162 research outputs found
Hypervelocity stars in the Gaia era: Runaway B stars beyond the velocity limit of classical ejection mechanisms
Young massive stars in the halo are assumed to be runaway stars from the
Galactic disk. Possible ejection scenarios are binary supernova ejections (BSE)
or dynamical ejections from star clusters (DE). Hypervelocity stars (HVSs) are
extreme runaway stars that are potentially unbound from the Galaxy. Powerful
acceleration mechanisms such as the tidal disruption of a binary system by a
supermassive black hole (SMBH) are required to produce them. Therefore, HVSs
are believed to originate in the Galactic center (GC), the only place known to
host an SMBH. The second Gaia data release (DR2) offers the opportunity of
studying HVSs in an unprecedented manner. We revisit some of the most
interesting high-velocity stars, that is, 15 stars for which proper motions
with the Hubble Space Telescope were obtained in the pre-Gaia era, to unravel
their origin. By carrying out kinematic analyses based on revised
spectrophotometric distances and proper motions from Gaia DR2, kinematic
properties were obtained that help constrain the spatial origins of these
stars. Stars that were previously considered (un)bound remain (un)bound in
Galactic potentials favored by Gaia DR2 astrometry. For nine stars (five
candidate HVSs plus all four radial velocity outliers), the GC can be ruled out
as spatial origin at least at confidence level, suggesting that a
large portion of the known HVSs are disk runaway stars launched close to or
beyond Galactic escape velocities. The fastest star in the sample, HVS3, is
confirmed to originate in the Large Magellanic Cloud. Because the ejection
velocities of five of our non-GC stars are close to or above the upper limits
predicted for BSE and DE, another powerful dynamical ejection mechanism (e.g.,
involving massive perturbers such as intermediate-mass black holes) is likely
to operate in addition to the three classical scenarios mentioned above.Comment: Accepted for publication in A&A (Astronomy and Astrophysics
Hot subluminous stars
Hot subluminous stars of spectral type B and O are core helium-burning stars
at the blue end of the horizontal branch or have evolved even beyond that
stage. Strikingly, the distribution in the Hertzsprung-Russell diagram of
He-rich vs. He-poor hot subdwarf stars of the globular clusters omega Cen and
NGC~2808 differ from that of their field counterparts. The metal-abundance
patterns of hot subdwarfs are typically characterized by strong deficiencies of
some lighter elements as well as large enrichments of heavy elements. A large
fraction of sdB stars are found in close binaries with white dwarf or very
low-mass main sequence companions, which must have gone through a
common-envelope phase of evolution.They provide a clean-cut laboratory to study
this important but yet purely understood phase of stellar evolution. Substellar
companions to sdB stars have also been found. For HW~Vir systems the companion
mass distribution extends from the stellar into the brown dwarf regime. A giant
planet to the pulsator V391 Peg was the first discovery of a planet that
survived the red giant evolution of its host star. Several types of pulsating
star have been discovered among hot subdwarf stars, the most common are the
gravity-mode sdB pulsators (V1093 Her) and their hotter siblings, the p-mode
pulsating V361 Hya stars. Another class of multi-periodic pulsating hot
subdwarfs has been found in the globular cluster omega Cen that is unmatched by
any field star. The masses of hot subdwarf stars are the key to understand the
stars' evolution. A few pulsating sdB stars in eclipsing binaries have been
found that allow mass determination. The results are in good agreement with
predictions from binary population synthesis. New classes of binaries, hosting
extremely low mass (ELM) white dwarfs (M<0.3 Msun), have recently been
discovered, filling a gap in the mosaic of binary stellar evolution.
(abbreviated)Comment: 216 pages, 79 figures, PASP invited review, accepted 04/19/201
Heavy-metal enrichment in the intermediate He-sdOB pulsator Feige 46
The intermediate He-enriched hot subdwarf star Feige 46 was recently reported
as the second member of the V366 Aqr (or He-sdOBV) pulsating class. Feige 46 is
very similar to the prototype of the class, LS IV14116, not only in terms of
pulsational properties, but also in terms of atmospheric parameters and
kinematic properties. LS IV14116 is additionally characterized by a very
peculiar chemical composition, with extreme overabundances of the trans-iron
elements Ge, Sr, Y, and Zr. In this paper, we investigate the possibility that
the similitude between both pulsators extends to their chemical composition. We
retrieved archived optical and UV spectroscopic observations of Feige 46 and
perform an abundance analysis using model atmospheres and synthetic spectra
computed with TLUSTY and SYNSPEC. In total, we derive abundances for 16
metallic elements and provide upper limits for four additional elements. From
absorption lines in the optical spectrum of the star we measure an enrichment
of more than 10 000 solar for yttrium and zirconium. As for strontium,
the UV spectrum revealed it to be equally enriched. Our results confirm that
Feige 46 is not only a member of the now growing group of "heavy-metal"
subdwarfs, but also has an abundance pattern remarkably similar to that of LS
IV14116.Comment: 12 pages, Accepted for publication in A&
The origin of early-type runaway stars from open clusters
Runaway stars are ejected from their place of birth in the Galactic disk,
with some young B-type runaways found several tens of kiloparsecs from the
plane traveling at speeds beyond the escape velocity. Young open clusters are a
likely place of origin, and ejection may be either through N-body interactions
or in binary supernova explosions. The excellent quality of Gaia astrometry
opens up the path to study the kinematics of young runaway stars to such a high
precision that the place of origin in open stellar clusters can be identified
uniquely. We developed an efficient minimization method to calculate whether
two or more objects may come from the same place, which we tested against
samples of Orion runaways. Our fitting procedure was then used to calculate
trajectories for known runaway stars where we used Gaia data and updated radial
velocities. We found that only half of the sample could be classified as
runaways while the others were walkaway stars. Most of the latter stars turned
out to be binaries. We identified parent clusters for runaways based on their
trajectories and then used cluster age and flight time of the stars to
investigate whether the ejection was likely due to a binary supernova or due to
a dynamical ejection. In particular, we show that the classical runaways AE
Aurigae and Columbae might not have originated together, with
Columbae having an earlier ejection from Collinder 69, a cluster near the ONC.
