HCN Line Research Articles

Observations of chemical differentiation in clumpy molecular clouds

We have extensively mapped a sample of dense molecular clouds (L1512, TMC-1C, L1262, Per 7, L1389, L1251E) in lines of HC3N, CH3OH, SO and C^{18}O. We demonstrate that a high degree of chemical differentiation is present in all of the observed clouds. We analyse the molecular maps for each cloud, demonstrating a systematic chemical differentiation across the sample, which we relate to the evolutionary state of the cloud. We relate our observations to the cloud physical, kinematical and evolutionary properties, and also compare them to the predictions of simple chemical models. The implications of this work for understanding the origin of the clumpy structures and chemical differentiation observed in dense clouds are discussed.

Hot Organic Molecules toward a Young Low-Mass Star: A Look at Inner Disk Chemistry

Spitzer Space Telescopespectra of the low-mass young stellar object (YSO) IRS 46 ( ) in Ophiuchus L ≈ 0.6 L bol , reveal strong vibration-rotation absorption bands of gaseous C 2H2, HCN, and CO2. This is the only source out of a sample of ∼100 YSOs that shows these features, and this is the first time that they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra reveals excitation temperatures of 350 K and abundances of 10 6 to 10 5 with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN line is barely detected with the J p 4–3 James Clerk Maxwell Telescope (JCMT), indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limit the mass and temperature of any remnant collapsing envelope to less than 0.01 M, and 100 K, respectively. This excludes a hot-core–type region as found in high-mass YSOs. The most plausible origin of this hot gas rich in organic molecules is in the inner (!6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edgeon two-dimensional disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe the organic chemistry, gas temperatures, and gas kinematics in the planet-forming zones close to a young star. Subject headings: infrared: ISM — ISM: individual (IRS 46) — ISM: jets and outflows — ISM: molecules — planetary systems: protoplanetary disks — stars: formation

Probing the Inner 200 AU of Low‐Mass Protostars with the Submillimeter Array: Dust and Organic Molecules in NGC 1333 IRAS 2A

The Submillimeter Array has opened a new window to study the innermost warm and dense regions of the envelopes and disks around deeply embedded protostars. This paper presents high-angular resolution (< 2'') submillimeter observations of the class 0 young stellar object NGC1333-IRAS2A. Dust continuum emission and lines of complex organic molecules such as CH3OCH3 and CH3OCHO, high excitation CH3OH transitions, deuterated methanol CH3OD as well as lines of CO, HCN, H13CN, SO and SO2 are detected on < 200 AU scales. The observations are interpreted using detailed radiative transfer models of the physical and chemical structure, consistent with both single-dish and interferometer data. The continuum emission is explained by an extended envelope and a compact but resolved component, presumably a circumstellar disk with a diameter of 200-300 AU and a mass of a few times 0.01-0.1 M_sun. If related to the rotation of the envelope, then the size of this disk suggests a centrifugal barrier of 200-300 AU, which implies that the temperature in the envelope does not increase above 100 K. Its large size also suggests that the build-up of disks proceeds rapidly throughout the early protostellar stages. The smaller (< 100 AU) disks found around other deeply embedded protostars may be a result of tidal truncation. The high-resolution observations of SO can be explained with a simple constant abundance, ~1E-9, constrained through single-dish observations, whereas those of H13CN and the organic species require high abundances, increased by one to two orders of magnitude, or an additional compact source of emission at small scales. The compact molecular emission could originate in a hot core region of the inner envelope, but a more likely reservoir is the circumstellar disk.

