Dense Gas Tracers Research Articles

Probing the feeding and feedback of activity near and far

High-resolution CO maps are an essential tool to search for observational evidence of AGN fueling in galaxy nuclei. While their capabilities will be surpassed by ALMA, current mm-interferometers can already provide relevant information on scales which are critical for the process of angular momentum transfer in fueling the AGN. In this context we present the latest results issued from the NUclei of GAlaxies (NUGA) project, a high-resolution (0.5′′–1′′) CO survey of low luminosity AGNs conducted with the IRAM Plateau de Bure interferometer (PdBI). The use of more specific molecular tracers of dense gas can probe the feedback influence of activity on the chemistry and energy balance in the interstellar medium of nearby galaxies, a prerequisite to understanding how feedback operate at higher redshift galaxies. We discuss the results obtained in an ongoing study devoted to probe the feedback of activity from nearby Seyferts to high-redshift QSO.

The Dense Gas in the Central Kiloparsec of NGC 6946

We present observations of the HCN and HCO -->+ -->J = 1–0 transitions in the center of the nearby spiral galaxy NGC 6946 made with the BIMA and CARMA interferometers. Using the BIMA SONG CO map, we investigate the change in the -->IHCN/ICO and -->IHCO+/ICO integrated intensity ratios as a function of radius in the central kiloparsec of the galaxy and find that they are strongly concentrated at the center. We use the 2MASS -->Ks-band image to find the stellar surface density and then construct a map of the hydrostatic midplane pressure. We apply a PDR model to the observed -->IHCN/IHCO+ integrated intensity ratio to calculate the number density of molecular hydrogen in the dense gas tracer emitting region and find that it is roughly constant at -->105 cm -->−3 across our map. We explore two hypotheses for the distribution of the dense gas. If the HCN and HCO -->+ emission comes from self-gravitating density peaks inside of a less dense gas distribution, there is a linear proportionality between the internal velocity dispersion of the dense gas and the size of the density peak. Alternatively, the HCN and HCO -->+ emission could come from dense gas homogeneously distributed throughout the center and bound by ambient pressure, similar to what is observed toward the center of the Milky Way. We find both of these hypotheses to be plausible. We fit the relationships between -->IHCN, IHCO+, and -->ICO. Correlations between the hydrostatic midplane pressure and -->IHCN and -->IHCO+ are demonstrated, and power-law fits are provided. We confirm the validity of a relation found by Blitz & Rosolowsky between pressure and the molecular to atomic gas ratio in the high hydrostatic midplane pressure regime ( -->106- -->108 cm -->−3 K).

Detection of Emission from the CN Radical in the Cloverleaf Quasar at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $z=2.56$ \end{document}

We report the detection of CN(N = 3 → 2) emission toward the Cloverleaf quasar (z = 2.56) based on observations with the IRAM Plateau de Bure Interferometer. This is the first clear detection of emission from this radical at high redshift. CN emission is a tracer of dense molecular hydrogen gas [n(H2) > 104 cm-3] within star-forming molecular clouds, in particular, in regions where the clouds are affected by UV radiation. The HCN/CN intensity ratio can be used as a diagnostic for the relative importance of photodissociation regions (PDRs) in a source and as a sensitive probe of optical depth, the radiation field, and photochemical processes. We derive a lensing-corrected CN(N = 3 → 2) line luminosity of L = (4.5 ± 0.5) × 109 K km s-1 pc2. The ratio between CN luminosity and far-infrared luminosity falls within the scatter of the same relationship found for low-z (ultra-) luminous infrared galaxies. Combining our new results with CO(J = 3 → 2) and HCN(J = 1 → 0) measurements from the literature and assuming thermal excitation for all transitions, we find a CO/CN luminosity ratio of 9.3 ± 1.9 and a HCN/CN luminosity ratio of 0.95 ± 0.15. However, we find that the CN(N = 3 → 2) line is likely only subthermally excited, implying that those ratios may only provide upper limits for the intrinsic 1 → 0 line luminosity ratios. We conclude that, in combination with other molecular gas tracers like CO, HCN, and HCO+, CN is an important probe of the physical conditions and chemical composition of dense molecular environments at high redshift.

