14,176 research outputs found
High-velocity gas towards the LMC resides in the Milky Way halo
To explore the origin of high-velocity gas in the direction of the Large
Magellanic Cloud (LMC) we analyze absorption lines in the ultraviolet spectrum
of a Galactic halo star that is located in front of the LMC at d=9.2 kpc
distance. We study the velocity-component structure of low and intermediate
metal ions in the spectrum of RXJ0439.8-6809, as obtained with the Cosmic
Origins Spectrograph (COS) onboard HST, and measure equivalent widths and
column densities for these ions. We supplement our COS data with a
Far-Ultraviolet Spectroscopic Explorer spectrum of the nearby LMC star Sk-69 59
and with HI 21cm data from the Leiden-Argentina-Bonn (LAB) survey. Metal
absorption towards RXJ0439.8-6809 is unambiguously detected in three different
velocity components near v_LSR=0,+60, and +150 km/s. The presence of absorption
proves that all three gas components are situated in front of the star, thus
being located in the disk and inner halo of the Milky Way. For the
high-velocity cloud (HVC) at v_LSR=+150 km/s we derive an oxygen abundance of
[O/H]=-0.63 (~0.2 solar) from the neighbouring Sk-69 59 sightline, in
accordance with previous abundance measurements for this HVC. From the observed
kinematics we infer that the HVC hardly participates in the Galactic rotation.
Our study shows that the HVC towards the LMC represents a Milky Way halo cloud
that traces low-column density gas with relatively low metallicity. It rules
out scenarios in which the HVC represents material close to the LMC that stems
from a LMC outflow.Comment: 4 pages, 3 figures; submitted to A&A Letter
Further search for a neutral boson with a mass around 9 MeV/c2
Two dedicated experiments on internal pair conversion (IPC) of isoscalar M1
transitions were carried out in order to test a 9 MeV/c2 X-boson scenario. In
the 7Li(p,e+e-)8Be reaction at 1.1 MeV proton energy to the predominantly T=0
level at 18.15 MeV, a significant deviation from IPC was observed at large pair
correlation angles. In the 11B(d,n e+e-)12C reaction at 1.6 MeV, leading to the
12.71 MeV 1+ level with pure T=0 character, an anomaly was observed at 9
MeV/c2. The compatibility of the results with the scenario is discussed.Comment: 12 pages, 5 figures, 2 table
Stellar Associations and their Field East of LMC 4 in the Large Magellanic Cloud
We report about the stellar content and the luminosity and mass functions of
three stellar associations and their field located on the north-east edge of
the super-bubble LMC 4 in the Large Magellanic Cloud.Comment: To be appeared in the meeting Proceedings of ``Modes of Star
Formation and the Origin of Field Populations'', Heidelberg, Germany, October
2000; to be published in the ASP Conference Series, edited by E. K. Grebel
and W. Brandne
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
ORFEUS II echelle spectra: Absorption by H_2 in the LMC
We present the first detection of molecular hydrogen (H_2) UV absorption
profiles on the line of sight to the LMC. The star LH 10:3120 in the LMC was
measured with the ORFEUS telescope and the Tuebingen echelle spectrograph
during the space shuttle mission of Nov./Dec. 1996. 16 absorption lines from
the Lyman band are used to derive the column densities of H_2 for the lowest 5
rotational states in the LMC gas. For these states we find a total column
density of N(H_2)=6.6 x 10^18$ cm^-2 on this individual line of sight. We
obtain equivalent excitation temperatures of T < 50 K for the rotational ground
state and T = 470 K for 0 < J < 6 by fitting the population densities of the
rotational states to theoretical Boltzmann distributions. We conclude that UV
pumping dominates the population of the higher rotational levels, as known from
the H_2 gas in the Milky Way. (Research supported in part by the DARA)Comment: Astronomy & Astrophysics, Letter, in pres
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