Millimeter Observations and Modeling of the AB Aurigae System

We present the results of millimeter observations and a suitable chemical and radiative transfer model of the AB Aurigae (HD 31293) circumstellar disk and surrounding envelope. The integral molecular content of this system is studied by observing CO, C18O, CS, HCO+, DCO+, H2CO, HCN, HNC, and SiO rotational lines with the IRAM 30 m antenna, while the disk is mapped in the HCO+ (1-0) transition with the Plateau de Bure Interferometer. Using a flared disk model with a vertical temperature gradient and an isothermal spherical envelope model with a shadowed midplane and two unshielded cones together with a gas-grain chemical network, time-dependent abundances of observationally important molecules are calculated. Then a two-dimensional non-LTE line radiative transfer code is applied to compute excitation temperatures of several rotational transitions of HCO+, CO, C18O, and CS molecules. We synthesize the HCO+ (1-0) interferometric map along with single-dish CO (2-1), C18O (2-1), HCO+ (1-0), HCO+ (3-2), CS (2-1), and CS (5-4) spectra and compare them with the observations. Our disk model successfully reproduces observed interferometric HCO+ (1-0) data, thereby constraining the following disk properties: (1) the inclination angle i=17+6-3 deg, (2) the position angle φ=80deg+/-30deg, (3) the size Rout=400+/-200 AU, (4) the mass Mdisk=1.3×10-2 Msolar (with a factor of ~7 uncertainty), and (5) that the disk is in Keplerian rotation. Furthermore, indirect evidence for a local inhomogeneity of the envelope at >~600 AU is found. The single-dish spectra are synthesized for three different cases, namely, for the disk model, for the envelope model, and for their combination. An overall reasonable agreement between all modeled and acquired line intensities, widths, and profiles is achieved for the latter model, with the exception of the CS (5-4) data that require the presence of high-density clumpy structures in the model. This allows us to constrain the physical structure of the AB Aur inner envelope: (1) its mass-average temperature is about 35+/-14 K; (2) the density goes inversely down with the radius, ρ~r-1.0+/-0.3, starting from an initial value n0~3.9×105 cm-3 at 400 AU; and (3) the mass of the shielded region within 2200 AU is about 4×10-3 Msolar (the latter two quantities are uncertain by a factor of ~7). In addition, evolutionary nature and lifetime for dispersal of the AB Aur system and Herbig Ae/Be systems in general are discussed.

Photon-dominated Chemistry in the Nucleus of M82: Widespread HOC + Emission in the Inner 650 Parsec Disk

The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C2H, c-C3H2, CH3C2H, HC3N, and HOC+ using the IRAM 30 m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured [CN]/[HCN] ~ 5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC+ 1 → 0 line with an intensity similar to that of the H13CO+ 1 → 0 line. This implies an [HCO+]/[HOC+] ratio of ~40. These results corroborate the existence of a giant photodissociation region (PDR) in the nucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained if the nucleus of M82 is formed by small (r < 0.02-0.2 pc) and dense (n ~ a few times 104-105 cm-3) clouds immersed in an intense UV field (G0 ~ 104 in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.

Molecular Line Observations of Carbon‐Chain–Producing Regions L1495B and L1521B

We present the first comprehensive study of physical and chemical properties of quiescent starless cores L1495B and L1521B, which are known to be rich in carbon-chain molecules like the cyanopolyyne peak of TMC-1 and L1521E. We have detected radio spectral lines of various carbon-chain molecules, such as CCS, C3S, C4H, HC3N, and HC5N. On the other hand, the NH3 lines are weak and the N2H+ lines are not detected. According to our mapping observations of the HC3N, CCS, and C3S lines, the dense cores in L1495B and L1521B are compact, with radius 0.063 and 0.044 pc, respectively, and have a simple elliptical structure. The distributions of CCS seem to be different from those of well-studied starless cores, L1498 and L1544, where the distribution of CCS shows a shell-like structure. Since the H13CO+, HN13C, and C34S lines are detected in L1495B and L1521B, the densities of these cores are high enough to excite the NH3 and N2H+ lines. Therefore, the abundances of NH3 and N2H+ relative to carbon-chain molecules are apparently deficient, as observed in L1521E. We found that longer carbon-chain molecules, such as HC5N and C4H, are more abundant in TMC-1 than in L1495B and L1521B, while those of sulfur-bearing molecules, such as C34S, CCS, and C3S, are comparable. Both distributions and abundances of the observed molecules in L1495B and L1521B are quite similar to those in L1521E, strongly suggesting that L1495B and L1521B are in a very early stage of physical and chemical evolution.

HCN(1-0) OBSERVATIONS OF STARLESS CORES

We present a progress report on HCN(1-0) line observations toward starless cores to probe inward motions. We have made a single pointing survey toward the central regions of 85 starless cores and performed mapping observations of 6 infall candidate starless cores. The distributions of the velocity difference between HCN(1-0) hyperfine lines and the optically thin tracer (1-0) are significantly skewed to the blue, meaning that HCN(1-0) frequently detects inward motions. Their skewness to the blue is even greater than that of CS(2-1) Lee et al., possibly implying more infall occurrence than CS(1-0). We identify 19 infall candidates by using several characteristics illustrating spectral infall asymmetry seen in HCN(1-0) hyperfine lines, CS(3-2), CS(2-1), and observations. The HCN(1-0) F(O-l) with the least optical depth usually shows a similar intensity distribution to that of which closely traces the density distribution of the cores, indicating that HCN(1-0) is less chemically affected and so believed to reflect kinematics occurring in rather inner regions of the cores. Detailed radiative transfer model fits of the spectra are underway to analyze central infall kinematics in starless cores.

High-Redshift HCN Emission: Dense Star-forming Molecular Gas in IRAS F10214+4724

Hydrogen cyanide emission in the J = 1–0 transition has been detected at redshift z = 2.2858 in IRAS F10214+4724 using the Green Bank Telescope. This is the second detection of HCN emission at high redshift. The large HCN line luminosity in F10214 is similar to that in the Cloverleaf (z = 2.6) and the ultraluminous infrared galaxies Mrk 231 and Arp 220. This is also true of the ratio of HCN to CO luminosities. The ratio of far-infrared luminosity to HCN luminosity, an indicator of the star formation rate per solar mass of dense gas, follows the correlation found for normal spirals and infrared luminous starburst galaxies. F10214 clearly contains a starburst that contributes, together with its embedded quasar, to its overall infrared luminosity. A new technique for removing spectral baselines in the search for weak, broad emission lines is presented.

The effect of the ionization rate on the chemical composition of dense cores of dark molecular clouds

The effect of variations in the rate of ionization of neutral chemical species by cosmic rays, ζ, on the abundances of some observed molecules in the dense cores of dark molecular clouds is studied. Changes in molecular abundances accompanying an increased (decreased) ionization rate have a single origin: the acceleration (deceleration) of processes that are affected directly or indirectlybychemical reactions with charged species. In addition to affecting the gas-phase chemistry, an increased cosmic-ray flux leads to the more efficient destruction of dust-grain mantles and also accelerates the freezing of some components onto dust. In particular, in a model with an increased ζ, the destruction of the volatile N2 molecule by ionized helium leads to the rapid accumulation of nitrogen atoms in dust-phase ammonia, which has a higher desorption energythan N2. As a result, the gas-phase abundance of NH3 and N2H+ decreases significantly. This mechanism can explain the unusual chemical structures of some dense globules, such as B68, where surprisingly low abundances of nitrogen-bearing molecules are observed together with a central drop in the NH3 and N2H+ column densities. Observations of clouds in HCN and HNC lines can discriminate between the two possible origins of the reduced NH3 and N2H+ abundances: an increased cosmic-ray flux or N2 freezing due to the higher desorption energy of this molecule.

The Slowly Expanding Envelope of CRL 618 Probed with HC3N Rotational Ladders

Lines from HC3N and isotopic substituted species in ground and vibrationally excited states produce crowded millimeter and submillimeter wave spectra in the C-rich protoplanetary nebula CRL 618. The complete sequence of HC3N rotational lines from J = 9-8 to J = 30-29 has been observed with the IRAM 30 m telescope toward this object. Lines from a total of 15 different vibrational states (including the fundamental), with energies up to 1100 cm-1, have been detected for the main HC3N isotopomer. In addition, the Caltech Submillimeter Observatory telescope has been used to complement this study in the range J = 31-30 to J = 39-38, with detections in five of these states, all of them below 700 cm-1. Only the rotational lines of HC3N in its ground vibrational state display evidence of the well-known CRL 618 high-velocity outflow. Vibrationally excited HC3N rotational lines exhibit P Cygni profiles at 3 mm, evolving to pure emission line shapes at shorter wavelengths. This evolution of the line profile shows little dependence on the vibrational state from which the rotational lines arise. The absorption features are formed against the continuum emission, which has been successfully characterized in this work as a result of the large frequency coverage. The fluxes range from 1.75 to 3.4 Jy in the frequency range 90-240 GHz. These values translate to an effective continuum source with a size between 022 and 027, an effective temperature at 200 GHz ranging from 3900 to 6400 K, and a spectral index between -1.15 and -1.12. We have made an effort to simultaneously fit a representative set of observed HC3N lines through a model with an expanding shell around the central star and its associated H II region, assuming that LTE prevails for HC3N. The simulations show that the slowly expanding inner envelope has expansion and turbulence velocities of ~5-18 and ~3.5 km s-1, respectively, and that it is possibly elongated. Its inclination with respect to the line of sight has also been explored. The HC3N column density in front of the continuum source has been determined by comparing the output of an array of models with the data. The best fits are obtained for column densities in the range × 1017 cm-2, consistent with previous estimates from Infrared Space Observatory (ISO) data, and TK in the range 250-275 K, in very good agreement with estimates made from the same ISO data.