Dense molecular gas in a sample of LIRGs and ULIRGs: The low-redshift connection to the huge high-redshift starbursts and AGNs

The sample of nearby LIRGs and ULIRGs for which dense molecular gas tracers have been measured is building up, allowing for the study of the physical and chemical properties of the gas in the variety of objects in which the most intense star formation and/or AGN activity in the local universe is taking place. This characterisation is essential to understand the processes involved, discard others and help to interpret the powerful starbursts and AGNs at high redshift that are currently being discovered and that will routinely be mapped by ALMA. We have studied the properties of the dense molecular gas in a sample of 17 nearby LIRGs and ULIRGs through millimeter observations of several molecules (HCO+, HCN, CN, HNC and CS) that trace different physical and chemical conditions of the dense gas in these extreme objects. In this paper we present the results of our HCO+ and HCN observations. We conclude that the very large range of measured line luminosity ratios for these two molecules severely questions the use of a unique molecular tracer to derive the dense gas mass in these galaxies.

Millimeter dust continuum emission revealing the true mass of giant molecular clouds in the Small Magellanic Cloud

Context. CO observations have been the best way so far to trace molecular gas in external galaxies, but in low metallicity environments the gas mass deduced could be largely underestimated due to enhanced photodissociation of the CO molecule. Large envelopes of H2 could therefore be missed by CO observations. Aims. At present, the kinematic information of CO data cubes are used to estimate virial masses and trace the total mass of the molecular clouds. Millimeter dust emission can also be used as a dense gas tracer and could unveil H2 envelopes lacking CO. These different tracers must be compared in different environments. Methods. This study compares virial masses to masses deduced from millimeter emission, in two GMC samples: the local molecular clouds in our Galaxy (10 4 −10 5 M� ), and their equivalents in the Small Magellanic Cloud (SMC), one of the nearest low metallicity dwarf galaxies. Results. In our Galaxy, mass estimates deduced from millimeter (FIRAS) emission are consistent with masses deduced from gamma ray analysis and therefore trace the total mass of the clouds. Virial masses are systematically larger (twice on average) than mass estimates from millimeter dust emission. This difference decreases toward high masses and has been reported in previous studies. This is not the case for SMC giant molecular clouds: molecular cloud masses deduced from SIMBA millimeter observations are systematically higher (twice on average for conservative values of the dust to gas ratio and dust emissivity) than the virial masses from SEST CO observations. The observed excess cannot be accounted for by any plausible change of dust properties. Taking a general form for the virial theorem, we show that a magnetic field strength of ∼15 µG in SMC clouds could provide additional support for the clouds and explain the difference observed. Conclusions. We conclude that masses of SMC molecular clouds have so far been underestimated. Magnetic pressure may contribute significantly to their support.

Methanol and other molecular tracers of outflows and dense gas in the molecular cloud G345.01+1.79

The results of SEST millimeter observations of the molecular cloud G345.01+1.79 are presented. Spectra of CH3OH, SO2, SiO, HCO+, C18O, C33S, C34S, HCN, and DCN lines have been obtained. Mapping of the cloud in CH3OH, SiO, and C34S lines indicates that the maximum integrated intensity in the SiO and C34S lines and in low-excitation CH3OH transitions coincide with the northern group of methanol masers, while the corresponding maximum for high-excitation CH3OH transitions coincides with the southern methanol-maser group. The physical parameters are estimated from the quasi-thermal CH3OH lines under the large-velocity-gradient approximation, and their distribution on the sky derived. The density and temperature are higher toward the southern group of methanol masers than in the northern group. This may indicate that star formation is in an earlier stage of evolution in the northern than toward the southern group. A maser component can be distinguished in 14 (of 71) CH3OH lines. We have detected for the first time weak, probably maser, emission in the CH3OH lines at 148.11, 231.28, 165.05, 165.06, and 165.07 GHz. A blue wing is clearly visible in the CH3OH, SiO, C18O, and SO2 lines. The emission in this wing is probably associated with a compact source whose velocity is characteristic of the CH3OH maser emission in the southern group of masers.

