1,770 research outputs found
Chemical tracers of episodic accretion in low-mass protostars
Aims: Accretion rates in low-mass protostars can be highly variable in time.
Each accretion burst is accompanied by a temporary increase in luminosity,
heating up the circumstellar envelope and altering the chemical composition of
the gas and dust. This paper aims to study such chemical effects and discusses
the feasibility of using molecular spectroscopy as a tracer of episodic
accretion rates and timescales.
Methods: We simulate a strong accretion burst in a diverse sample of 25
spherical envelope models by increasing the luminosity to 100 times the
observed value. Using a comprehensive gas-grain network, we follow the chemical
evolution during the burst and for up to 10^5 yr after the system returns to
quiescence. The resulting abundance profiles are fed into a line radiative
transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at
a series of time steps. We compare these spectra to observations taken from the
literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the
Herschel Space Observatory.
Results: The bursts are strong enough to evaporate CO throughout the
envelope, which in turn enhances the abundance of HCO+ and reduces that of
N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+
and N2H+ to return to normal. The chemical effects of the burst remain visible
in the rotational spectra for as long as 10^5 yr after the burst has ended,
highlighting the importance of considering luminosity variations when analyzing
molecular line observations in protostars. The spherical models are currently
not accurate enough to derive robust timescales from single-dish observations.
As follow-up work, we suggest that the models be calibrated against spatially
resolved observations in order to identify the best tracers to be used for
statistically significant source samples.Comment: Accepted by A&A; 12 pages, 7 figure
A developmental investigation of the relationship between appraisals and peer self-esteem in children experiencing peer-aggression
Transactional models of stress and coping emphasize the role played by cognitive appraisals in determining psychological adjustment (Lazarus, 1999). This proposition has been supported by research examining young people's adjustment in relation to family conflict and break-up (Grych et al., 1992). Furthermore, this literature suggests that there is a change in the relationship between appraisals and adjustment at around 10 years of age: specificity of appraisal type (e.g. threat, blame) becomes relevant to outcome after 10 years, whereas before 10 there are either no effects of appraisal on adjustment or a diffuse effect of 'negative' appraisals more generally (Jouriles et al., 2000). However, it is currently unclear whether this developmental progression can be generalized from familial- to social-stressors experienced by children and young people. The current study therefore evaluates the model within the context of a commonly experienced social childhood stressor: peer-aggression
Tentative detection of ethylene glycol toward W51/e2 and G34.3+0.2
How complex organic - and potentially prebiotic - molecules are formed in
regions of low- and high-mass star-formation remains a central question in
astrochemistry. In particular, with just a few sources studied in detail, it is
unclear what role environment plays in complex molecule formation. In this
light, a comparison of relative abundances of related species between sources
might be useful to explain observed differences. We seek to measure the
relative abundance between three important complex organic molecules, ethylene
glycol ((CHOH)), glycolaldehyde (CHOHCHO) and methyl formate
(HCOOCH), toward high-mass protostars and thereby provide additional
constraints on their formation pathways. We use IRAM 30-m single dish
observations of the three species toward two high-mass star-forming regions -
W51/e2 and G34.3+0.2 - and report a tentative detection of (CH2OH)2 toward both
sources. Assuming that (CHOH), CHOHCHO and HCOOCH spatially
coexist, relative abundance ratios, HCOOCH/(CHOH), of 31 and 35 are
derived for G34.3+0.2 and W51/e2, respectively. CHOHCHO is not detected,
but the data provide lower limits to the HCOOCH/CHOHCHO abundance
ratios of 193 for G34.3+0.2 and 550 for W51/e2. A comparison of these
results to measurements from various sources in the literature indicates that
the source luminosities may be correlated with the HCOOCH/(CHOH)
and HCOOCH/CHOHCHO ratios. This apparent correlation may be a
consequence of the relative timescales each source spend at different
temperatures-ranges in their evolution. Furthermore, we obtain lower limits to
the ratio of (CHOH)/CH2OHCHO for G34.3+0.2 (6) and W51/e2
(16). This result confirms that a high (CHOH)/CHOHCHO
abundance ratio is not a specific property of comets, as previously speculated.Comment: Accepted for publication by A&
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Molecule sublimation as a tracer of protostellar accretion: Evidence for accretion bursts from high angular resolution C18O images
The accretion histories of embedded protostars are an integral part of
descriptions of their physical and chemical evolution. In particular, are the
accretion rates smoothly declining from the earlier toward later stages or in
fact characterized by variations such as intermittent bursts? We aim to
characterize the impact of possible accretion variations in a sample of
embedded protostars by measuring the size of the inner regions of their
envelopes where CO is sublimated and relate those to their temperature profiles
dictated by their current luminosities. Using observations from the
Submillimeter Array we measure the extents of the emission from the C18O
isotopologue toward 16 deeply embedded protostars. We compare these
measurements to the predicted extent of the emission given the current
luminosities of the sources through dust and line radiative transfer
calculations. Eight out of sixteen sources show more extended C18O emission
than predicted by the models. The modeling shows that the likely culprit for
these signatures is sublimation due to increases in luminosities of the sources
by about a factor five or more during the recent 10,000 years - the time it
takes for CO to freeze-out again on dust grains. For four of those sources the
increase would have had to have been a factor 10 or more. The compact emission
seen toward the other half of the sample suggests that C18O only sublimates
when the temperature exceeds 30 K - as one would expect if CO is mixed with H2O
in the grain ice-mantles. The small-number statistics from this survey suggest
that protostars undergo significant bursts about once every 20,000 years. This
also illustrates the importance of taking the physical evolutionary histories
into account for descriptions of the chemical structures of embedded
protostars.Comment: Accepted by A&A; 11 pages, 5 figure
Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments
It is not known whether the original carriers of Earth's nitrogen were
molecular ices or refractory dust. To investigate this question, we have used
data and results of Herschel observations towards two protostellar sources: the
high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422.
