140 research outputs found
Astronomical Oxygen Isotopic Evidence for Supernova Enrichment of the Solar System Birth Environment by Propagating Star Formation
New infrared absorption measurements of oxygen isotope ratios in CO gas from
individual young stellar objects confirm that the solar system is anomalously
high in its 18O/17O ratio compared with extra-solar oxygen in the Galaxy. We
show that this difference in oxygen isotope ratios is best explained by 1 per
cent enrichment of the proto-solar molecular cloud by ejecta from type II
supernovae from a cluster having of order a few hundred stars that predated the
Sun by at least 10 to 20 Myr. The likely source of exogenous oxygen was the
explosion of one or more B stars during a process of propagating star
formation
On the oxygen isotopic composition of the Solar System
The 18O/17O ratio of the Solar System is 5.2 while that of the interstellar
medium (ISM) and young stellar objects is ~4. This difference cannot be
explained by pollution of the Sun's natal molecular cloud by 18O-rich supernova
ejecta because (1) the necessary B-star progenitors live longer than the
duration of star formation in molecular clouds; (2) the delivery of ejecta gas
is too inefficient and the amount of dust in supernova ejecta is too small
compared to the required pollution (2% of total mass or ~20% of oxygen); and
(3) the predicted amounts of concomitant short-lived radionuclides (SLRs)
conflicts with the abundances of 26Al and 41Ca in the early Solar System.
Proposals for the introduction of 18O-rich material must also be consistent
with any explanation for the origin of the observed slope-one relationship
between 17O/16O and 18O/16O in the high-temperature components of primitive
meteorites. The difference in 18O/17O ratios can be explained by enrichment of
the ISM by the 17O-rich winds of asymptotic giant branch (AGB) stars, the
sequestration of comparatively 18O-rich gas from star-forming regions into
long-lived, low-mass stars, and a monotonic decrease in the 18O/17O ratio of
interstellar gas. At plausible rates of star formation and gas infall, Galactic
chemical evolution does not follow a slope-one line in an three-isotope plot,
but instead moves along a steeper trajectory towards an 17O-rich state.
Evolution of the ISM and star-forming gas by AGB winds also explains the
difference in the carbon isotope ratios of the Solar System and ISM.Comment: accepted to ApJ Letter
The origin of short-lived radionuclides and the astrophysical environment of solar system formation
Based on early solar system abundances of short-lived radionuclides (SRs),
such as Al (T Myr) and Fe (T Myr),
it is often asserted that the Sun was born in a large stellar cluster, where a
massive star contaminated the protoplanetary disk with freshly
nucleosynthesized isotopes from its supernova (SN) explosion. To account for
the inferred initial solar system abundances of short-lived radionuclides, this
supernova had to be close ( 0.3 pc) to the young ( 1 Myr)
protoplanetary disk.
Here we show that massive star evolution timescales are too long, compared to
typical timescales of star formation in embedded clusters, for them to explode
as supernovae within the lifetimes of nearby disks. This is especially true in
an Orion Nebular Cluster (ONC)-type of setting, where the most massive star
will explode as a supernova 5 Myr after the onset of star formation,
when nearby disks will have already suffered substantial photoevaporation
and/or formed large planetesimals.
We quantify the probability for {\it any} protoplanetary disk to receive SRs
from a nearby supernova at the level observed in the early solar system. Key
constraints on our estimate are: (1) SRs have to be injected into a newly
formed ( 1 Myr) disk, (2) the disk has to survive UV
photoevaporation, and (3) the protoplanetary disk must be situated in an
enrichment zone permitting SR injection at the solar system level without disk
disruption. The probability of protoplanetary disk contamination by a supernova
ejecta is, in the most favorable case, 3 10
Stellar Kinematics of Young Clusters in Turbulent Hydrodynamic Simulations
The kinematics of newly-formed star clusters are interesting both as a probe
of the state of the gas clouds from which the stars form, and because they
influence planet formation, stellar mass segregation, cluster disruption, and
other processes controlled in part by dynamical interactions in young clusters.