HCN versus HCO+as Dense Molecular Gas Mass Tracers in Luminous Infrared Galaxies

It has recently been argued that the HCN J = 1-0 line emission may not be an unbiased tracer of dense molecular gas (n 104 cm-3) in luminous infrared galaxies (LIRGs; LFIR > 1011 L☉) and that HCO+ J = 1-0 may constitute a better tracer instead, casting doubt onto earlier claims supporting the former as a good tracer of such gas. In this paper new sensitive HCN J = 4-3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J = 1-0 transition from either species a poor proxy of its mass. Moreover, the well-known sensitivity of the HCO+ abundance to the ionization degree of molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO+ as dense gas tracers) may severely reduce the HCO+ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result in the decreasing HCO+/HCN J = 1-0 line ratios with increasing IR luminosity found in LIRGs, and it casts doubts on HCO+ rather than HCN as a good dense molecular gas tracer. Multitransition observations of both molecules are needed to identify the best such tracer and its relation to ongoing star formation, and to constrain what may be a considerable range of dense gas properties in such galaxies.

Warm SiO gas in molecular bullets associated with protostellar outflows

Context. We present the first SiO multiline analysis (from J = 2-1 to J = 11-10) of the molecular bullets along the outflows of the Class 0 sources L1448-mm and LI 157-mm, obtained through observations with IRAM and JCMT. Aims. We investigate the physical properties of different types of bullets, in order to explore their excitation conditions and constrain their formation and evolution. Methods. We have computed the main physical parameters (n H2 , T kin , N SiO ) in each bullet and compared them with other tracers of warm and dense gas and with models for the SiO excitation in shocks. Results. The bullets close to L1448-mm, associated with high velocity gas, have higher excitation conditions (n H2 ∼ 10 6 cm -3 , T ≥ 500 K) than the L1157 bullets (n H2 ∼ 1-5 x 10 5 cm -3 , T ∼ 100-300 K). In both the sources, there is clear evidence of the presence of velocity components having different excitation conditions, with the denser and/or warmer gas associated with the gas at the higher speed. In L1448 the bulk of the emission is due to the high-excitation and high velocity gas, while in L1157 most of the emission comes from the low excitation gas at ambient velocity. The observed velocity-averaged line ratios are well reproduced by shocks with speeds v s larger than ∼30 km s -1 and densities ∼10 5 -10 6 cm -3 . Plane-parallel shock models, however, fail to predict all the observed line profiles and in particular the very similar profiles shown by both low and high excitation lines. Conclusions. The observations support the idea that the L1157 clumps are shock interaction events older than the L1448 bullets close to the driving source. In the latter objects, the velocity structure and the variations of physical parameters with the velocity very closely resemble those found in optical/IR jets near the protostar, suggesting that similar launching and excitation mechanisms are also at the origin of collimated jets seen at millimetre wavelengths.

Searching for gas-rich disks around T Tauri stars in Lupus

Aims. We characterize the molecular environment of classical T Tauri stars in Lupus observed with the Spitzer Space Telescope in our search for gas-rich disks toward these sources. Methods. Submillimeter observations of 12 CO, 13 CO, and C 18 O in the $J=3$–2 and 2–1 lines were obtained with the James Clerk Maxwell Telescope toward a sample of 21 T Tauri stars with disks in the Lupus molecular clouds. Pointings at the sources and at selected off-positions are presented in order to disentangle material associated with disks from ambient cloud material. Results. One source, IM Lup (Sz 82), was found with the double-peaked 12 CO and 13 CO profiles characteristic of a large rotating gas disk. The inclination of the disk is found to be ~$20^\circ$, with an outer radius of 400–700 AU. For most other sources, including GQ Lup with its substellar companion, the single-dish 12 CO lines are dominated by extended cloud emission with a complex velocity structure. No evidence of molecular outflows is found. Due to dense circumstellar material, compact C 18 O emission was detected toward only two sources. Future searches for gas-rich disks in Lupus should either use interferometers or perform very deep single-dish integrations in dense gas tracers to separate the cloud and disk emission.

