2,449 research outputs found
Distributions of Long-Lived Radioactive Nuclei Provided by Star Forming Environments
Radioactive nuclei play an important role in planetary evolution by providing
an internal heat source, which affects planetary structure and helps facilitate
plate tectonics. A minimum level of nuclear activity is thought to be necessary
--- but not sufficient --- for planets to be habitable. Extending previous work
that focused on short-lived nuclei, this paper considers the delivery of
long-lived radioactive nuclei to circumstellar disks in star forming regions.
Although the long-lived nuclear species are always present, their abundances
can be enhanced through multiple mechanisms. Most stars form in embedded
cluster environments, so that disks can be enriched directly by intercepting
ejecta from supernovae within the birth clusters. In addition, molecular clouds
often provide multiple episodes of star formation, so that nuclear abundances
can accumulate within the cloud; subsequent generations of stars can thus
receive elevated levels of radioactive nuclei through this distributed
enrichment scenario. This paper calculates the distribution of additional
enrichment for K, the most abundant of the long-lived radioactive
nuclei. We find that distributed enrichment is more effective than direct
enrichment. For the latter mechanism, ideal conditions lead to about 1 in 200
solar systems being directly enriched in K at the level inferred for the
early solar nebula (thereby doubling the abundance). For distributed enrichment
from adjacent clusters, about 1 in 80 solar systems are enriched at the same
level. Distributed enrichment over the entire molecular cloud is more
uncertain, but can be even more effective.Comment: 24 pages, 8 figures, accepted for publication in Ap
The Massive Star-forming Regions Omnibus X-ray Catalog
We present the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog
(MOXC), a compendium of X-ray point sources from {\em Chandra}/ACIS
observations of a selection of MSFRs across the Galaxy, plus 30 Doradus in the
Large Magellanic Cloud. MOXC consists of 20,623 X-ray point sources from 12
MSFRs with distances ranging from 1.7 kpc to 50 kpc. Additionally, we show the
morphology of the unresolved X-ray emission that remains after the catalogued
X-ray point sources are excised from the ACIS data, in the context of \Spitzer\
and {\em WISE} observations that trace the bubbles, ionization fronts, and
photon-dominated regions that characterize MSFRs. In previous work, we have
found that this unresolved X-ray emission is dominated by hot plasma from
massive star wind shocks. This diffuse X-ray emission is found in every MOXC
MSFR, clearly demonstrating that massive star feedback (and the
several-million-degree plasmas that it generates) is an integral component of
MSFR physics.Comment: Accepted to ApJS, March 3, 2014. 51 pages, 25 figure
Young stars and reflection nebulae near the lower "edge" of the Galactic molecular disc
We investigate the star formation occurring in a region well below the
Galactic plane towards the optical reflection nebula ESO 368-8 (IRAS
07383-3325). We confirm the presence of a small young stellar cluster (or
aggregate of tens of YSOs) identified earlier, embedded in a molecular cloud
located near the lower "edge" of the Galactic disc, and characterise the young
stellar population. We report the discovery of a near-infrared nebula, and
present a CO map revealing a new dense, dynamic cloud core. We used
near-infrared JHKs images, millimetre CO spectra and optical V-band images.
This star formation region displays an optical reflection nebula (ESO 368-8)
and a near-infrared nebula located about 46" (1.1 pc) from each other. The two
nebulae are likely to be coeval and to represent two manifestations of the same
single star formation episode with about 1 Myr age. The near-IR nebula reveals
an embedded, optically and near-IR invisible source whose light scatters off a
cavity carved by previous stellar jets or molecular outflows and into our
line-of-sight. The molecular cloud is fully covered by our CO(J=1-0) maps and,
traced by this line, extends over a region of 7.8 x 7.8 pc^2, exhibiting an
angular size 5.4' x 5.4' and shape (close to circular) similar to spherical (or
slightly cometary) globules. Towards the direction of the near-IR nebula, the
molecular cloud contains a dense core where the molecular gas exhibits large
line widths indicative of a very dynamical state, with stirred gas and
supersonic motions. Our estimates of the mass of the molecular gas in this
region range from 600 to 1600 solar masses. The extinction Av towards the
positions of the optical reflection nebula and of the near-IR nebula was found
to be Av=3-4 mag and Av=12-15 mag, respectively.Comment: 11 pages, 13 figure
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