The Star Formation Rate and Dense Molecular Gas in Galaxies

HCN luminosity is a tracer of dense molecular gas, n(H(2)) greater than or similar to3 x 10(4) cm(-3), associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, L(IR) greater than or similar to 10(12) L(circle dot)), 22 luminous infrared galaxies (LIGs, 10(11) L(circle dot) < L(IR) less than or similar to 10(12) L(circle dot)), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities L(IR) and L(HCN) (in the log-log plot) with a correlation coefficient R = 0.94, and an almost constant average ratio L(IR)/L(HCN) = 900 L(circle dot) (K km s(-1) pc(2))(-1). The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas.

Australia Telescope Compact Array imaging of circumstellar HCN line emission from R Scl

We present radio-interferometric observations of HCN $J=1\rightarrow0$ line emission from the carbon star R Scl, obtained with the interim 3-mm receivers of the Australia Telescope Compact Array. The emission is resolved into a central source with a Gaussian FWHM of ~1´´, which we identify as the present mass loss envelope. Using a simple photodissociation model and constraints from single-dish HCN spectra, we argue that the present mass-loss rate is low, ${\sim} 2\, \times\, 10^{-7}~M_\odot$ yr -1 , supporting the idea that R Scl had to experience a brief episode of intense mass loss in order to produce the detached CO shell at ~10´´ radius inferred from single-dish observations. Detailed radiative transfer modelling yields an abundance of HCN relative to H 2 , $f_{\mathrm{HCN}}$, of ${\sim}10^{-5}$ in the present-day wind. There appears to be a discrepancy between model results obtained with higher transition single-dish data included and those from the $J=1\rightarrow0$ interferometer data alone, in that the interferometer data suggest a smaller envelope size and larger HCN abundance than the single-dish data. The lack of HCN in the detached shell, $f_{\mathrm{HCN}}\la 2\times 10^{-7}$, is consistent with the rapid photodissociation of HCN into CN as it expands away from the star.

The essential signature of a massive starburst in a distant quasar.

Observations of carbon monoxide emission in high-redshift (zeta > 2) galaxies indicate the presence of large amounts of molecular gas. Many of these galaxies contain an active galactic nucleus powered by accretion of gas onto a supermassive black hole, and a key question is whether their extremely high infrared luminosities result from the active galactic nucleus, from bursts of massive star formation (associated with the molecular gas), or both. In the Milky Way, high-mass stars form in the dense cores of interstellar molecular clouds, where gas densities are n(H2) > 10(5) cm(-3) (refs 1, 2). Recent surveys show that virtually all galactic sites of high-mass star formation have similarly high densities. The bulk of the cloud material traced by CO observations, however, is at a much lower density. For galaxies in the local Universe, the HCN molecule is an effective tracer of high-density molecular gas. Here we report observations of HCN emission from the infrared-luminous 'Cloverleaf' quasar (at a redshift zeta = 2.5579). The HCN line luminosity indicates the presence of 10 billion solar masses of very dense gas, an essential feature of an immense starburst, which contributes, together with the active galactic nucleus it harbours, to its high infrared luminosity.

