456 research outputs found
Open clusters in AurOB2
We study the area around the HII region Sh 2-234, including the young open
cluster Stock 8, to investigate the extent and definition of the association
Aur OB2 and the possible role of triggering in massive cluster formation. We
obtained Str\"omgren and J,H,Ks photometry for Stock 8 and Str\"omgren
photometry for two other cluster candidates in the area, which we confirm as
young open clusters and name Alicante 11 and Alicante 12. We took spectroscopy
of 33 early-type stars in the area, including the brightest cluster members. We
calculate a common distance of kpc for the three open
clusters and surrounding association. We derive an age 4-6 Ma for Stock 8, and
do not find a significantly different age for the other clusters or the
association. The star LSV +34 23, with spectral type O8 II(f), is likely the
main source of ionization of Sh 2-234. We observe an important population of
pre-main sequence stars, some of them with disks, associated with the B-type
members lying on the main-sequence. We interpret the region as an area of
recent star formation with some residual and very localized ongoing star
formation. We do not find evidence for sequential star formation on a large
scale. The classical definition of Aur OB2 has to be reconsidered, because its
two main open clusters, Stock 8 and NGC 1893, are not at the same distance.
Stock 8 is probably located in the Perseus arm, but other nearby HII regions
whose distances also place them in this arm show quite different distances and
radial velocities and, therefore, are not connected.Comment: 29 pages, 17 figures Accepted for publication in MNRA
A representative sample of Be Stars I: Sample Selection, Spectral Classification and Rotational Velocities
We present a sample of 58 Be stars containing objects of spectral types O9 to
B8.5 and luminosity classes III to V. We have obtained 3670 - 5070 Angstrom
spectra of the sample which are used to derive spectral types and rotational
velocities. We discuss the distribution of spectral types and rotational
velocities obtained and conclude that there are no significant selection
effects in our sample.Comment: 10 Pages, 9 Figures, Accepted for publication in A&A
The nature of 1WGA J1958.2+3232: a new intermediate polar
We present low and intermediate resolution spectroscopy of the optical counterpart to the recently discovered pulsating X-ray source 1WGA J1958.2+3232. The presence of strong HI , HeI and HeII emission lines together with the absence of absorption features rules out the possibility that the object is a massive star, as had recently been suggested. The observed X-ray and optical characteristics are consistent with the object being an intermediate polar. The double-peaked structure of the emission lines indicates that an accretion disc is present
SXP 323 - an unusual X-ray binary system in the Small Magellanic Cloud
Spectroscopic observations taken with the VLT/UVES telescope/instrument are
presented of the unusual Small Magellanic Cloud (SMC) X-ray binary system SXP
323 = AX J0051-733. This system shows a clear modulation at 0.71d in long term
optical photometry which has been proposed as the binary period of this system.
The high resolution optical spectra, taken at a range of phases during the
0.71d cycle, rule out this possibility. Instead it is suggested that this
long-term effect is due to Non Radial Pulsations (NRP) in the Be star companion
to SXP 323. In addition, the spectra show clear evidence for major changes in
the (V/R) ratio of the double peaks of the Balmer lines indicative of
asymmetries in the circumstellar disk. The complex structure of the
interstellar lines are also discussed in the context of the SMC structure.Comment: Accepted in MNRA
The massive binary population of the starburst cluster Westerlund 1
We present initial results from a long-baseline radial velocity survey for massive binaries in the cluster Westerlund 1. Four systems are examined: the dust-producing WC binary W239, the double-lined eclipsing binary W13, and the single-lined B0 supergiants W43a and W3003. Finally, the evolutionary implications for the population of massive stars in Westerlund 1 are discussed
The Be/X-ray Transient V0332+53: Evidence for a tilt between the orbit and the equatorial plane?
We present optical and infrared observations of BQ Cam, the optical
counterpart to the Be/X-ray transient system V0332+53. BQ Cam is shown to be an
O8-9Ve star, which places V0332+53 at a distance of ~7 kpc. H-alpha
spectroscopy and infrared photometry are used to discuss the evolution of the
circumstellar envelope. Due to the low inclination of the system, parameters
are strongly constrained. We find strong evidence for a tilt of the orbital
plane with respect to the circumstellar disc (pressumably on the equatorial
plane). Even though the periastron distance is only ~ 10 R_*, during the
present quiescent state the circumstellar disc does not extend to the distance
of periastron passage. Under these conditions, X-ray emission is effectively
prevented by centrifugal inhibition of accretion. The circumstellar disc is
shown to be optically dense at optical and infrared wavelengths, which together
with its small size, is taken as an indication of tidal truncation.Comment: 9 pages, 3 figures, uses mn.sty and epsfig Accepted for publication
in MNRA
Stellar tracers of the Cygnus Arm. II: A young open cluster in Cam OB3
Cam OB3 is the only defined OB association believed to belong to the Outer
Galactic Arm or Cygnus Arm. Very few members have been observed and the
distance modulus to the association is not well known. We attempt a more
complete description of the population of Cam OB3 and a better determination of
its distance modulus. We present uvby photometry of the area surrounding the
O-type stars BD +56 864 and LS I +57 138, finding a clear sequence of
early-type stars that define an uncatalogued open cluster, which we call
Alicante 1. We also present spectroscopy of stars in this cluster and the
surrounding association. From the spectral types for 18 very likely members of
the association and UBV photometry found in the literature, we derive
individual reddenings, finding a extinction law close to standard and an
average distance modulus DM=13.0+-0.4. This value is in excellent agreement
with the distance modulus to the new cluster Alicante 1 found by fitting the
photometric sequence to the ZAMS. In spite of the presence of several O-type
stars, Alicante 1 is a very sparsely populated open cluster, with an almost
total absence of early B-type stars. Our results definitely confirm Cam OB3 to
be located on the Cygnus Arm and identify the first open cluster known to
belong to the association.Comment: Accepted for publication in Astronomy & Astrophysics. Tables 7 & 8 to
appear only in electronic forma
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