44 research outputs found
Observations of H3+ in the Diffuse Interstellar Medium
Surprisingly large column densities of H3+ have been detected using infrared
absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12,
Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating
that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard
model of diffuse cloud chemistry, our H3+ column densities imply unreasonably
long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary
millimeter-wave, infrared, and visible observations of related species suggest
that the chemical model is incorrect and that the number density of H3+ must be
increased by one to two orders of magnitude. Possible solutions include a
reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller
value of the H3+ dissociative recombination rate constant than implied by
laboratory experiments.Comment: To be published in Astrophysical Journal, March 200
The nature of the high Galactic latitude O-star HD93521: new results from X-ray and optical spectroscopy
Owing to its unusual location and its isolation, the nature of the high
Galactic latitude O9.5Vp object HD93521 is still uncertain. We have collected
X-ray and optical observations to characterize the star and its surroundings.
X-ray images and spectra are analyzed to search for traces of a recent star
formation event around HD93521 and to search for the signature of a possible
compact companion. Optical echelle spectra are analysed with plane-parallel
model atmosphere codes, assuming either a spherical star or a gravity darkened
rotationally flattened star, to infer the effective temperature and surface
gravity, and to derive the He, C, N and O abundances of HD93521. The X-ray
images reveal no traces of a population of young low-mass stars coeval with
HD93521. The X-ray spectrum of HD93521 is consistent with a normal late O-type
star although with subsolar metallicity. No trace of a compact companion is
found in the X-ray data. In the optical spectrum, He and N are found to be
overabundant, in line with the effect of rotational mixing in this very fast
rotator, whilst C and O are subsolar. A critical comparison with the properties
of subdwarf OB stars, indicates that, despite some apparent similarities,
HD93521 does not belong to this category. Despite some ambiguities on the
runaway status of the star, the most likely explanation is that HD93521 is a
Population I massive O-type star that was ejected from the Galactic plane
either through dynamical interactions or a result of a supernova event in a
binary system.Comment: Accepted for publication in Astronomy & Astrophysic
High resolution optical spectroscopy of an LBV-candidate inside the CygOB2 association
For the first time, we obtained the high-resolution (R=15000 and 60000)
optical spectra for the extremely luminous star No.12, associated with the
IR-source IRAS20308+4104, a member of the CygOB2 association. We have found
about 200 spectral features in range 4552-7939AA, including the interstellar
NaI, KI lines and numerous DIBs, which are the strongest absorption lines in
the spectrum, along with the HeI, CII, and SiII lines. A two-dimensional
spectral classification indicates that the spectral type is B5+/-0.5 Ia+. Our
analysis of the Vr data shows the presence of a Vr gradient in the stellar
atmosphere, caused by the infall of matter onto the star. The strong Halpha
emission displays broad Thompson wings and time-variable core absorption,
providing evidence that the stellar wind is inhomogeneous, and a slightly
blue-shifted PCyg type absorption profile. We concluded that the wind is
variable in time.Comment: 20 pages, 5 figures, 2 table
IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery
We report on the discovery of over 50 strong H alpha emitting objects towards the large OB association Cyg OB2 and the H II region DR 15 on its southern periphery. This was achieved using the INT Photometric H alpha Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r', i' and H alpha photometry from IPHAS, and near-infrared J, H and K photometry from Two Micron All Sky Survey. The position of the objects in the (J - H) versus (H - K) diagram strongly suggests most of them are young. Many show Ca II infrared triplet emission indicating that they are in a pre-main-sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the H II region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable. The emission-line objects towards Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main-sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery