140 research outputs found
IRAS 16293-2422: Evidence for Infall onto a Counter-Rotating Protostellar Accretion Disk
We report high spatial resolution VLA observations of the low-mass
star-forming region IRAS 16293-2422 using four molecular probes: ethyl cyanide
(CHCHCN), methyl formate (CHOCHO), formic acid (HCOOH), and the
ground vibrational state of silicon monoxide (SiO). Ethyl cyanide emiss ion has
a spatial scale of and encompasses binary cores A and B as
determined by continuum emission peaks. Surrounded by formic acid emission,
methyl formate emission has a spatial scale of and is confined to core
B. SiO emission shows two velocity components with spatial scales less than
2 that map northeast of the A and B symmetry axis. The redshifted
SiO is northwest of blueshifted SiO along a position angle of
which is approximately parallel to the A and B symmetry axis. We
interpret the spatial position offset in red and blueshifted SiO emission as
due to rotation of a protostellar accretion disk and we derive 1.4
M interior to the SiO emission. In the same vicinity, Mundy et al.
(1986) also concluded rotation of a nearly edge-on disk from OVRO observations
of much stronger and ubiquitous CO emission but the direction of
rotation is opposite to the SiO emission findings. Taken together, SiO and
CO data suggest evidence for a counter-rotating disk. Moreover, archival
BIMA array CO data show an inverse P Cygni profile with the strongest
absorption in close proximity to the SiO emission, indicating unambiguous
material infall toward the counter-rotating protostellar disk at a new source
location within the IRAS 16293-2422 complex. The details of these observations
and our interpretations are discussed.Comment: 18 pages, 5 figures, accepted for publication in the Astrophysical
Journa
A Search for Hydroxylamine (NH2OH) toward Select Astronomical Sources
Observations of 14 rotational transitions of hydroxylamine (NH2OH) using the
NRAO 12 m Telescope on Kitt Peak are reported towards IRC+10216, Orion KL,
Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although recent models
suggest the presence of NH2OH in high abundance, these observations resulted in
non-detection. Upper limits are calculated to be as much as six orders of
magnitude lower than predicted by models. Possible explanations for the lower
than expected abundance are explored.Comment: 18 pages, 3 figures, 3 table
A Search for Hydroxylamine (NH_2OH) toward Select Astronomical Sources
Observations of 14 rotational transitions of hydroxylamine (NH_2OH) using the NRAO 12 m telescope on Kitt Peak
are reported toward IRC+10216, Orion KL, Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although
recent models suggest the presence of NH_2OH in high abundance, these observations resulted in non-detection.
Upper limits are calculated to be as much as six orders of magnitude lower than those predicted by models. Possible
explanations for the lower-than-expected abundance are explored
Cyclopropenone (c-H2C3O): A New Interstellar Ring Molecule
The three-carbon keto ring cyclopropenone (c-H2C 3O) has been detected largely in absorption with the 100 m Green Bank Telescope (GBT) toward the star-forming region Sagittarius B2(N) by means of a number of rotational transitions between energy levels that have energies less than 10 K. Previous negative results from searches for interstellar c-H2C3O by other investigators attempting to detect rotational transitions that have energy levels ~10 K or greater indicate no significant hot core component. Thus, we conclude that only the low-energy levels of c-H2C3O are populated because the molecule state temperature is low, suggesting that c-H2C3O resides in a star-forming core halo region that has a widespread arcminute spatial scale. Toward Sagittarius B2(N), the GBT was also used to observe the previously reported, spatially ubiquitous, three-carbon ring cyclopropenylidene (c-C3H2 ), which has a divalent carbon that makes it highly reactive in the laboratory. The presence of both c-C3H2 and c-H2C3O toward Sagittarius B2(N) suggests that gas-phase oxygen addition may account for the synthesis of c-H 2C3O from c-C3H2. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO)
Weak maser emission of methyl formate toward Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey
A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3)
rotational lines is presented for the first time using a set of theoretical
collisional rate coefficients. These coefficients have been computed in the
temperature range 5-30 K by combining coupled-channel scattering calculations
with a high accuracy potential energy surface for HCOOCH3-He. The results are
compared to observations toward the Sagittarius B2(N) molecular cloud using the
publicly available PRIMOS survey from the Green Bank Telescope. A total of 49
low-lying transitions of methyl formate, with upper levels below 25 K, are
identified. These lines are found to probe a presumably cold (~30 K),
moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The
derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the
column density of the trans conformer in the same source. Provided that the two
conformers have the same spatial distribution, this result suggests that
strongly non-equilibrium processes must be involved in their synthesis.