The second sample investigated comprises a set of distant runaway B stars in
the halo which have been studied carefully by quantitative spectral analyses.
We are able to identify candidate parent clusters for at least four stars
including the hyper-runaway candidate HIP 60350. The ejection events had to be
very violent, ejecting stars at velocities as large as 150 to 400 km/s
Heavy metals in intermediate He-rich hot subdwarfs: The chemical composition of HZ44 and HD127493
Hot subluminous stars can be spectroscopically classified as subdwarf B (sdB)
and O (sdO) stars. While the latter are predominantly hydrogen deficient, the
former are mostly helium deficient. The atmospheres of most sdOs are almost
devoid of hydrogen, whereas a small group of hot subdwarf stars of mixed H/He
composition exists, showing extreme metal abundance anomalies. Whether such
intermediate helium-rich (iHe) subdwarf stars provide an evolutionary link
between the dominant classes is an open question. The presence of strong Ge,
Sn, and Pb lines in the UV spectrum of HZ44 suggests a strong enrichment of
heavy elements in this iHe-sdO star and calls for a detailed quantitative
spectral analysis focusing on trans-iron elements. Non-LTE model atmospheres
calculated with TLUSTY are combined with high-quality optical, UV and FUV
spectra of HZ44 and its hotter sibling HD127493 to determine their
atmospheric parameters and metal abundance patterns. By collecting atomic data
from literature we succeeded to determine abundances of 29 metals in HZ44,
including the trans-iron elements Ga, Ge, As, Se, Zr, Sn, and Pb and provide
upper limits for 10 other metals. This makes it the best described hot subdwarf
in terms of chemical composition. For HD127493 the abundance of 15 metals,
including Ga, Ge, and Pb and upper limits for another 16 metals were derived.
Heavy elements turn out to be overabundant by one to four orders of magnitude
with respect to the Sun. Zr and Pb are among the most enriched elements. The C,
N, and O abundance for both stars can be explained by nucleosynthesis of
hydrogen burning in the CNO cycle along with their helium enrichment. On the
other hand, the heavy-element anomalies are unlikely to be caused by
nucleosynthesis. Instead diffusion processes are evoked with radiative
levitation overcoming gravitational settlement of the heavy elements.Comment: Accepted for publication in Astronomy & Astrophysic
Quantitative spectroscopy of extreme helium stars - Model atmospheres and a non-LTE abundance analysis of BD+102179?
Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of
spectral type A and B. They are believed to result from mergers in double
degenerate systems. In this paper we present a detailed quantitative non-LTE
spectral analysis for BD+102179, a prototype of this rare class of
stars, using UVES and FEROS spectra covering the range from 3100 to 10
000 {\AA}. Atmosphere model computations were improved in two ways. First,
since the UV metal line blanketing has a strong impact on the
temperature-density stratification, we used the Atlas12 code. Additionally, We
tested Atlas12 against the benchmark code Sterne3, and found only small
differences in the temperature and density stratifications, and good agreement
with the spectral energy distributions. Second, 12 chemical species were
treated in non-LTE. Pronounced non-LTE effects occur in individual spectral
lines but, for the majority, the effects are moderate to small. The
spectroscopic parameters give = 17 300300 K and
= 2.800.10, and an evolutionary mass of 0.550.05 . The star
is thus slightly hotter, more compact and less massive than found in previous
studies. The kinematic properties imply a thick-disk membership, which is
consistent with the metallicity Fe/H and -enhancement.
The refined light-element abundances are consistent with the white dwarf merger
scenario. We further discuss the observed helium spectrum in an appendix,
detecting dipole-allowed transitions from about 150 multiplets plus the most
comprehensive set of known/predicted isolated forbidden components to date.
Moreover, a so far unreported series of pronounced forbidden He I components is
detected in the optical-UV.Comment: Accepted for publication in MNRAS, 26 pages, 19 Figure
A new method for an objective, -based spectroscopic analysis of early-type stars
A precise quantitative spectral analysis - encompassing atmospheric parameter
and chemical elemental abundance determination - is time consuming due to its
iterative nature and the multi-parameter space to be explored, especially when
done "by eye". A robust automated fitting technique that is as trustworthy as
traditional methods would allow for large samples of stars to be analyzed in a
consistent manner in reasonable time. We present a semi-automated quantitative
spectral analysis technique for early-type stars based on the concept of
minimization. The method's main features are: far less subjective than
typical "by eye" methods, correction for inaccurate continuum normalization,
consideration of the whole useful spectral range, simultaneous sampling of the
entire multi-parameter space (effective temperature, surface gravity,
microturbulence, macroturbulence, projected rotational velocity, radial
velocity, elemental abundances) to find the global best solution, applicable
also to composite spectra. The method is fast, robust and reliable as seen from
formal tests and from a comparison with previous analyses. Consistent
quantitative spectral analyses of large samples of early-type stars can be
performed quickly with very high accuracy.Comment: 32 pages, 4 figures, Astronomy and Astrophysics, accepte
- …