Molecular Line Emission from Gravitationally Unstable Protoplanetary Disks

In the era of high resolution submillimeter interferometers, it will soon be possible to observe the neutral circumstellar medium directly involved in gas giant planet (GGP) formation at physical scales previously unattainable. In order to explore possible signatures of gas giant planet formation via disk instabilities, we have combined a 3D, non-local thermodynamic equilibrium (LTE) radiative transfer code with a 3D, finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4 M_J self-gravitating proto-GGP. Here, we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCO+. Our main results are the following: 1. Very high lying HCO+ transitions (e.g. HCO+ J=7-6) can trace dense planet forming clumps around circumstellar disks. Depending on the molecular abundance, the proto-GGP may be directly imageable by the Atacama Large Millimeter Array (ALMA). 2. HCO+ emission lines are heavily self-absorbed through the proto-GGP's dense molecular core. This signature is nearly ubiquitous, and only weakly dependent on assumed HCO+ abundances. The self-absorption features are most pronounced at higher angular resolutions. Dense clumps that are not self-gravitating only show minor self-absorption features. 3. Line temperatures are highest through the proto-GGP at all assumed abundances and inclination angles. Conversely, due to self-absorption in the line, the velocity-integrated intensity may not be. High angular resolution interferometers such as the Submillimeter Array (SMA) and ALMA may be able to differentiate between competing theories of gas giant planet formation.

Colliding Clouds: The Star Formation Trigger of the Stellar Cluster around BD +40 4124

We present BIMA and SCUBA observations of the young cluster associated with BD +40 4124 in the dense molecular gas tracer CS J = 2 → 1 and the continuum dust emission at λ = 3.1 mm and 850 μm. The dense gas and dust in the system are aligned in a long ridge morphology extending ~0.4 pc with 16 gas clumps of estimated masses ranging from 0.14 to 1.8 M☉. A north-south variation in the CS center line velocity can be explained with a two-cloud model. We posit that the BD +40 4124 stellar cluster formed from a cloud-cloud collision. The largest line widths occur near V1318 Cyg S, a massive star affecting its natal environment. In contrast, the dense gas near the other, more evolved, massive stars displays no evidence for disruption; the material must either be processed into the star, dissipate, or relax fairly quickly. The more evolved low-mass protostars are more likely to be found near the massive stars. If the majority of low-mass stars are coeval, the seemingly evolved low-mass protostars are not older: the massive stars have eroded their structures. Finally, at the highest resolution, the λ = 3.1 mm dust emission is resolved into a flattened structure 3100 × 1500 AU with an estimated mass of 3.4 M☉. The continuum and CS emission are offset by 11 from the southern binary source. A simple estimate of the extinction due to the continuum emission structure is AV ~ 700 mag. From the offset and because the southern source is detected in the optical, the continuum emission is from a previously unknown very young, intermediate-mass, embedded stellar object.

Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?

We present the results of the first HCO+ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN(1-0) observations, had shown that LIRGs and ULIRGs posses a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO+ partly confirms this result, we have discovered an intriguing correlation between the HCN(1-0)/HCO+(1-0) luminosity ratio and the IR luminosity of the galaxy (L(IR)). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded AGN may play a dominant role in the chemistry of molecular gas at L(IR) > 1e12 Lsun. We discuss the implications of this result for the understanding of LIRGs, ULIRGs and high redshift gas-rich galaxies.