However, to date there have been no attempts to use simulations of star cluster
formation to investigate how the kinematics of young stars change in response
to variations in the properties of their parent molecular clouds. In this
letter we report the results of turbulent self-gravitating simulations of
cluster formation in which we consider both clouds in virial balance and those
undergoing global collapse. We find that stars in these simulations generally
have velocity dispersions smaller than that of the gas by a factor of ~ 5,
independent of the dynamical state of the parent cloud, so that subvirial
stellar velocity dispersions arise naturally even in virialized molecular
clouds. The simulated clusters also show large-scale stellar velocity gradients
of ~0.2-2 km s pc and strong correlations between the centroid
velocities of stars and gas, both of which are observed in young clusters. We
conclude that star clusters should display subvirial velocity dispersions,
large-scale velocity gradients, and strong gas-star velocity correlations
regardless of whether their parent clouds are in virial balance, and,
conversely, that observations of these features cannot be used to infer the
dynamical state of the parent gas clouds.Comment: 5 pages, 4 figures, accepted to ApJ
Spitzer Observations of the HII Region NGC 2467: An Analysis of Triggered Star Formation
We present new Spitzer Space Telescope observations of the region NGC 2467,
and use these observations to determine how the environment of an HII region
affects the process of star formation. Our observations comprise IRAC (3.6,
4.5, 5.8, and 8.0 um) and MIPS (24 um) maps of the region, covering
approximately 400 square arcminutes. The images show a region of ionized gas
pushing out into the surrounding molecular cloud, powered by an O6V star and
two clusters of massive stars in the region. We have identified as candidate
Young Stellar Objects (YSOs) 45 sources in NGC 2467 with infrared excesses in
at least two mid-infrared colors. We have constructed color-color diagrams of
these sources and have quantified their spatial distribution within the region.
We find that the YSOs are not randomly distributed in NGC 2467; rather, over
75% of the sources are distributed at the edge of the HII region, along
ionization fronts driven by the nearby massive stars. The high fraction of YSOs
in NGC 2467 that are found in proximity to gas that has been compressed by
ionization fronts supports the hypothesis that a significant fraction of the
star formation in NGC 2467 is triggered by the massive stars and the expansion
of the HII region. At the current rate of star formation, we estimate at least
25-50% of the total population of YSOs formed by this process.Comment: Accepted to the Astrophysical Journal, set to appear in Volume 701;
18 pages, 13 figures, 4 tables. This version reflects a few major changes
made in the accepted version, including new figure
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Overview of the results of the organics PET Study of the cometary samples returned from comet Wild 2 by the Stardust mission
This presenation will provide an overview of the efforts and results produced by the Organics Preliminary Examination Team during their studies of the samples returned from comet Wild 2 by the Stardust spacecraft
Short-lived Nuclei in the Early Solar System: Possible AGB Sources
(Abridged) We review abundances of short-lived nuclides in the early solar
system (ESS) and the methods used to determine them. We compare them to the
inventory for a uniform galactic production model. Within a factor of two,
observed abundances of several isotopes are compatible with this model. I-129
is an exception, with an ESS inventory much lower than expected. The isotopes
Pd-107, Fe-60, Ca-41, Cl-36, Al-26, and Be-10 require late addition to the
solar nebula. Be-10 is the product of particle irradiation of the solar system
as probably is Cl-36. Late injection by a supernova (SN) cannot be responsible
for most short-lived nuclei without excessively producing Mn-53; it can be the
source of Mn-53 and maybe Fe-60. If a late SN is responsible for these two
nuclei, it still cannot make Pd-107 and other isotopes. We emphasize an AGB
star as a source of nuclei, including Fe-60 and explore this possibility with
new stellar models. A dilution factor of about 4e-3 gives reasonable amounts of
many nuclei. We discuss the role of irradiation for Al-26, Cl-36 and Ca-41.
Conflict between scenarios is emphasized as well as the absence of a global
interpretation for the existing data. Abundances of actinides indicate a
quiescent interval of about 1e8 years for actinide group production in order to
explain the data on Pu-244 and new bounds on Cm-247. This interval is not
compatible with Hf-182 data, so a separate type of r-process is needed for at
least the actinides, distinct from the two types previously identified. The
apparent coincidence of the I-129 and trans-actinide time scales suggests that
the last actinide contribution was from an r-process that produced actinides
without fission recycling so that the yields at Ba and below were governed by
fission.Comment: 92 pages, 14 figure files, in press at Nuclear Physics
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