Towards Orion KL, our analysis of the molecular inventory of Crockett et al.
(2014) indicates that HCN is the organic molecule that contains by far the most
nitrogen, carrying of nitrogen-in-organics. Following this
evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar
analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra
of HCN, and fit these observations against "jump" abundance models of IRAS
16293-2422's protostellar envelope. We find an inner-envelope HCN abundance
and an outer-envelope HCN
abundance . We also find the
sublimation temperature of HCN to be ~K; this
measured enables us to predict an HCN binding energy
~K. Based on a comparison of the HCN/H2O ratio
in these protostars to N/H2O ratios in comets, we find that HCN (and, by
extension, other organics) in these protostars is incapable of providing the
total bulk N/H2O in comets. We suggest that refractory dust, not molecular
ices, was the bulk provider of nitrogen to comets. However, interstellar dust
is not known to have 15N enrichment, while high 15N enrichment is seen in both
nitrogen-bearing ices and in cometary nitrogen. This may indicate that these
15N-enriched ices were an important contributor to the nitrogen in
planetesimals and likely to the Earth.Comment: Accepted to ApJ; 21 pages, 4 figure
Atomic jet from SMM1 (FIRS1) in Serpens uncovers non-coeval binary companion
We report on the detection of an atomic jet associated with the protostellar
source SMM1 (FIRS1) in Serpens. The jet is revealed in [FeII] and [NeII] line
maps observed with Spitzer/IRS, and further confirmed in HiRes IRAC and MIPS
images. It is traced very close to SMM1 and peaks at ~5 arcsec" from the source
at a position angle of $\sim 125 degrees. In contrast, molecular hydrogen
emission becomes prominent at distances > 5" from the protostar and extends at
a position angle of 160 degrees. The morphological differences suggest that the
atomic emission arises from a companion source, lying in the foreground of the
envelope surrounding the embedded protostar SMM1. In addition the molecular and
atomic Spitzer maps disentangle the large scale CO (3-2) emission observed in
the region into two distinct bipolar outflows, giving further support to a
proto-binary source setup. Analysis at the peaks of the [FeII] jet show that
emission arises from warm and dense gas (T ~1000 K, n(electron) 10^5 - 10^6
cm^-3). The mass flux of the jet derived independently for the [FeII] and
[NeII] lines is 10^7 M(sun)/yr, pointing to a more evolved Class~I/II protostar
as the driving source. All existing evidence converge to the conclusion that
SMM1 is a non-coeval proto-binary source.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in Astronomy
\& Astrophysic
Detection of 6 K gas in Ophiuchus D
Cold cores in interstellar molecular clouds represent the very first phase in
star formation. The physical conditions of these objects are studied in order
to understand how molecular clouds evolve and how stellar masses are
determined. The purpose of this study is to probe conditions in the dense,
starless clump Ophichus D (Oph D). The ground-state (1(10)-1(11)) rotational
transition of ortho-H2D+ was observed with APEX towards the density peak of Oph
D. The width of the H2D+ line indicates that the kinetic temperature in the
core is about 6 K. So far, this is the most direct evidence of such cold gas in
molecular clouds. The observed H2D+ spectrum can be reproduced with a
hydrostatic model with the temperature increasing from about 6 K in the centre
to almost 10 K at the surface. The model is unstable against any increase in
the external pressure, and the core is likely to form a low-mass star. The
results suggest that an equilibrium configuration is a feasible intermediate
stage of star formation even if the larger scale structure of the cloud is
thought to be determined by turbulent fragmentation. In comparison with the
isothermal case, the inward decrease in the temperature makes smaller, i.e.
less massive, cores susceptible to externally triggered collapse.Comment: 7 pages, 5 figures, accepted for Astronomy and Astrophysic
Hot water in the inner 100 AU of the Class 0 protostar NGC1333 IRAS2A
Evaporation of water ice above 100 K in the inner few 100 AU of low-mass
embedded protostars (the so-called hot core) should produce quiescent water
vapor abundances of ~10^-4 relative to H2. Observational evidence so far points
at abundances of only a few 10^-6. However, these values are based on spherical
models, which are known from interferometric studies to be inaccurate on the
relevant spatial scales. Are hot cores really that much drier than expected, or
are the low abundances an artifact of the inaccurate physical models? We
present deep velocity-resolved Herschel-HIFI spectra of the 3(12)-3(03) lines
of H2-16O and H2-18O (1097 GHz, Eup/k = 249 K) in the low-mass Class 0
protostar NGC1333 IRAS2A. A spherical radiative transfer model with a power-law
density profile is unable to reproduce both the HIFI data and existing
interferometric data on the H2-18O 3(13)-2(20) line (203 GHz, Eup/k = 204 K).
Instead, the HIFI spectra likely show optically thick emission from a hot core
with a radius of about 100 AU. The mass of the hot core is estimated from the
C18O J=9-8 and 10-9 lines. We derive a lower limit to the hot water abundance
of 2x10^-5, consistent with the theoretical predictions of ~10^-4. The revised
HDO/H2O abundance ratio is 1x10^-3, an order of magnitude lower than previously
estimated.Comment: Accepted by ApJ; 12 pages in emulateapj format; 7 figure
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