars" at these
wavelengths. On the other hand, the first component of the IPP residual light
represents the diffuse Galactic light (DGL) - scattered starlight by the
interstellar dust. We derive the mean DGL-to-100 um brightness ratios of 2.1 x
10^(-3) and 4.6 x 10^(-3) at the two bands, which are roughly consistent with
the previous observations toward denser dust regions. Extended red emission in
the diffuse interstellar medium is also confirmed.Comment: Accepted for publication in the Astrophysical Journal; Typos
correcte
The Frequency of Rapid Rotation Among K Giant Stars
We present the results of a search for unusually rapidly rotating giant stars
in a large sample of K giants (~1300 stars) that had been spectroscopically
monitored as potential targets for the Space Interferometry Mission's
Astrometric Grid. The stars in this catalog are much fainter and typically more
metal-poor than those of other catalogs of red giant star rotational
velocities, but the spectra generally only have signal-to-noise (S/N) of
~20-60, making the measurement of the widths of individual lines difficult. To
compensate for this, we have developed a cross-correlation method to derive
rotational velocities in moderate S/N echelle spectra to efficiently probe this
sample for rapid rotator candidates. We have discovered 28 new red giant rapid
rotators as well as one extreme rapid rotator with a vsini of 86.4 km/s. Rapid
rotators comprise 2.2% of our sample, which is consistent with other surveys of
brighter, more metal-rich K giant stars. Although we find that the temperature
distribution of rapid rotators is similar to that of the slow rotators, this
may not be the case with the distributions of surface gravity and metallicity.
The rapid rotators show a slight overabundance of low gravity stars and as a
group are significantly more metal-poor than the slow rotators, which may
indicate that the rotators are tidally-locked binaries.Comment: Accepted for publication in ApJ. 25 pages, 9 figures, 3 tables.
Tables 1 and 2 are provided in their full form as plain text ancillary file
Discovery of a Faint Companion to Alcor Using MMT/AO 5 m Imaging
We report the detection of a faint stellar companion to the famous nearby A5V
star Alcor (80 UMa). The companion has M-band ( = 4.8 m)
magnitude 8.8 and projected separation 1".11 (28 AU) from Alcor. The companion
is most likely a low-mass (0.3 \msun) active star which is responsible
for Alcor's X-ray emission detected by ROSAT (L 10
erg/s). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d
25 pc, age 0.5 Gyr), and has been occasionally mentioned as a
possible distant (709") companion of the stellar quadruple Mizar ( UMa).
Comparing the revised Hipparcos proper motion for Alcor with the mean motion
for other UMa nuclear members shows that Alcor has a peculiar velocity of 1.1
km/s, which is comparable to the predicted velocity amplitude induced by the
newly-discovered companion (1 km/s). Using a precise dynamical parallax
for Mizar and the revised Hipparcos parallax for Alcor, we find that Mizar and
Alcor are physically separated by 0.36 0.19 pc (74 39 kAU; minimum
18 kAU), and their velocity vectors are marginally consistent (
probability 6%). Given their close proximity and concordant motions we suggest
that the Mizar quadruple and the Alcor binary be together considered the 2nd
closest stellar sextuplet. The addition of Mizar-Alcor to the census of stellar
multiples with six or more components effectively doubles the local density of
such systems within the local volume (d 40 pc).Comment: 28 pages, 2 figures, AJ, in press; emulateapj short version at
http://www.pas.rochester.edu/~emamajek/alcor.pd
Evidence for the White Dwarf Nature of Mira B
The nature of the accreting companion to Mira --- the prototypical pulsating
asymptotic giant branch star --- has been a matter of debate for more than 25
years. Here we use a quantitative analysis of the rapid optical brightness
variations from this companion, Mira B, which we observed with the Nickel
telescope at Lick Observatory, to show that it is a white dwarf (WD). The
amplitude of aperiodic optical variations on time scales of minutes to tens of
minutes (approximately 0.2 mag) is consistent with that of accreting WDs in
cataclysmic variables on these same time scales. It is significantly greater
than that expected from an accreting main-sequence star. With Mira B identified
as a WD, its ultraviolet (UV) and optical luminosities, along with constraints
on the WD effective temperature from the UV, indicate that it accretes at
~1e-10 solar masses per year. We do not find any evidence that the accretion
rate is higher than predicted by Bondi-Hoyle theory. The accretion rate is high
enough, however, to explain the weak X-ray emission, since the accretion-disk
boundary layer around a low-mass WD accreting at this rate is likely to be
optically thick and therefore to emit primarily in the far or extreme UV.
Furthermore, the finding that Mira B is a WD means that it has experienced, and
will continue to experience nova explosions, roughly every million years. It
also highlights the similarity between Mira AB and other jet-producing
symbiotic binaries such as R Aquarii, CH Cygni, and MWC 560, and therefore
raises the possibility that Mira B launched the recently discovered bipolar
streams from this system.Comment: Accepted for publication in Ap
High Resolution Spectroscopy of the high galactic latitude RV Tauri star CE Virginis
Analysis of the surface composition of the suspected cool RV Tauri star CE
Vir shows no systematic trend in depletions of elements with respect to
condensation temperature. However, there is a significant depletion of the
elements with respect to the first ionization potential of the element. The
derived Li abundance of log (Li) = 1.50.2 indicates production
of Li in the star. Near infrared colours indicate sporadic dust formation close
to the photosphere.Comment: 12 pages, including 8 pages: Accepted for publication in MNRA
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