1,021 research outputs found
First results of an Hα based search of classical Be stars in the Perseus Arm and beyond
We investigate a region of the Galactic plane, between 120° †l †140° and-1° †bâ€+4°, and uncover a population of moderately reddened (E(B-V) ~ 1) classical Be stars within and beyond the Perseus and Outer Arms. 370 candidate emission-line stars (13âČrâČ16) selected from the Isaac Newton Telescope Photometric Ha Survey of the Northern Galactic plane have been followed up spectroscopically. A subset of these, 67 stars with properties consistent with those of classical Be stars, have been observed at sufficient spectral resolution (Ύλ â 2-4 Ă
) at blue wavelengths to narrow down their spectral types. We determine these to a precision estimated to be ±1 subtype and then we measure reddenings via spectral energy distribution fitting with reference to appropriate model atmospheres. Corrections for contribution to colour excess from circumstellar discs are made using an established scaling to Ha emission equivalent width. Spectroscopic parallaxes are obtained after luminosity class has been constrained via estimates of distances to neighbouring A/F stars with similar reddenings. Overwhelmingly, the stars in the sample are confirmed as luminous classical Be stars at heliocentric distances ranging from 2 kpc up to ~12 kpc. However, the errors are presently too large to enable the cumulative distribution function with respect to distance to distinguish between models placing the stars exclusively in spiral arms, or in a smooth exponentially declining distribution.Peer reviewe
Discovery of a Transition to Global Spin-up in EXO 2030+375
EXO 2030+375, a 42-second transient X-ray pulsar with a Be star companion,
has been observed to undergo an outburst at nearly every periastron passage for
the last 13.5 years. From 1994 through 2002, the global trend in the pulsar
spin frequency was spin-down. Using RXTE data from 2003 September, we have
observed a transition to global spin-up in EXO 2030+375. Although the spin
frequency observations are sparse, the relative spin-up between 2002 June and
2003 September observations, along with an overall brightening of the outbursts
since mid 2002 observed with the RXTE ASM, accompanied by an increase in
density of the Be disk, indicated by infrared magnitudes, suggest that the
pattern observed with BATSE of a roughly constant spin frequency, followed by
spin-up, followed by spin-down is repeating. If so this pattern has
approximately an 11 year period, similar to the 15 +/- 3 year period derived by
Wilson et al. (2002) for the precession period of a one-armed oscillation in
the Be disk. If this pattern is indeed repeating, we predict a transition from
spin-up to spin-down in 2005.Comment: Accepted for publication in ApJ Letters, 4 pages, 5 figures, using
emulateapj.cl
A Photometric Technique to Search for Be Stars in Open Clusters
We describe a technique to identify Be stars in open clusters using Stromgren
b, y, and narrow-band Halpha photometry. We first identify the B-type stars of
the cluster using a theoretical isochrone fit to the (b-y, y) color-magnitude
diagram. The strongest Be stars are easily identified in a (b-y, y-Halpha)
color-color diagram, but those with weaker Halpha emission (classified as
possible Be star detections) may be confused with evolved or foreground stars.
Here we present such photometry plus Halpha spectroscopy of members of the
cluster NGC 3766 to demonstrate the accuracy of our technique. Statistical
results on the relative numbers of Be and B-type stars in additional clusters
will be presented in a future paper.Comment: 15 pages, 6 figures, 1 table. Accepted by Ap
Unveiling the nature of IGR J16493-4348 with IR spectroscopy
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is discovering
a large number of new hard X-ray sources, many of them being HMXBs. The
identification and spectral characterization of their optical/infrared
counterparts is a necessary step in undertaking a detailed study of these
systems. In a previous paper, we presented spectral analyses and
classifications of six newly discovered INTEGRAL sources. In this paper, we
extend the analysis to IGR J16493-4348. We used the ESO/VLT ISAAC spectrograph
to observe the proposed IR counterpart to the source, obtaining a Ks
medium-resolution spectrum (R = 500) with a signal-to-noise ratio (S/N) >150.
We classified the source by comparing with published atlases. We spectrally
classified the source as a B0.5-1 supergiant and estimated its interstellar
extinction. We compared the extinction derived from X-ray data with effective
interstellar extinction obtained from our data, discussing the absorption
component associated with the circumstellar environment.Comment: 4 pages, 1 figure, 3 tables. Accepted for publication in Astronomy
and Astrophysics
Photometry of Be Stars in the vicinity of COROT primary targets for asteroseismology
We present differential photometry of Be stars close to potential COROT
primary targets for asteroseismology. Several stars are found to be short
period variables. We propose them to be considered as secondary targets in the
COROT asteroseismology fields.Comment: 7 pages, 6 figures, submitted to Communications in Asteroseismolog
NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HII Region N 66
In the third part of our photometric study of the star-forming region NGC
346/N~66 and its surrounding field in the Small Magellanic Cloud (SMC), we
focus on the large number of low-mass pre-main sequence (PMS) stars revealed by
the Hubble Space Telescope Observations with the Advanced Camera for Surveys.
We investigate the origin of the observed broadening of the pre-main sequence
population in the , CMD. The most likely explanations are either the
presence of differential reddening or an age spread among the young stars.
Assuming the latter, simulations indicate that we cannot exclude the
possibility that stars in NGC 346 might have formed in two distinct events
occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are
not homogeneously distributed across NGC 346, but instead are grouped in at
least five different clusters. On spatial scales from 0.8 to 8 (0.24 to
2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed
by a two-point angular correlation function is self-similar with a power law
slope . The clustering properties are quite similar to
Milky Way star forming regions like Orion OB or Oph. Thus molecular
cloud fragmentation in the SMC seems to proceed on the same spatial scales as
in the Milky Way. This is remarkable given the differences in metallicity and
hence dust content between SMC and Milky Way star forming regions.Comment: Accepted for publication in ApJ. 16 pages, 13 (low-resolution)
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