687 research outputs found
Hypervelocity Stars in the Gaia era. Revisiting the most extreme stars from the MMT survey
The hypervelocity star (HVS) survey conducted at the Multiple Mirror
Telescope (MMT) identified 42 B-type stars in the Galactic halo whose radial
velocity in the Galactic rest-frame exceeds kms. In order
to unravel the nature and origin of those high-velocity outliers, their
complete six-dimensional phase space information is needed. To this end, we
complemented positions and proper motions from the second data release of {\it
Gaia} with revised radial velocities and spectrophotometric distances that are
based on a reanalysis of the available MMT spectra of 40 objects using
state-of-the-art model spectra and a tailored analysis strategy. The resulting
position and velocity vectors for 37 stars were then used as input for a
subsequent kinematic investigation to obtain as complete a picture as possible.
The combination of projected rotational velocity, position in the Kiel diagram,
and kinematic properties suggests that all objects in the sample except two
(B576, B598) are very likely to be main sequence stars. While the available
data are still not precise enough to constrain the place of origin for 19
program stars, we identified eight objects that either come from the outer rim
of the Galactic disk or not from the disk at all, along with ten that
presumably stem from the Galactic disk. For almost all of those 18 targets with
more or less well-constrained spatial origin, the Galactic center (GC) is
disqualified as a possible place of origin. The most notable exception is B576,
the origin of which coincides extremely well with the GC when assuming a blue
horizontal branch (BHB) nature for it. HVS22 is by far the most extreme
object in the sample. Although its origin is completely unconstrained, an
ejection from the GC by the Hills mechanism is the most plausible explanation
for its current Galactic rest-frame velocity of
kms
NLTE spectroscopic analysis of the He anomaly in subluminous B-type stars
Several B-type main-sequence stars show chemical peculiarities. A
particularly striking class are the He stars, which exhibit a remarkable
enrichment of He with respect to He. This isotopic anomaly has also
been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which
are helium-core burning stars of the extreme horizontal branch. Using a hybrid
local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars,
we analyzed high-quality spectra of two known He BHBs and nine known He
sdBs to determine their isotopic helium abundances and He/He abundance
ratios. We redetermined their atmospheric parameters and analyzed selected
neutral helium lines, including 4922 and 6678
, which are very sensitive to He/He. Most of the He
sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are
helium deficient in accordance with previous LTE analyses. BD+48 2721
is reclassified as a BHB star because of its low temperature
( 20700 K). Whereas He is almost absent
(He/He 0.25) in most of the known He stars, other sample stars
show abundance ratios up to He/He2.51. A search for He stars in
the ESO SPY survey led to the discovery of two new He sdB stars (HE
0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and
of three sdBs are not matched by chemically homogeneous atmospheres, but hint
at vertical helium stratification. This phenomenon has been seen in other
peculiar B-type stars, but is found for the first time for sdBs. We estimate
helium to increase from the outer to the inner atmosphere by factors ranging
from 1.4 (SB 290) up to 8.0 (BD+48 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic
Hot subdwarf stars and their connection to thermonuclear supernovae
Hot subdwarf stars (sdO/Bs) are evolved core helium-burning stars with very
thin hydrogen envelopes, which can be formed by common envelope ejection. Close
sdB binaries with massive white dwarf (WD) companions are potential progenitors
of thermonuclear supernovae type Ia (SN Ia). We discovered such a progenitor
candidate as well as a candidate for a surviving companion star, which escapes
from the Galaxy. More candidates for both types of objects have been found by
crossmatching known sdB stars with proper motion and light curve catalogues.
The Gaia mission will provide accurate astrometry and light curves of all the
stars in our hot subdwarf sample and will allow us to compile a much larger
all-sky catalogue of those stars. In this way we expect to find hundreds of
progenitor binaries and ejected companions.Comment: Proceedings of the 11th Pacific Rim Conference on Stellar
Astrophysics, Hong Kong 2015, Journal of Physics: Conference Series, in pres
Candidate hypervelocity stars of spectral type G and K revisited
Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy.
When they were first discovered in 2005, dynamical ejection from the
supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as
their origin. The two dozen HVSs known today are young massive B stars, mostly
of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in
the Segue G and K dwarf sample, but none of them originates from the GC. We
embarked on a kinematic analysis of the Segue HVS candidate sample using the
full 6D phase space information based on new proper motion measurements. Their
orbital properties can then be derived by tracing back their trajectories in
different mass models of our Galaxy. We present the results for 14 candidate
HVSs, for which proper motion measurements were possible. Significantly lower
proper motions than found in the previous study were derived. Considering three
different Galactic mass models we find that all stars are bound to the Galaxy.
We confirm that the stars do not originate from the GC. The distribution of
their proper motions and radial velocities is consistent with predictions for
runaway stars ejected from the Galactic disk by the binary supernova mechanism.
However, their kinematics are also consistent with old disk membership.
Moreover, most stars have rather low metallicities and strong -element
enrichment as typical for thick disk and halo stars, whereas the metallicity of
the three most metal-rich stars could possibly indicate that they are runaway
stars from the thin disk. One star shows halo kinematics.Comment: A&A letter accepte
Recommended from our members
Crystal structure of bis(1,3-bis[{4-methyl-pyridin-2-yl}amido]-1,1,3,3-tetramethyldisiloxane)dichromium dichloride, [(C16H24N4OSi2)CrCl]2
C32H48Cl2Cr2N8O2Si=, monoclinic, P121/n1 (no. 14), a = 12.416(2) Å, b = 13.668(3) Å, c = 13.172(3) Å, β = 113.83(3)°, V= 2044.8 A3, Z = 2, Rgt(F) = 0.052, wRref(F2) = 0.110, T = 200 K. © 2014 Oldenbourg Wissenschaftsverlag GmbH, Rosenheimer Str. 145, 81671 München. All rights reserved
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