VLA and BIMA Observations toward the Exciting Source of the Massive HH 80–81 Outflow

We present high angular resolution VLA and BIMA observations of NH$_3$, HCO$^+$, HCN and SO molecular emission and 1.4, 3.5 and 7 mm continuum emission toward the exciting source of the HH~80-81 system. This object is one of the few massive protostars known to be driving a collimated outflow. We report the first detection of SO 5$_5$--4$_4$ molecular emission toward the exciting source of HH 80-81, suggesting that this transition may be a good tracer of molecular gas near massive protostars. We also detected toward this source dust continuum emission at 1.4 and 3.5 mm. From the SO molecular emission and the dust emission we roughly estimated that the molecular mass associated with the circumstellar surroundings of the exciting source of the thermal jet is in the range 1 -- 3 M$_\odot$. Weak and broad (2,2) ammonia emission was also found in the direction of the jet suggesting the presence of small amounts of molecular gas at high temperatures ($>$ 50 K). The VLA observations show the presence of three ammonia components toward the HH 80-81 region. The brightest component peaks at $\sim8^{\prime\prime}$ to the NE of the thermal jet and is associated with the H$_2$O maser spots in the region. A second ammonia clump is located about 25$^{\prime\prime}$ to the NE of the jet and is associated with class I methanol masers. The third ammonia component is located 1$^\prime$ to the south from the thermal jet and may be a molecular core yet without stellar formation. The BIMA observations show that the strongest emission in the HCO$^+$ and HCN lines originate close to the H$_2$O maser, and cover the same spatial region and velocity range as the brightest ammonia component.

The outgassing and composition of Comet 19P/Borrelly from radio observations

Radio spectroscopic observations of Comet 19P/Borrelly were performed during the 1994 apparition and at, and near, the time of the Deep Space 1 flyby in 2001. HCN, CS, CH 3OH, and H 2CO were detected using the 30-m telescope of the Institut de Radioastronomie Millimétrique and the James Clerk Maxwell Telescope, and their production rates relative to water are estimated to be 0.06–0.11, 0.07, 1.7, and 0.4%, respectively. Only upper limits are derived for H 2S and CO. The upper limit for CO/H 2O (<15%) is not very constraining, while the upper limit for the H 2S/H 2O ratio of 0.45% is near the bottom of the range of values measured for other comets. Observations of the OH radical at the Nançay radio telescope provide water production rates a few weeks before the 1994 and 2001 perihelia. Observations of the 1 10–1 01 water line at 557 GHz with the Odin satellite yield a water production rate of (2.5±0.5)×10 28 s −1 on September 22, 2001, at the time of the Deep Space 1 encounter, and (3.3±0.6)×10 28 s −1 averaged over the September 22–24, 2001 period. The line shapes are asymmetric and blueshifted by V 0∼−0.18 km s −1 for the best observed HCN lines recorded one week after perihelion. The HCN line shapes, and the similar OH and HCN velocity shifts over the September–November 1994 and August–September 2001 periods, favor anisotropic outgassing towards the Sun. Strong outgassing directed along the primary dust jet seen on visible images is not excluded by the HCN line shapes, but unrealistically high gas expansion velocities are required to explain the line shapes in that case.

Star forming cores in L 1251: Maps and molecular abundances

We have mapped the dense parts of the cometary-shaped, star-forming dark cloud L 1251 in the rotational lines of HCN, HNC, HCO + and CS at 3 mm, and observed selected positions in SO, CH 3 CCH and rare isotopomers of the mapped molecules. Using the CS line we detected 15 cores with sizes of ∼0.1-0.3 pc. New estimates of the fraction of dense gas in the cores yield a revised average SFE of ∼10%. Although 3 times lower than the previous estimate, this high SFE still points to externally triggered star formation in the cloud. Around IRAS 22343+7501, the source proposed to drive a previously detected extended CO outflow, our data suggest the existence of either a rotating HCO + disk or a dense outflow with a dynamical age of ∼2 x 10 4 years. A stability check seems to rule out the disk interpretation. We suggest that both continuum sources of Beltran et al. are protostars each driving its own outflow. Using methyl acetylene as a thermometer we find indications that at lower temperatures the A and E species are defined by different partition functions. A gradient was found in the cloud, with the highest temperature detected in the region. The column density ratios derived from these observations and the previously published NH 3 data show in general little variations, but for two exceptional locations. One of these is in the tip of the head with high relative SO and NH 3 abundances, and the other is in the tail with low CO and HCO + column densities with respect to HNC, HCN and NH 3 . In the first case the abundance ratios can probably be explained by an advanced stage of chemical evolution assisted by an elevated temperature. The second location is likely to be an example of CO and HCO + depletion, and the implication is that also HNC and HCN belong to the molecules which are more resistant against freezing-out than CO and HCO + .