Connecting Dense Gas Tracers of Star Formation in our Galaxy to High- z Star Formation

Observations have revealed prodigious amounts of star formation in starburst galaxies as traced by dust and molecular emission, even at large redshifts. Recent work shows that for both nearby spiral galaxies and distant starbursts, the global star formation rate, as indicated by the infrared luminosity, has a tight and almost linear correlation with the amount of dense gas as traced by the luminosity of HCN. Our surveys of Galactic dense cores in HCN 1-0 emission show that this correlation continues to a much smaller scale, with nearly the same ratio of infrared luminosity to HCN luminosity found over 7-8 orders of magnitude in LIR, with a lower cutoff around 104.5 L☉ of infrared luminosity. The linear correlation suggests that we may understand distant star formation in terms of the known properties of local star-forming regions. Both the correlation and the luminosity cutoff can be explained if the basic unit of star formation in galaxies is a dense core, similar to those studied in our Galaxy.

HCN and HCO+emission in the disk of M 31

We report observations made with the IRAM 30 m radiotelescope in the HCN(1-0) and HCO + (1-0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy (M 31). The targets were identified bright CO GMCs selected from the IRAM 30 m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5 kpc. The HCN and HCO + emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO + emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO + emission is weaker than the HCN towards a quiescent cloud in the inner part of M 31, which could be due to a lower abundance of HCO + . We derive IHCN/ICO ratios between 0.008 and 0.03 and IHCO+ /ICO ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M 31. Unlike our Galaxy the HCO + /CO ratio is lower in the center of M 31 than in the arms, which can be explained by both a lower abundance of HCO + and different conditions of excitation. Furthermore the HCN/CO and HCO + /CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.

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).

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.

An H13CO+Survey for Dense Envelopes around Low‐Mass Embedded Sources in Taurus

A dense gas survey was carried out toward 10 low-mass embedded young sources in Taurus with the Nobeyama Millimeter Array (NMA) to investigate protostellar evolution. All the sources were observed in the H13CO+(J = 1-0) line, a high-density tracer. Significant H13CO+ emission (≥4.5 σ) was detected toward six of them. The H13CO+ emission is distributed roughly perpendicular to the molecular outflow axes, indicating that the H13CO+ line traces the dense envelopes associated with the central stars. The sizes and masses of the dense envelopes are estimated to be (1-7) × 103 AU and 0.01-0.2 M☉, respectively. The 10 sources are divided into the following three classes based on their H13CO+ intensities of the NMA maps, and their properties are studied using our own and other available data. Class A sources have H13CO+ emission centered on the star with its elongation perpendicular to the molecular outflow axes. These sources also have dense outflowing gas and centrally condensed parent cores. Class B sources have H13CO+ emission near the source positions and dense outflowing gas. The parent cores around class B sources, however, have a shallower density profile. Class C sources have neither H13CO+ emission nor wing emission in dense gas tracers. From these properties, we conclude that low-mass protostars evolve from class A, B to C sources by dissipating their parent cloud cores, which is consistent with the widely accepted ideas of star formation. In addition, these observational data suggest that significant dispersion of a parent core by a molecular outflow and main accretion phase ends at the early protostellar phase. Six of the 10 sources are detected in continuum emission at 87 GHz. The intensities of the H13CO+ emission do not correlate with the flux densities at 87 GHz. This is because our continuum maps trace compact disks on a 102 AU scale and not dense gas on a 103-4 AU scale.

Thermally Dominated Carbon Monoxide Emission in the Taurus Molecular Cloud Complex

We analyze the structure of a region of the Taurus molecular cloud containing star-forming dense cores, as traced by emission from the J = 1-0 rotational transition of the carbon monoxide isotopomer, C18O. While the spatial structure of the velocity-integrated emission is rather featureless, there is substantial structure in the emission integrated over small velocity widths, and the entire three-dimensional data set (right ascension, declination, and velocity) can be broken into a collection of nearly discrete components. We construct a cloud model consisting of nine individual Gaussian components and find that the data are best replicated when these components have sizes of ~0.1 pc and velocity dispersions comparable to or smaller than the thermal velocity dispersion of molecular hydrogen at a temperature of 10 K. We find that nearly all of the molecular mass of the cloud is contained within these quiescent structures. At least two of the structures are detected in observations in the J = 4-3 line of HC3N; these two structures are likely associated with the two forming stars in the region. Our results suggest that thermally dominated structures may be common in regions containing dense cores, but that many of these structures are insufficiently dense to be detected with dense gas tracers.

