637 research outputs found
Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant
We present high resolution (R~75,000), high signal-to-noise (S/N~100) Ca II
3933.663 and Na I 5889.951, 5895.924 spectra of 68
stars in the direction of the Vela supernova remnant. The spectra comprise the
most complete high resolution, high S/N, optical survey of early type stars in
this region of the sky. A subset of the sight lines has been observed at
multiple epochs, 1993/1994 and 1996. Of the thirteen stars observed twice,
seven have spectra revealing changes in the equivalent width and/or velocity
structure of lines, most of which arise from remnant gas. Such time variability
has been reported previously for the sight lines towards HD 72089 and HD 72997
by Danks & Sembach (1995) and for HD 72127 by Hobbs et al. (1991). We have
confirmed the ongoing time variability of these spectra and present new
evidence of variability in the spectra of HD 73658, HD 74455, HD 75309 and HD
75821. We have tabulated Na I and Ca II absorption line information for the
sight lines in our sample to serve as a benchmark for further investigations of
the dynamics and evolution of the Vela SNR.Comment: 8 pages of text, 4 tables, 16 pages of figures Accepted and to be
published in ApJ
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
K2-136: A Hyades Binary Star with a Neptune-sized Planet
We report the discovery of a Neptune-size planet (Rp = 3.0 Re) in the Hyades
Cluster. The host star is in a binary system, comprising a K5V star and M7/8V
star with a projected separation of 40 AU. The planet orbits the primary star
with an orbital period of 17.3 days and a transit duration of 3 hours. The host
star is bright (V=11.2, J=9.1) and so may be a good target for precise radial
velocity measurements. K2-136A c is the first Neptune-sized planet to be found
orbiting in a binary system within an open cluster. The Hyades is the nearest
star cluster to the Sun, has an age of 625-750 Myr, and forms one of the
fundamental rungs in the distance ladder; understanding the planet population
in such a well-studied cluster can help us understand and set constraints on
the formation and evolution of planetary systems.Comment: 13 pages, 8 figures, 2 tables. Accepted to AAS Journal
High resolution spectroscopy of symbiotic stars. VI. Orbital and stellar parameters for AR Pav
We present new dynamical parameters of the AR Pav binary system.
Ourobservations consist of a series of high resolution optical/NIR spectra from
which we derive the radial velocity curve of the red giant as well as its
rotation velocity. Assuming co-rotation, we determine the stellar radius (130
R_solar) of the red giant. Based on this we derive the red giant's luminosity
and mass (2.0 M_solar) as well as the distance of the system (4.9 kpc). The
binary mass function finally yields the companion's mass (0.75 M_solar) and the
binary separation (1.95 AU). We find that the red giant does not fill its Roche
lobe. We review the radial velocity data of Thackeray and Hutchings (1974), and
compare it with our red giant's orbit. We find that their RV curves of the blue
absorption system and the permitted emission lines are in anti-phase with the
red giant, and that the forbidden emission lines are shifted by a quarter of a
period. The blue absorptions and the permitted emission lines are associated
with the hot companion but not in a straightforward way. The blue absorption
system only tracks the hot component's orbital motion whilst it is in front of
the red giant, whereas at other phases line blanketing by interbinary material
leads to perturbations. We finally present UV light curves based on IUE archive
spectra. We clearly detect eclipses in the continuum at all wavelengths. The
eclipse light curves are unusual in that they show a slow and gradual decline
prior to eclipse which is followed by a sharp increase after eclipse.Comment: 9 pages, 5 figure
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
An analysis of ultraviolet spectra of Extreme Helium Stars and new clues to their origins
Abundances of about 18 elements including the heavy elements Y and Zr are
determined from Hubble Space Telescope Space Telescope Imaging Spectrograph
ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179,
V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of
the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The
abundance analyses is done using LTE line formation and LTE model atmospheres
especially constructed for these EHe stars. The stellar parameters derived from
an EHe's UV spectrum are in satisfactory agreement with those derived from its
optical spectrum. Adopted abundances for the seven EHes are from a combination
of the UV and optical analyses. Published results for an additional ten EHes
provide abundances obtained in a nearly uniform manner for a total of 17 EHes,
the largest sample on record. The initial metallicity of an EHe is indicated by
the abundance of elements from Al to Ni; Fe is adopted to be the representative
of initial metallicity. Iron abundances range from approximately solar to about
one-hundredth of solar. Clues to EHe evolution are contained within the H, He,
C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for
some stars is close to the predicted initial abundance yet the N abundance
indicates almost complete conversion of initial C, N, and O to N by the
CNO-cycles; (ii) three of the seven stars with UV spectra show a strong
enhancement of Y and Zr attributable to an s-process. The observed compositions
are discussed in light of expectations from accretion of a He white dwarf by a
CO white dwarf.Comment: 126 pages, 15 figures, 20 Tables, accepted for publication in the Ap
The Distances to Open Clusters from Main-Sequence Fitting. IV. Galactic Cepheids, the LMC, and the Local Distance Scale
We derive the basic properties of seven Galactic open clusters containing
Cepheids and construct their period-luminosity (P-L) relations. For our cluster
main-sequence fitting we extend previous Hyades-based empirical
color-temperature corrections to hotter stars using the Pleiades as a template.