The ISM and Stellar Distributions Near Sgr A*

HST-NICMOS imaging in the 1.87μmPα line and 1.90μm continuum and OVRO interferometry of the 3mm HCN line provide maps of the ionized and molecular gas at 0.2 and 3″ resolution, respectively. Comparison of the observed Pα emission with the 6-cm radio continuum yields a map of the foreground extinction. High apparent extinction is seen along the periphery of the ionized disk and is well-correlated with the molecular torus. The median extinction is AV = 31.1 mag. This extinction distribution is then used to correct the observed Pα emission map to yield an ‘extinction-corrected’ image of the ionized gas at 0.2″ resolution. The stellar distribution near Sgr A* is also analyzed from the 1.9μm continuum image – indicating a drop in the surface brightness distribution of star light within 1″ radius of SgrA*. This could be the result of stellar collisions which deplete giant stars within this radius or indicate the presence of a secondary lower mass black hole.

Long-term evolution of CO, CS and HCN in Jupiter after the impacts of comet Shoemaker-Levy 9

Various trace species were identified for the first time in Jupiter's stratosphere in the aftermath of the collision of comet Shoemaker-Levy 9 in July 1994. In the millimeter and submillimeter ranges strong lines of CO, CS and HCN were measured at the JCMT 15-m and IRAM 30-m telescopes. We report subsequent monitoring observations of the evolution of these lines, conducted at both telescopes from 1995 to 1998. Regular global mapping of the Jovian disk allowed for a detailed study of the spreading of newly created molecular compounds in the planet's stratosphere. Temperatures and abundances have been inferred in the atmospheric region above 0.2 mbar . The results are interpreted using a model of spatial and temporal dispersion specially developed for our investigations. Implications for stratospheric dynamics are outlined. Origins of C-, O-, N-, and S-bearing compounds are reviewed in the light of our abundance determinations.

Indications of Inflow Motions in Regions Forming Massive Stars

Observational evidence of inflowing motions in massive star-forming regions has been extremely rare. We have made a spectroscopic survey of a sample of 28 massive star-forming cores associated with water masers. An optically thick line of HCN (3-2) was used in combination with optically thin lines [H13CN (3-2) or C34S (5-4), (3-2), and (2-1)] to identify "blue" line profiles that can indicate inflow. Comparing intensities for 18 double-peaked line profiles yields 11 blue and three red profiles that are statistically significant. In the full sample of 28 sources, 12 show blue profiles, and six show red profiles that are statistically significant based on the velocity offsets of lines that are optically thick from those that are optically thin. These results indicate that HCN (3-2) emission may trace inflow in regions forming high-mass stars.

Enhanced HCN (1-0) Emission in the Type-1 Seyfert Galaxy NGC 1097

Abstract The central kpc region of the low-luminosity type-1 Seyfert galaxy NGC 1097 has been observed in the CO (1–0) and HCN (1–0) lines with the Nobeyama Millimeter Array and the NRO 45 m telescope. We have found a striking enhancement of the HCN emission toward the active nucleus of NGC 1097; a large fraction of the CO emission comes from a circumnuclear starburst ring ($r \sim 10''$ or 700 pc at $D = 14.5 \,\mathrm{Mpc}$), whereas the HCN emission is dominated by a strong unresolved peak at the nucleus. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, $R_\mathrm{HCN/CO}$, is about 0.11 in the aperture of the 45 m telescope beams (${15{}^{\mathrm {\prime \prime }}}$ for CO and ${19{}^{\mathrm {\prime \prime }}}$ for HCN), but reaches 0.34 in the smaller aperture (${10{}^{\mathrm {\prime \prime }}} \times {4\rlap {.}{}^{\mathrm {\prime \prime }}4}$). These CO and HCN properties in NGC 1097 are similar to those in the type-2 Seyfert galaxies NGC 1068 and NGC 5194 (M 51).