916 research outputs found
Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?
IRAM 30m observations reveal that the deeply obscured IR-luminous galaxy
NGC4418 has a rich molecular chemistry - including unusually luminous HC3N line
emission. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0.
3-2, HCN 3-2, HNC 1-0, 3-2 (and tentatively OCS 12-11). The HCN, HCO+, H2CO and
CN line emission can be fitted to densities of n=5 x 10E4 - 10E5 cm-3 and gas
temperatures Tk=80-150 K. Both HNC and HC3N are, however, significantly more
excited than the other species which requires higher gas densities - or
radiative excitation through e.g. mid-IR pumping. The HCN line intensity is
fainter than that of HCO+ and HNC for the 3-2 transition, in contrast to
previous findings for the 1-0 lines where the HCN emission is the most
luminous. We tentatively suggest that the observed molecular line emission is
consistent with a young starburst, where the emission can be understood as
emerging from dense, warm gas with an additional PDR component. We find that
X-ray chemistry is not required to explain the observed mm line emission,
including the HCN/HCO+ 1-0 and 3-2 line ratios. The luminous HC3N line emission
is an expected signature of dense, starforming gas. A deeply buried AGN can not
be excluded, but its impact on the surrounding molecular medium is then
suggested to be limited. However, detailed modelling of HC3N abundances in
X-ray dominated regions (XDRs) should be carried out. The possibility of
radiative excitation should also be further investigatedComment: 7 pages, one eps figure, uses aa.cls, submitted to Astronomy and
Astrophysic
CH^+(1â0) and ^(13)CH^+(1â0) absorption lines in the direction of massive star-forming regions
We report the detection of the ground-state rotational transition of the methylidyne cation CH^+ and its isotopologue ^(13)CH^+ toward the remote
massive star-forming regions W33A, W49N, and W51 with the HIFI instrument onboard the Herschel satellite. Both lines are seen only in
absorption against the dust continuum emission of the star-forming regions. The CH^+ absorption is saturated over almost the entire velocity
ranges sampled by the lines-of-sight that include gas associated with the star-forming regions (SFR) and Galactic foreground material. The CH^+
column densities are inferred from the optically thin components. A lower limit of the isotopic ratio [^(12)CH^+]/[^(13)CH^+] > 35.5 is derived from
the absorptions of foreground material toward W49N. The column density ratio, N(CH^+)/N(HCO^+), is found to vary by at least a factor 10,
between 4 and >40, in the Galactic foreground material. Line-of-sight ^(12)CH^+ average abundances relative to total hydrogen are estimated. Their
average value, N(CH^+)/N_H > 2.6 Ă 10^(â8), is higher than that observed in the solar neighborhood and confirms the high abundances of CH^+ in
the Galactic interstellar medium. We compare this result to the predictions of turbulent dissipation regions (TDR) models and find that these high
abundances can be reproduced for the inner Galaxy conditions. It is remarkable that the range of predicted N(CH^+)/N(HCO^+) ratios, from 1 to
~50, is comparable to that observed
A survey of HC_3N in extragalactic sources: Is HC_3N a tracer of activity in ULIRGs?
Context. HC_3N is a molecule that is mainly associated with Galactic star-forming regions, but it has also been detected in extragalactic environments.
Aims. To present the first extragalactic survey of HC_3N, when combining earlier data from the literature with six new single-dish detections, and to compare HC_3N with other molecular tracers (HCN, HNC), as well as other properties (silicate absorption strength, IR flux density ratios, C_(II) flux, and megamaser activity).
Methods. We present mm IRAM 30 m, OSO 20 m, and SEST observations of HC_3N rotational lines (mainly the J = 10â9 transition) and of the J = 1â0 transitions of HCN and HNC. Our combined HC_3N data account for 13 galaxies (excluding the upper limits reported for the non-detections), while we have HCN and HNC data for more than 20 galaxies.
Results. A preliminary definition âHC_3N-luminous galaxyâ is made based upon the HC_3N/HCN ratio. Most (~80%) HC_3N-luminous galaxies seem to be deeply obscured galaxies and (U)LIRGs. A majority (~60% or more) of the HC3N-luminous galaxies in the sample present OH mega- or strong kilomaser activity. A possible explanation is that both HC_3N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC_3N. A high silicate absorption strength is also found in several of the HC_3N-luminous objects, which may help the HC3N to survive. Finally, we find that a high HC_3N/HCN ratio is related to a high dust temperature and a low C_(II) flux
Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6â0.4 (W31C), W49N, and W51
We report the detection of the ground state N, J = 1, 3/2 â 1, 1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with
the Herschel/HIFI instrument along the sight-line to the massive star-forming regions G10.6â0.4 (W31C), W49N, and W51. While the molecular
cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption
against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles,
with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH,
CN, and HCO^+ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO^+ deviate from the mean trends,
giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better
tracers, with [CCH]/[H_2] ~3.2 ± 1.1 Ă 10^(â8). The observed [CN]/[CH], [CCH]/[CH] abundance ratios suggest that the bulk of the diffuse matter
along the lines of sight has gas densities n_H = n(H) + 2n(H_2) ranging between 100 and 1000 cm^(â3)
A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
We combine Herschel observations of a total of 12 sources to construct the
most uniform survey of HF and H2O in our Galactic disk. Both molecules are
detected in absorption along all sight lines. The high spectral resolution of
the Heterodyne Instrument for the Far-Infrared (HIFI) allows us to compare the
HF and H2O distributions in 47 diffuse cloud components sampling the disk. We
find that the HF and H2O velocity distributions follow each other almost
perfectly and establish that HF and H2O probe the same gas-phase volume. Our
observations corroborate theoretical predictions that HF is a sensitive tracer
of H2 in diffuse clouds, down to molecular fractions of only a few percent.
Using HF to trace H2 in our sample, we find that the N(H2O)-to-N(HF) ratio
shows a narrow distribution with a median value of 1.51. Our results further
suggest that H2O might be used as a tracer of H2 -within a factor 2.5- in the
diffuse interstellar medium. We show that the measured factor of ~2.5 variation
around the median is driven by true local variations in the H2O abundance
relative to H2 throughout the disk. The latter variability allows us to test
our theoretical understanding of the chemistry of oxygen-bearing molecules in
the diffuse gas. We show that both gas-phase and grain-surface chemistry are
required to reproduce our H2O observations. This survey thus confirms that
grain surface reactions can play a significant role in the chemistry occurring
in the diffuse interstellar medium n_H < 1000 cm^-3.Comment: 53 pages; 12 figures, accepted for publication in ApJ main journa
Excitation and abundance of C_3 in star forming cores: Herschel/HIFI observations of the sight-lines to W31C and W49N
We present spectrally resolved observations of triatomic carbon (C_3) in several ro-vibrational transitions between the vibrational ground state and
the low-energy Îœ_2 bending mode at frequencies between 1654â1897 GHz along the sight-lines to the submillimeter continuum sources W31C
and W49N, using Herschelâs HIFI instrument. We detect C_3 in absorption arising from the warm envelope surrounding the hot core, as indicated
by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C_3 absorption due to diffuse foreground clouds.
From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_(rot))
of ~50â70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state
are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on
the derived T_(rot), we get column densities N(C_3) ~ 7â9 Ă 10^(14) cm^(â2) and abundance x(C_3) ~ 10^(â8) with respect to H_2. For W31C, using a radiative
transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find
that a model with x(C_3) = 10^(â8), T_(kin) = 30â50 K, N(C_3) = 1.5 Ă 10^(15) cm^(â2) fits the observations reasonably well and provides parameters in very
good agreement with the simple excitation analysis
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