15,563 research outputs found
Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)
Seven of ten candidate H-alpha emission-line stars found in an objective
grism survey of a 1 square degree region in MBM 18, were observed
spectroscopically. Four of these have weak H-alpha emission, and 6 out of 7
have spectral types M1-M4V. One star is of type F7-G1V, and has H-alpha in
absorption. The spectra of three of the M-stars may show an absorption line of
LiI, although none of these is an unambiguous detection. For the six M-stars a
good fit is obtained with pre-main-sequence isochrones indicating ages between
7.5 and 15Myr. The molecular cloud mass, derived from the integrated 12CO(1-0)
emission, is 160Mo (for a distance of 120pc), much smaller than the virial mass
(10^3Mo), and the cloud is not gravitationally bound. Nor are the individual
clumps we identified through a clump-finding routine. Considering the relative
weakness or absence of the H-alpha emission, the absence of other emission
lines, and the lack of clear LiI absorption, the targets are not T Tauri stars.
With ages between 7.5 and 15Myr they are old enough to explain the lack of
lithium in their spectra. Based on the derived distances (60-250pc), some of
the stars may lie inside the molecular cloud (120-150pc). From the fact that
the cloud as a whole, as well as the individual clumps, are not gravitationally
bound, in combination with the ages of the stars we conclude that it is not
likely that (these) stars were formed in MBM 18.Comment: Accepted for publication in Astronomy & Astrophysics (20 pages
The Distance and Age of the SNR Kes 73 and AXP 1E 1841-045
We provide a new distance estimate to the supernova remnant (SNR) Kes 73 and
its associated anomalous X-ray pulsar (AXP) 1E 1841-045. 21 cm HI images and HI
absorption/ emission spectra from new VLA observations, and 13CO emission
spectra of Kes 73 and two adjacent compact HII regions (G27.276+0.148 and
G27.491+0.189) are analyzed. The HI images show prominent absorption features
associated with Kes 73 and the HII regions. The absorption appears up to the
tangent point velocity giving a lower distance limit to Kes 73 of 7.5 kpc,
which has previously been given as the upper limit. Also, G27.276+0.148 and
G27.491+0.189 are at the far kinematic distances of their radio recombination
line velocities. There is prominent HI emission in the range 80--90 km/s for
all three objects. The two HII regions show HI absorption at ~ 84 km/s, but
there is no absorption in the Kes 73 absorption spectrum. This implies an upper
distance limit of ~ 9.8 kpc to Kes 73. This corrected larger distance to Kes
73/ AXP 1E 1841-045 system leads to a refined age of the SNR of 500 to 1000 yr,
and a ~ 50% larger AXP X-ray luminosity.Comment: 10 pages, 2 figures, ApJ, dol:10.1086/"529120
Shocked H2 and Fe+ Dynamics in the Orion Bullets
Observations of H2 velocity profiles in the two most clearly defined Orion
bullets are extremely difficult to reconcile with existing steady-state shock
models. We have observed [FeII] 1.644um velocity profiles of selected bullets
and H2 1-0 S(1) 2.122um velocity profiles for a series of positions along and
across the corresponding bow-shaped shock fronts driven into the surrounding
molecular cloud. Integrated [FeII] velocity profiles of the brightest bullets
are consistent with theoretical bow shock predictions. However, observations of
broad, singly-peaked H2 1-0 S(1) profiles at a range of positions within the
most clearly resolved bullet wakes are not consistent with molecular shock
modelling. A uniform, collisionally broadened background component which
pervades the region in both tracers is inconsistent with fluorescence due to
the ionizing radiation of the Trapezium stars alone.Comment: 20 pages including 18 figures, published in MNRA
Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of
the Galactic HII region RCW 79. The millimetre continuum data reveal the
presence of massive fragments located in a dust emission ring surrounding the
ionized gas. The two most massive fragments are diametrically opposite each
other in the ring. The near-IR data, centred on the compact HII region located
at the south-eastern border of RCW 79, show the presence of an IR-bright
cluster containing massive stars along with young stellar objects with near-IR
excesses. A bright near- and mid-IR source is detected towards maser emissions,
1.2 pc north-east of the compact HII region centre. Additional information,
extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of
the bright IR sources observed towards RCW 79. Twelve luminous Class I sources
are identified towards the most massive millimetre fragments. All these facts
strongly indicate that the massive-star formation observed at the border of the
HII region RCW 79 has been triggered by its expansion, most probably by the
collect and collapse process.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images
have been highly compressed for astro-ph. A version of this paper with
higher-resolution figures is available at
http://www.oamp.fr/matiere/rcw79.pd
Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219
Context. Massive-star formation triggered by the expansion of HII regions.
Aims. To understand if sequential star formation is taking place at the
periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line
observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta,
Spain). Results. In the optical, Sh2-219 is spherically symmetric around its
exciting star; furthermore it is surrounded along three quarters of its
periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at
infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies
at the southwestern border of Sh2-219, in the HI gap. Two molecular
condensations, elongated along the ionization front, probably result from the
interaction between the expanding HII region and the molecular cloud. In this
region of interaction there lies a cluster containing many highly reddened
stars, as well as a massive star exciting an ultracompact HII region. More
surprisingly, the brightest parts of the molecular cloud form a `chimney',
perpendicular to the ionization front. This chimney is closed at its south-west
extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5
pc long. A luminous H-alpha emission-line star, lying at one end of the chimney
near the ionization front, may be responsible for this structure. Confrontation
of the observations with models of HII region evolution shows that Sh2-219 is
probably 10^5 yr old. The age and origin of the near-IR cluster observed on the
border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&
A star cluster at the edge of the Galaxy
We study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789) through NIR (JHK), molecular line-, and dust continuum observations. The kinematic distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of about 20.2 kpc, this object is at the edge of the (molecular) disk of the Galaxy. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ca. 1.3 pc and an average stellar surface density of ca. 12 pc^{-2}. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded in a 1000 Mo molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is < 40 K, and the total mass of the swept-up material is < 10 Mo. Near the center of the flow, indications of much higher temperatures are found, probably due to shocks. A spectrum of one of the probable cluster members shows a tentative likeness to that of a K3III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud
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