233 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
Interplay between chemistry and dynamics in embedded protostellar disks
Context. A fundamental part of the study of star formation is to place young stellar objects in an evolutionary sequence. Establishing a robust evolutionary classification scheme allows us not only to understand how the Sun was born but also to predict what kind of main sequence star a given protostar will become. Traditionally, low-mass young stellar objects are classified according to the shape of their spectral energy distributions. Such methods are, however, prone to misclassification due to degeneracy and do not constrain the temporal evolution. More recently, young stellar objects have been classified based on envelope, disk, and stellar masses determined from resolved images of their continuum and line emission at submillimeter wavelengths.
Aims. Through detailed modeling of two Class I sources, we aim at determining accurate velocity profiles and explore the role of freeze-out chemistry in such objects.
Methods. We present new Submillimeter Array observations of the continuum and HCO+ line emission at 1.1 mm toward two protostars, IRS 63 and IRS 43 in the Ophiuchus star forming region. The sources were modeled in detail using dust radiation transfer to fit the SED and continuum images and line radiation transfer to produce synthetic position-velocity diagrams. We used a χ2 search algorithm to find the best model fit to the data and to estimate the errors in the model variables.
Results. Our best fit models present disk, envelope, and stellar masses, as well as the HCO+ abundance and inclination of both sources. We also identify a ring structure with a radius of about 200 AU in IRS 63.
Conclusions. We find that freeze-out chemistry is important in IRS 63 but not for IRS 43. We show that the velocity field in IRS 43 is consistent with Keplerian rotation. Owing to molecular depletion, it is not possible to draw a similar conclusion for IRS 63. We identify a ring-shaped structure in IRS 63 on the same spatial scale as the disk outer radius. No such structure is seen in IRS 43
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
High DO/HDO ratio in the inner regions of the low-mass protostar NGC1333 IRAS2A
Water plays a crucial role both in the interstellar medium and on Earth. To
constrain its formation mechanisms and its evolution through the star formation
process, the determination of the water deuterium fractionation ratios is
particularly suitable. Previous studies derived HDO/HO ratios in the warm
inner regions of low-mass protostars. We here report a detection of the DO
1-1 transition toward the low-mass protostar NGC1333 IRAS2A
with the Plateau de Bure interferometer: this represents the first
interferometric detection of DO - and only the second solar-type protostar
for which this isotopologue is detected. Using the observations of the HDO
5-6 transition simultaneously detected and three other HDO
lines previously observed, we show that the HDO line fluxes are well reproduced
with a single excitation temperature of 21821 K and a source size of
0.5 arcsec. The DO/HDO ratio is (1.20.5)
10, while the use of previous HO observations give an
HDO/HO ratio of (1.70.8) 10, i.e. a factor of 7
lower than the DO/HDO ratio. These results contradict the predictions of
current grain surface chemical models and indicate that either the surface
deuteration processes are poorly understood or that both sublimation of grain
mantles and water formation at high temperatures (230 K) take place in
the inner regions of this source. In the second scenario, the thermal
desorption of the grain mantles would explain the high DO/HDO ratio, while
water formation at high temperature would explain significant extra production
of HO leading to a decrease of the HDO/HO ratio.Comment: Accepted for publication in ApJ Letters; 12 pages, 2 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
- …