367 research outputs found
An object that defies stereotypes : X-ray observations of SBS 1150+599A - the binary nucleus of PN G135.9+55.9
We present X-ray observations of the close binary nucleus of the planetary nebula (PN) PN G135.9+55.9 obtained with the XMM satellite. The nebula is the most oxygen-poor PN known to date and is located in the Galactic halo. It is known to harbor a close binary nucleus of which only one component can be observed in optical-UV range. New X-ray observations show that the invisible component is a very hot compact star. This finding allows us to reconstruct the immediate past of the object and predict its future. The parameters of the binary components we determine strongly suggest that the precursor was a symbiotic supersoft X-ray source that finished its life by Roche lobe overflow. PN G135.9+55.9 is an excellent candidate for a future type Ia supernova
Massive stars in the hinterland of the young cluster, Westerlund 2
Accepted for publication in MNRAS, 13 July 2018. 16 pages, plus one-page table in an appendix.An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al.: these are located inside a box of 1.5 × 1.5 deg 2 and are selected on the basis of their extinctions and K magnitudes.We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 M ·. Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are -29.4 ± 1.7 and -14.4 ± 2.2 km s -1, respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected fromWesterlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to ~ 25 per cent. On sky, the positions of these stars and their proper motions show a near N-S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.Peer reviewe
Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalog
For some years now, we have been gathering optical spectra of DA white dwarfs
in an effort to study and define the empirical ZZ Ceti instability strip.
However, we have recently expanded this survey to include all the DA white
dwarfs in the McCook & Sion catalog down to a limiting visual magnitude of
V=17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs
from this ongoing survey. We have several specific areas of interest most
notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM
binary systems. Furthermore, we present a comparison of the ensemble properties
of our sample with those of other large surveys of DA white dwarfs, paying
particular attention to the distribution of mass as a function of effective
temperature.Comment: 8 pages, 7 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
A Far-UV Spectroscopic Analysis of the Central Star of the Planetary Nebula Longmore 1
We have performed a non-LTE spectroscopic analysis using far-UV and UV data
of the central star of the planetary nebula K1-26 (Longmore 1), and found Teff
= 120+/-10 kK, logg = 6.7 +0.3/-0.7, and y = 0.10. The temperature is
significantly hotter than previous results based on optical line analyses,
highlighting the importance of analyzing the spectra of such hot objects at
shorter wavelengths. The spectra show metal lines (from, e.g, carbon, oxygen,
sulfur, and iron). The signatures of most elements can be fit adequately using
solar abundances, confirming the classification of Longmore 1 as a high gravity
O(H) object. Adopting a distance of 800 pc, we derive R = 0.04 Rsun, L = 250
Lsun, and M = 0.6 Msun. This places the object on the white dwarf cooling
sequence of the evolutionary tracks with an age of ~= 65 kyr.Comment: 14 pages, 4 color figures. Accepted for publication in PAS
High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216
LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We
perform a comprehensive spectral analysis of high-resolution, high-S/N
ultraviolet observations obtained with FUSE and STIS aboard the HST as well as
the optical spectrum of LS V +4621 by means of state-of-the-art NLTE
model-atmosphere techniques in order to compare its photospheric properties to
theoretical predictions from stellar evolution theory as well as from diffusion
calculations.
From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization
equilibria, we determined an effective temperature of 95 +/- 2 kK with high
precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained
discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and
the parallax distance d = 129 +6/-5 pc of LS V +4621. For the first time, we
have identified Mg IV and Ar VI absorption lines in the spectrum of a
hydrogen-rich central star and determined the Mg and Ar abundances as well as
the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With
the realistic treatment of metal opacities up to the iron group in the
model-atmosphere calculations, the so-called Balmer-line problem (found in
models that neglect metal-line blanketing) vanishes.
Spectral analysis by means of NLTE model atmospheres has presently arrived at
a high level of sophistication, which is now hampered largely by the lack of
reliable atomic data and accurate line-broadening tables. Strong efforts should
be made to improve upon this situation.Comment: 13 pages, 20 figure
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