199 research outputs found
Ammonia in the hot core W51-IRS2: 12 new maser lines and a maser component with a velocity drift
With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines have
been detected toward the prominent massive star forming region W51-IRS2. Eleven
of these inversion lines, the (J,K) = (6,2), (5,3), (7,4), (8,5), (7,6), (7,7),
(9,7), (10,7), (9,9), (10,9), and (12,12) transitions, are classified as masers
for the first time in outer space. All detected masers are related to highly
excited inversion doublets. The (5,4) maser originates from an inversion
doublet 340 K above the ground state, while the (12,12) transition, at 1450 K,
is the most highly excited NH3 maser line so far known. Strong variability is
seen not only in ortho- but also in para-NH3 transitions. Bright narrow
emission features are observed, for the first time, in (mostly) ortho-ammonia
transitions, at V ~ 45 km/s, well separated from the quasi-thermal emission
near 60 km/s. These features were absent 25 years ago and show a velocity drift
of about +0.2 km/s/yr. The component is likely related to the SiO maser source
in W51-IRS2 and a possible scenario explaining the velocity drift is outlined.
The 57 km/s component of the (9,6) maser line is found to be strongly linearly
polarized. Maser emission in the (J,K) to (J+1,K) inversion doublets is
strictly forbidden by selection rules for electric dipole transitions in the
ground vibrational state. However, such pairs (and even triplets with (J+2,K))
are common toward W51-IRS2. Similarities in line widths and velocities indicate
that such groups of maser lines arise from the same regions, which can be
explained by pumping through vibrational excitation. The large number of NH3
maser lines in W51-IRS2 is most likely related to the exceptionally high
kinetic temperature and NH3 column density of this young massive star forming
region.Comment: Accepted for publication in Astronomy & Astrophysics, 11 pages, 12
postscript figures, 1 tabl
First detection of ammonia in M82
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion
lines of ammonia (NH3) towards the south--western molecular lobe in M82. The
relative intensities of the ammonia lines are characterized by a rotational
temperature of T_rot=29+/-5 K which implies an average kinetic temperature of
T_kin~60 K. A Gaussian decomposition of the observed spectra indicates
increasing kinetic temperatures towards the nucleus of M82, consistent with
recent findings based on CO observations. The observations imply a very low NH3
abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a
decreasing NH3 abundance towards the central active regions in M82 and
interpret this abundance gradient in terms of photodissociation of NH3 in PDRs.
The low temperature derived here from NH3 also explains the apparent
underabundance of complex molecules like CH_3OH and HNCO, which has previously
been reported.Comment: 4 pages, 4 figures, accepted by ApJ
Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342
We have imaged emission from the millimeter lines of eight molecules--C2H,
C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of the
nearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made with
OVRO. Using these maps we obtain a picture of the chemistry within the nuclear
region on the sizescales of individual GMCs. Bright emission is detected from
all but SO. There are marked differences in morphology for the different
molecules. A principal component analysis is performed to quantify similarities
and differences among the images. This analysis reveals that while all
molecules are to zeroth order correlated, that is, they are all found in dense
molecular clouds, there are three distinct groups of molecules distinguished by
the location of their emission within the nuclear region. N2H+, C18O, HNC and
HCN are widespread and bright, good overall tracers of dense molecular gas. C2H
and C34S, tracers of PDR chemistry, originate exclusively from the central
50-100 pc region, where radiation fields are high. The third group of
molecules, CH3OH and HNCO, correlates well with the expected locations of
bar-induced orbital shocks. The good correlation of HNCO with the established
shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has
been uncertain, is indeed produced by processing of grains. HC3N is observed to
correlate tightly with 3mm continuum emission, demonstrating that the young
starbursts are the sites of the warmest and densest molecular gas. We compare
our HNC images with the HCN images of Downes et al. (1992) to produce the first
high resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity
across the nucleus and the correlation of both of these gas tracers with the
star formation is excellent. (Abridged).Comment: 54 pages including 10 figures and 8 tables. Accepted for publication
in Ap
On carbon and oxygen isotope ratios in starburst galaxies: New data from NGC253 and Mrk231 and their implications
Using the IRAM 30-m telescope, CN and CO isotopologues have been measured
toward the central regions of the nearby starburst galaxy NGC253 and the
prototypical ultraluminous infrared galaxy Mrk231. In NGC253, the 12C/13C ratio
is 40+-10. Assuming that the ratio also holds for the CO emitting gas, this
yields 16O/18O = 145+-36 and 16O/17O = 1290+-365 and a 32S/34S ratio close to
that measured for the local interstellar medium (20-25). No indication for
vibrationally excited CN is found. Peak line intensity ratios between NGC253
and Mrk231 are ~100 for 12C16O and 12C18O J=1-0, while the ratio for 13C16O
J=1-0 is ~250. This and similar 13CO and C18O line intensities in the J=1-0 and
2-1 transitions of Mrk231 suggest 12C/13C ~ 100 and 16O/18O ~ 100, in agreement
with values obtained for the less evolved ultraluminous merger Arp220. Also
accounting for other extragalactic data, 12C/13C ratios appear to vary over a