L1544: A Starless Dense Core with Extended Inward Motions

We present a multiline study of the dense core L1544 in the Taurus molecular complex. Although L1544 does not harbor an embedded star, it presents several characteristics of cores that have already undergone star formation, suggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting dichotomy, with the thick dense gas tracers su†ering very strong self absorption while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and kinematics of the gas in detail. A simple analysis shows that the core is almost isothermal and that the self absorptions are due to very subthermal excitation of the dense gas tracers in the outer layers. The density has to decrease outward rapidly, and a detailed radiative transfer calculation that simultaneously -ts three iso- topes of CO and two of CS shows that the density approximately follows a r~1.5 power law. The self absorptions, in addition, allow us to measure the relative velocity between the inner and outer layers of the core, and we -nd that there is a global pattern of inward motions (background and foreground approaching each other). The relative speed between the foreground and background changes with posi- tion, and we use a simple two-layer model to deduce that while the foreground gas has a constant veloc- ity, the background material presents systematic velocity changes that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center and propagates outward (as in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that sug- gests an advanced age) and the lack of a star at the core center (that suggests extreme youth). Ambipolar di†usion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s~1) and the fact that ionized species move with speeds similar to those of the neutrals. Other infall models seem also to have problems -tting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star formation. Our study of L1544 illustrates how little is still known about the physical conditions that precede star formation and how detailed studies of starless cores are urgently needed. Subject headings: ISM: individual (L1544) E ISM: kinematics and dynamics E stars: formation

An Out‐of‐Plane CO (J= 2–1) Survey of the Milky Way. II. Physical Conditions of Molecular Gas

Physical conditions of molecular gas in the first quadrant of the Galaxy are examined through comparison of the CO J = 2-1 data of the Tokyo-Nobeyama Radio Observatory survey with the CO J = 1-0 data of the Columbia survey. A gradient of the CO J = 2-1/J = 1-0 intensity ratio (≡ R2-1/1-0) with Galactocentric distance is reported. The ratio varies from 0.75 at 4 kpc to 0.6 at 8 kpc in Galactocentric distance. This confirms the early in-plane results reported by Handa et al. We classify molecular gas into three categories in terms of R2-1/1-0 on the basis of a large velocity gradient model calculation. Very high ratio gas (VHRG; R2-1/1-0 > 1.0) is either dense, warm, and optically thin gas or externally heated, dense gas. High ratio gas (HRG; R2-1/1-0 = 0.7-1.0) is warm and dense gas with high-excitation temperature of the J = 2-1 transition (Tex 10 K), and it is often observed in central regions of giant molecular clouds. Low ratio gas (LRG; R2-1/1-0 < 0.7) has low-excitation temperature of the J = 2-1 transition (Tex 10 K) because of low density or low kinetic temperature, or both, and is often observed in dark clouds and outer envelopes of giant molecular clouds. It is shown that the CO J = 2-1 emission is better characterized as a tracer of dense gas rather than a tracer of warm gas for molecular gas with kinetic temperature higher than 10 K. The observed large-scale decrease in R2-1/1-0 as a function of Galactocentric distance is ascribed to the fractional decrease of HRG and VHRG from 40% near 5 kpc to 20% near the solar circle. The HRG and VHRG are found predominantly along the Sagittarius and Scutum arms, probably in their downstream. This fact and the deficiency of atomic gas compared with molecular gas in the inner Galaxy indicate that physical conditions of interstellar gas are affected by grand-design, nonlinear processes, such as compression by spiral density waves followed by gravitational collapse, and not by dissociation of low-density molecular gas by young stars.