Finally, our calculations show that all detected emission lines with a
frequency below 30 GHz are (collisionally pumped) weak masers amplifying the
continuum of Sgr B2(N). This result demonstrates the importance and generality
of non-LTE effects in the rotational spectra of complex organic molecules at
centimetre wavelengths.Comment: 33 pages, 9 figures, accepted in The Astrophysical Journal (january 4
2014
Green Bank Telescope Detection of New Interstellar Aldehydes: Propenal and Propanal
The new interstellar molecules propenal (CH2CHCHO) and propanal (CH3CH2CHO) have been detected largely in absorption toward the star-forming region Sagittarius B2(N) by means of rotational transitions observed with the 100 m Green Bank Telescope (GBT) operating in the range from 18 GHz (λ ~ 1.7 cm) to 26 GHz (λ ~ 1.2 cm). The GBT was also used to observe the previously reported interstellar aldehyde propynal (HC2CHO) in Sagittarius B2(N), which is a known source of large molecules presumably formed on interstellar grains. The presence of these three interstellar aldehydes toward Sagittarius B2(N) strongly suggests that simple hydrogen addition on interstellar grains accounts for successively larger molecular species: from propynal to propenal and from propenal to propanal. Energy sources within Sagittarius B2(N) likely permit the hydrogen addition reactions on grain surfaces to proceed. This work demonstrates that successive hydrogen addition is probably an important chemistry route in the formation of a number of complex interstellar molecules. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO)
First Acetic Acid Survey with CARMA in Hot Molecular Cores
Acetic acid (CHCOOH) has been detected mainly in hot molecular cores
where the distribution between oxygen (O) and nitrogen (N) containing molecular
species is co-spatial within the telescope beam. Previous work has presumed
that similar cores with co-spatial O and N species may be an indicator for
detecting acetic acid. However, does this presumption hold as higher spatial
resolution observations become available of large O and N-containing molecules?
As the number of detected acetic acid sources is still low, more observations
are needed to support this postulate. In this paper, we report the first acetic
acid survey conducted with the Combined Array for Research in Millimeter-wave
Astronomy (CARMA) at 3 mm wavelengths towards G19.61-0.23, G29.96-0.02 and IRAS
16293-2422. We have successfully detected CHCOOH via two transitions toward
G19.61-0.23 and tentatively confirmed the detection toward IRAS 16293-2422 A.
The determined column density of CHCOOH is 2.0(1.0)
cm and the abundance ratio of CHCOOH to methyl formate (HCOOCH)
is 2.2(0.1) toward G19.61-0.23. Toward IRAS 16293 A, the
determined column density of CHCOOH is 1.6
cm and the abundance ratio of CHCOOH to methyl formate (HCOOCH)
is 1.0 both of which are consistent with abundance
ratios determined toward other hot cores. Finally, we model all known line
emission in our passband to determine physical conditions in the regions and
introduce a new metric to better reveal weak spectral features that are blended
with stronger lines or that may be near the 1-2 detection limit.Comment: 28 pages, 8 figures, accepted for publication in the ApJ; Revised
citation in session 2, references remove
Non-Thermal Continuum toward SGRB2(N-LMH)
An analysis of continuum antenna temperatures observed in the Green Bank
Telescope (GBT) spectrometer bandpasses is presented for observations toward
SgrB2(N-LMH). Since 2004, we have identified four new prebiotic molecules
toward this source by means of rotational transitions between low energy
levels; concurrently, we have observed significant continuum in the GBT
spectrometer bandpasses centered at 85 different frequencies in the range of 1
to 48 GHz. The continuum heavily influences the molecular spectral features
since we have observed far more absorption lines than emission lines for each
of these new molecular species. Hence, it is important to understand the
nature, distribution, and intensity of the underlying continuum in the GBT
bandpasses for the purposes of radiative transfer, i.e. the means by which
reliable molecular abundances are estimated. We find that the GBT spectrometer
bandpass continuum is consistent with optically-thin, non thermal (synchrotron)
emission with a flux density spectral index of -0.7 and a Gaussian source size
of ~143" at 1 GHz that decreases with increasing frequency as nu^(-0.52). Some
support for this model is provided by high frequency Very Large Array (VLA)
observations of SgrB2.Comment: Accepted for Publication in the Astrophysical Journal Letter
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