130 research outputs found
Protostellar accretion traced with chemistry: Comparing synthetic C18O maps of embedded protostars to real observations
Context: Understanding how protostars accrete their mass is a central
question of star formation. One aspect of this is trying to understand whether
the time evolution of accretion rates in deeply embedded objects is best
characterised by a smooth decline from early to late stages or by intermittent
bursts of high accretion.
Aims: We create synthetic observations of deeply embedded protostars in a
large numerical simulation of a molecular cloud, which are compared directly to
real observations. The goal is to compare episodic accretion events in the
simulation to observations and to test the methodology used for analysing the
observations.
Methods: Simple freeze-out and sublimation chemistry is added to the
simulation, and synthetic CO line cubes are created for a large number
of simulated protostars. The spatial extent of CO is measured for the
simulated protostars and compared directly to a sample of 16 deeply embedded
protostars observed with the Submillimeter Array. If CO is distributed over a
larger area than predicted based on the protostellar luminosity, it may
indicate that the luminosity has been higher in the past and that CO is still
in the process of refreezing.
Results: Approximately 1% of the protostars in the simulation show extended
CO emission, as opposed to approximately 50% in the observations,
indicating that the magnitude and frequency of episodic accretion events in the
simulation is too low relative to observations. The protostellar accretion
rates in the simulation are primarily modulated by infall from the larger
scales of the molecular cloud, and do not include any disk physics. The
discrepancy between simulation and observations is taken as support for the
necessity of disks, even in deeply embedded objects, to produce episodic
accretion events of sufficient frequency and amplitude.Comment: Accepted for publication in A&A, 11 pages, 8 figures; v2 contains
minor updates to the languag
Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view
Measuring the water deuterium fractionation in the inner warm regions of
low-mass protostars has so far been hampered by poor angular resolution
obtainable with single-dish ground- and space-based telescopes. Observations of
water isotopologues using (sub)millimeter wavelength interferometers have the
potential to shed light on this matter. Observations toward IRAS 16293-2422 of
the 5(3,2)-4(4,1) transition of H2-18O at 692.07914 GHz from Atacama Large
Millimeter/submillimeter Array (ALMA) as well as the 3(1,3)-2(2,0) of H2-18O at
203.40752 GHz and the 3(1,2)-2(2,1) transition of HDO at 225.89672 GHz from the
Submillimeter Array (SMA) are presented. The 692 GHz H2-18O line is seen toward
both components of the binary protostar. Toward one of the components, "source
B", the line is seen in absorption toward the continuum, slightly red-shifted
from the systemic velocity, whereas emission is seen off-source at the systemic
velocity. Toward the other component, "source A", the two HDO and H2-18O lines
are detected as well with the SMA. From the H2-18O transitions the excitation
temperature is estimated at 124 +/- 12 K. The calculated HDO/H2O ratio is (9.2
+/- 2.6)*10^(-4) - significantly lower than previous estimates in the warm gas
close to the source. It is also lower by a factor of ~5 than the ratio deduced
in the outer envelope. Our observations reveal the physical and chemical
structure of water vapor close to the protostars on solar-system scales. The
red-shifted absorption detected toward source B is indicative of infall. The
excitation temperature is consistent with the picture of water ice evaporation
close to the protostar. The low HDO/H2O ratio deduced here suggests that the
differences between the inner regions of the protostars and the Earth's oceans
and comets are smaller than previously thought.Comment: Accepted for publication in Astronomy & Astrophysic
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&
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
Externally heated protostellar cores in the Ophiuchus star-forming region
We present APEX 218 GHz observations of molecular emission in a complete
sample of embedded protostars in the Ophiuchus star-forming region. To study
the physical properties of the cores, we calculate HCO and c-CH
rotational temperatures, both of which are good tracers of the kinetic
temperature of the molecular gas. We find that the HCO temperatures range
between 16 K and 124 K, with the highest HCO temperatures toward the hot
corino source IRAS 16293-2422 (69-124 K) and the sources in the Oph A
cloud (23-49 K) located close to the luminous Herbig Be star S 1, which
externally irradiates the Oph A cores. On the other hand, the
c-CH rotational temperature is consistently low (7-17 K) in all
sources. Our results indicate that the c-CH emission is primarily
tracing more shielded parts of the envelope whereas the HCO emission (at
the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the
outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino
emission dominates. In some sources, a secondary velocity component is also
seen, possibly tracing the molecular outflow.Comment: 19 pages, 9 figures, accepted for publication in Ap
The Class 0 Protostar BHR71: Herschel Observations and Dust Continuum Models
We use Herschel spectrophotometry of BHR71, an embedded Class 0 protostar, to
provide new constraints on its physical properties. We detect 645 (non-unique)
spectral lines amongst all spatial pixels. At least 61 different spectral lines
originate from the central region. A CO rotational diagram analysis shows four
excitation temperature components, 43 K, 197 K, 397 K, and 1057 K. Low-J CO
lines trace the outflow while the high-J CO lines are centered on the infrared
source. The low-excitation emission lines of H2O trace the large-scale outflow,
while the high-excitation emission lines trace a small-scale distribution
around the equatorial plane. We model the envelope structure using the dust
radiative transfer code, Hyperion, incorporating rotational collapse, an outer
static envelope, outflow cavity, and disk. The evolution of a rotating
collapsing envelope can be constrained by the far-infrared/millimeter SED along
with the azimuthally-averaged radial intensity profile, and the structure of
the outflow cavity plays a critical role at shorter wavelengths. Emission at
20-40 um requires a cavity with a constant-density inner region and a power-law
density outer region. The best fit model has an envelope mass of 19 solar mass
inside a radius of 0.315 pc and a central luminosity of 18.8 solar luminosity.