We use BVI_{C}JHK_{s} data to test the reddening law, and include metallicity
effects to perform a more comprehensive study for our clusters than prior
efforts. The ratio of total to selective extinction R_V that we derive is
consistent with expectations. Assuming the LMC P-L slopes, we find =
-3.93 +/- 0.07 (statistical) +/- 0.14 (systematic) for 10-day period Cepheids,
which is generally fainter than those in previous studies. Our results are
consistent with recent HST and Hipparcos parallax studies when using the
Wesenheit magnitudes W(VI). Uncertainties in reddening and metallicity are the
major remaining sources of error in the V-band P-L relation, but a higher
precision could be obtained with deeper optical and near-infrared cluster
photometry. We derive distances to NGC4258, the LMC, and M33 of (m - M)_0 =
29.28 +/- 0.10, 18.34 +/- 0.06, and 24.55 +/- 0.28, respectively, with an
additional systematic error of 0.16 mag in the P-L relations. The distance to
NGC4258 is in good agreement with the geometric distance derived from water
masers [\Delta (m - M)_0 = 0.01 +/- 0.24]; our value for M33 is less consistent
with the distance from an eclipsing binary [\Delta (m - M)_0 = 0.37 +/- 0.34];
our LMC distance is moderately shorter than the adopted distance in the HST Key
Project, which formally implies an increase in the Hubble constant of 7% +/-
8%.Comment: 28 pages, 21 figures; accepted for publication in the Ap
Tomographic Separation of Composite Spectra. X. The Massive Close Binary HD 101131
We present the first orbital elements for the massive close binary, HD
101131, one of the brightest objects in the young, open cluster, IC 2944. This
system is a double-lined spectroscopic binary in an elliptical orbit with a
period of 9.64659 +/- 0.00012 days. It is a young system of unevolved stars
(approximately 2 million years old) that are well within their critical Roche
surfaces. We use a Doppler tomography algorithm to reconstruct the individual
component optical spectra, and we apply well known criteria to arrive at
classifications of O6.5 V((f)) and O8.5 V for the primary and secondary,
respectively. We compare the reconstructed spectra of the components to single
star, spectrum standards to determine a flux ratio of f_2/f_1 = 0.55 +/- 0.08
in the V-band. Both components are rotating faster than synchronously. We
estimate the temperatures and luminosities of the components from the observed
spectral classifications, composite V magnitude, and cluster distance modulus.
The lower limits on the masses derived from the orbital elements and the lack
of eclipses are 25 solar masses and 14 solar masses for the primary and
secondary, respectively. These limits are consistent with the larger masses
estimated from the positions of the stars in the Hertzsprung-Russell diagram
and evolutionary tracks for single stars.Comment: 17 pages, 3 figures, submitted to Ap
Recommended from our members
Lithium-oxide-stabilized alpha manganese dioxide for rechargeable lithium batteries
Objective was to increase the reversible cycling capacity of anhydrous {alpha}-MnO{sub 2} by stabilizing its structure. We have synthesized various {alpha}-MnO{sub 2} materials and modified their structures via lithia doping in order to determine what properties are important for controlling cycling performance. A set of new stabilized {alpha}-[xLi{sub 2}O]{center_dot}MnO{sub 2} (x<0.2) cathode materials was synthesized, structurally characterized, and electrochemically evaluated
A UBVR CCD Survey of the Magellanic Clouds
We present photometry and a preliminary interpretation of a UBVR survey of
the Large and Small Magellanic Clouds. We determine improved values for the
relative number of blue and red supergiants. We also compare the relative
number of Red Supergiants (RSGs) and Wolf-Rayet stars, demonstrating a strong,
tight correlation with metallicity, and reinvestigate the initial mass function
slope of massive stars found in the field.Comment: complete postscript (including embedded figures) can be found at:
ftp://ftp.lowell.edu/pub/massey/mcatlas.ps.gz Accepted for publication in the
ApJ
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