full order of magnitude, from >100 in ultraluminous high redshift galaxies to
~100 in more local such galaxies to ~40 in weaker starbursts not undergoing a
large scale merger to 25 in the Central Molecular Zone of the Milky Way. With
12C being predominantly synthesized in massive stars, while 13C is mostly
ejected by longer lived lower mass stars at later times, this is qualitatively
consistent with our results of decreasing carbon isotope ratios with time and
rising metallicity. It is emphasized, however, that both infall of poorly
processed material, initiating a nuclear starburst, as well as the ejecta from
newly formed massive stars (in particular in case of a top-heavy stellar
initial mass function) can raise the carbon isotope ratio for a limited amount
of time.Comment: Accepted by Astronomy & Astrophysics, 6 figures, 4 table
Interferometric Observations of the Nuclear Region of Arp220 at Submillimeter Wavelengths
We report the first submillimeter interferometric observations of an
ultraluminous infrared galaxy. We observed Arp220 in the CO J=3-2 line and
342GHz continuum with the single baseline CSO-JCMT interferometer consisting of
the Caltech Submillimeter Observatory (CSO) and the James Clerk Maxwell
Telescope (JCMT). Models were fit to the measured visibilities to constrain the
structure of the source. The morphologies of the CO J=3-2 line and 342GHz
continuum emission are similar to those seen in published maps at 230 and
110GHz. We clearly detect a binary source separated by about 1 arcsec in the
east-west direction in the 342GHz continuum. The CO J=3-2 visibility
amplitudes, however, indicate a more complicated structure, with evidence for a
compact binary at some velocities and rather more extended structure at others.
Less than 30% of the total CO J=3-2 emission is detected by the interferometer,
which implies the presence of significant quantities of extended gas. We also
obtained single-dish CO J=2-1, CO J=3-2 and HCN J=4-3 spectra. The HCN J=4-3
spectrum, unlike the CO spectra, is dominated by a single redshifted peak. The
HCN J=4-3/CO J=3-2, HCN J=4-3/HCN J=1-0 and CO J=3-2/2-1 line ratios are larger
in the redshifted (eastern) source, which suggests that the two sources may
have different physical conditions. This result might be explained by the
presence of an intense starburst that has begun to deplete or disperse the
densest gas in the western source, while the eastern source harbors undispersed
high density gas.Comment: 17 pages, 9 figures, 4 Tables. accepted by Ap
Molecular Gas in Candidate Double Barred Galaxies III. A Lack of Molecular Gas?
Most models of double-barred galaxies suggest that a molecular gas component
is crucial for maintaining long-lived nuclear bars. We have undertaken a CO
survey in an attempt to determine the gas content of these systems and to
locate double barred galaxies with strong CO emission that could be candidates
for high resolution mapping. We observed 10 galaxies in CO J=2-1 and J=3-2 and
did not detect any galaxies that had not already been detected in previous CO
surveys. We preferentially detect emission from galaxies containing some form
of nuclear activity. Simulations of these galaxies require that they contain 2%
to 10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for
the galaxies for which we have detections suggest that the gas mass fraction is
in agreement with these models requirements. The lack of emission in the other
galaxies suggests that they contain as little as 7 x 10^6 solar masses of
molecular material which corresponds to < 0.1% gas by mass. This result
combined with the wide variety of CO distributions observed in double barred
galaxies suggests the need for models of double-barred galaxies that do not
require a large, well ordered molecular gas component.Comment: 17 pages (3 figures embedded on pg 17). To appear in the March 10
issue of the Astrophysical Journa
Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers
We have observed 150 regions of massive star formation, selected originally
by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of
CS, and 49 regions in the same transitions of CS. Over 90% of the 150
regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were
detected in the J=5-4 transition. We have combined the data with the J = 7-6
data from our original survey (Plume et al. 1992) to determine the density by
analyzing the excitation of the rotational levels. Using Large Velocity
Gradient (LVG) models, we have determined densities and column densities for 71
of these regions. The gas densities are very high (the mean log of the density
is 5.9), but much less than the critical density of the J=7-6 line. Small maps
of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The
mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity
to virial mass (L/M) is 190, about 50 times higher than estimates using CO
emission, suggesting that star formation is much more efficient in the dense
gas probed in this study. The gas depletion time for the dense gas is roughly
1.3 x 10^7 yr. We find no statistically significant linewidth--size or
density--size relationships in our data. Instead, both linewidth and density
are larger for a given size than would be predicted by the usual relationships.
We find that the linewidth increases with density, the opposite of what would
be predicted by the usual arguments. We estimate that the luminosity of our
Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23
L_sun, considerably less than the luminosity in this line within the central
100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to
CS luminosity is higher in M82 than in any cloud in our sample.Comment: 26 pages, 6 postscript figures, 3 postscript tables. Uses AAS Latex
macros, accepted for Astrophysical Journa
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