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Rotational spectra in four new excited vibrational levels of the linear carbon chain radical C$_4$H radical were observed in the millimeter band between 69 and 364 GHz in a low pressure glow discharge, and two of these were observed in a supersonic molecular beam between 19 and 38 GHz. All have rotational constants within 0.4% of the $^2Sigma^+$ ground vibrational state of C$_4$H and were assigned to new bending vibrational levels, two each with $^2Sigma$ and $^2Pi$ vibrational symmetry. The new levels are tentatively assigned to the $1 u_6$ and $1 u_5$ bending vibrational modes (both with $^2Pi$ symmetry), and the $1 u_6 + 1 u_7$ and $1 u_5 + 1 u_6$ combination levels ($^2Sigma$ symmetry) on the basis of the derived spectroscopic constants, relative intensities in our discharge source, and published laser spectroscopic and quantum calculations. Prior spectroscopic constants in the $1 u_7$ and $2 u_7$ levels were refined. Also presented are interferometric maps of the ground state and the $1 u_7$ level obtained with the SMA near 257 GHz which show that C$_4$H is present near the central star in IRC+10216. We found no evidence with the SMA for the new vibrationally excited levels of C$_4$H at a peak flux density averaged over a $3^{primeprime}$ synthesized beam of $ge 0.15$ Jy/beam in the 294-296 and 304-306 GHz range, but it is anticipated that rotational lines in the new levels might be observed in IRC+10216 when ALMA attains its full design capability.
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Several rotational transitions of water have been identified toward evolved stars in the ground vibrational state as well as in the first excited state of the bending mode. In the latter vibrational state of water, the 658 GHz J = 1_1,0-1_0,1 rotational transition is often strong and seems to be widespread in late-type stars. Our main goals are to better characterize the nature of the 658 GHz emission, compare the velocity extent of the 658 GHz emission with SiO maser emission to help locate the water layers and, more generally, investigate the physical conditions prevailing in the excited water layers of evolved stars. Another goal is to identify new 658 GHz emission sources and contribute in showing that this emission is widespread in evolved stars. Eleven evolved stars were extracted from our mini-catalog of existing and potential 658 GHz sources for observations with the APEX telescope equipped with the SEPIA receiver. The 13CO J=6-5 line was placed in the same receiver sideband for simultaneous observation with the 658 GHz line of water. We have compared the 658 GHz line properties with our H2O radiative transfer models in stars and we have compared the velocity ranges of the 658 GHz and SiO J=2-1, v=1 maser lines. All stars show 658 GHz emission with a peak flux density in the range 50-70 Jy to 2000-3000 Jy. We have shown that the 658 GHz line is masing and we found that the 658 GHz velocity extent tends to be correlated with that of the SiO maser suggesting that both emission lines are excited in circumstellar layers close to the central star. Broad and stable line profiles are observed at 658 GHz. This could indicate maser saturation although we have tentatively provided first information on time variability at 658 GHz.
We investigate the molecular gas properties of the deeply obscured luminous infrared galaxy NGC 4418. We address the excitation of the complex molecule HC3N to determine whether its unusually luminous emission is related to the nature of the buried nuclear source. We use IRAM 30m and JCMT observations of rotational and vibrational lines of HC3N to model the excitation of the molecule by means of rotational diagrams. We report the first confirmed extragalactic detection of vibrational lines of HC3N. We detect 6 different rotational transitions ranging from J=10-9 to J=30-29 in the ground vibrational state and obtain a tentative detection of the J=38-37 line. We also detect 7 rotational transitions of the vibrationally excited states v6 and v7, with angular momenta ranging from J=10-9 to 28-27. The energies of the upper states of the observed transitions range from 20 to 850 K. In the optically thin regime, we find that the rotational transitions of the vibrational ground state can be fitted for two temperatures, 30 K and 260 K, while the vibrationally excited levels can be fitted for a rotational temperature of 90 K and a vibrational temperature of 500 K. In the inner 300 pc of NGC 4418, we estimate a high HC3N abundance, of the order of 10^-7. The excitation of the HC3N molecule responds strongly to the intense radiation field and the presence of warm, dense gas and dust at the center of NGC 4418. The intense HC3N line emission is a result of both high abundances and excitation. The properties of the HC3N emitting gas are similar to those found for hot cores in Sgr B2, which implies that the nucleus (< 300 pc) of NGC 4418 is reminiscent of a hot core. The potential presence of a compact, hot component (T=500 K) is also discussed.
The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A$ and $^2A$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3Sigma^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H$_2$ reaction is endothermic by $approx$ 9860 K, and requires at least two vibrational quanta on the H$_2$ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H$_2$($v=2$) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for $v=3$, 4, and 5 to cover all vibrationally excited H$_2$ levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH$^+$ column densities, in agreement with those inferred from recent ALMA observations of the Orion bar.
Rotational transitions in vibrationally excited AlO and TiO -- two possible precursors of dust -- were observed in the 300 GHz range (1 mm wavelength) towards the oxygen rich AGB stars R Dor and IK Tau with ALMA, and vibrationally excited AlO was observed towards the red supergiant VY CMa with the SMA. The $J=11 to 10$ transition of TiO in the $v=1~{rm{and}}~2$ levels, and the $N = 9 to 8$ transition in the $v=2$ level of AlO were identified towards R Dor; the $J=11 to 10$ line of TiO was identified in the $v=1$ level towards IK Tau; and two transitions in the $v=1~{rm{and}}~2$ levels of AlO were identified towards VY CMa. The newly-derived high vibrational temperature of TiO and AlO in R Dor of $1800 pm 200$ K, and prior measurements of the angular extent confirm that the majority of the emission is from a region within $lesssim2R_{star}$ of the central star. A full radiative transfer analysis of AlO in R Dor yielded a fractional abundance of $sim$3% of the solar abundance of Al. From a similar analysis of TiO a fractional abundance of $sim78$% of the solar abundance of Ti was found. The observations provide indirect evidence that TiO is present in a rotating disk close to the star. Further observations in the ground and excited vibrational levels are needed to determine whether AlO, TiO, and TiO$_2$ are seeds of the Al$_2$O$_3$ dust in R Dor, and perhaps in the gravitationally bound dust shells in other AGB stars with low mass loss rates.
This paper was withdrawed from the ApJ after the comments from the referee, please be carefully.
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