The time since collapse began is 24630-44000 yr, most likely around 36000 yr.
The corresponding mass infall rate in the envelope (1.2x10 solar mass
per year) is comparable to the stellar mass accretion rate, while the mass loss
rate estimated from the CO outflow is 20% of the stellar mass accretion rate.
We find no evidence for episodic accretion.Comment: Accepted for publication in ApJ. 33 pages; 34 figures; 4 table
The organic chemistry in the innermost, infalling envelope of the Class 0 protostar L483
Context: The protostellar envelopes, outflow and large-scale chemistry of
Class~0 and Class~I objects have been well-studied, but while previous works
have hinted at or found a few Keplerian disks at the Class~0 stage, it remains
to be seen if their presence in this early stage is the norm. Likewise, while
complex organics have been detected toward some Class~0 objects, their
distribution is unknown as they could reside in the hottest parts of the
envelope, in the emerging disk itself or in other components of the
protostellar system, such as shocked regions related to outflows.
Aims: In this work, we aim to address two related issues regarding
protostars: when rotationally supported disks form around deeply embedded
protostars and where complex organic molecules reside in such objects.
Methods: We observed the deeply embedded protostar, L483, using Atacama Large
Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a
high angular resolution down to ~0.1 (20~au) scales.
Results: We find that the kinematics of CS~-- and
HCN~-- are best fitted by the velocity profile from infall under
conservation of angular momentum and not by a Keplerian profile. The spatial
extents of the observed complex organics are consistent with an estimated ice
sublimation radius of the envelope at ~50~au, suggesting that the complex
organics exist in the hot corino of L483.
Conclusions: We find that L483 does not harbor a Keplerian disk down to at
least ~au in radius. Instead, the innermost regions of L483 are undergoing
a rotating collapse. This result highlights that some Class~0 objects contain
only very small disks, or none at all, with the complex organic chemistry
taking place on scales inside the hot corino of the envelope, in a region
larger than the emerging disk.Comment: 19 pages, 11 figure
The Co-evolution of Disk and Star in Embedded Stages: The Case of the Very Low-mass Protostar
We have observed the CCH (N=3-2, J=7/2-5/2, F=4-3 and 3-2) and SO (6_7-5_6)
emission at a 0"2 angular resolution toward the low-mass Class 0 protostellar
source IRAS 15398-3359 with ALMA. The CCH emission traces the
infalling-rotating envelope near the protostar with the outflow cavity extended
along the northeast-southwest axis. On the other hand, the SO emission has a
compact distribution around the protostar. The CCH emission is relatively weak
at the continuum peak position, while the SO emission has a sharp peak there.
Although the maximum velocity shift of the CCH emission is about 1 km s^-1 from
the systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for the
SO emission. This high velocity component is most likely associated with the
Keplerian rotation around the protostar. The protostellar mass is estimated to
be 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. With
this protostellar mass, the velocity structure of the CCH emission can be
explained by the model of the infalling-rotating envelope, where the radius of
the centrifugal barrier is estimated to be 40 au from the comparison with the
model. The disk mass evaluated from the dust continuum emission by assuming the
dust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting in
the Toomre Q parameter of 0.4-5. Hence, the disk structure may be partly
unstable. All these results suggest that a rotationally-supported disk can be
formed in the earliest stages of the protostellar evolution
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