We review recent determinations of the present-day mass function (PDMF) and initial mass function (IMF) in various components of the Galaxy—disk, spheroid, young, and globular clusters—and in conditions characteristic of early star formation. As a general feature, the IMF is found to depend weakly on the environment and to be well described by a power-law form forM , and a lognormal form below, except possibly for m!1 early star formation conditions. The disk IMF for single objects has a characteristic mass around M , m!0.08 c and a variance in logarithmic mass , whereas the IMF for multiple systems hasM , and . j!0.7 m!0.2 j!0.6 c The extension of the single MF into the brown dwarf regime is in good agreement with present estimates of L- and T-dwarf densities and yields a disk brown dwarf number density comparable to the stellar one, n!n! BD " pc !3 .T he IMF of young clusters is found to be consistent with the disk fi eld IMF, providing the same correction 0.1 for unresolved binaries, confirming the fact that young star clusters and disk field stars represent the same stellar population. Dynamical effects, yielding depletion of the lowest mass objects, are found to become consequential for ages!130 Myr. The spheroid IMF relies on much less robust grounds. The large metallicity spread in the local subdwarf photometric sample, in particular, remains puzzling. Recent observations suggest that there is a continuous kinematic shear between the thick-disk population, present in local samples, and the genuine spheroid one. This enables us to derive only an upper limit for the spheroid mass density and IMF. Within all the uncertainties, the latter is found to be similar to the one derived for globular clusters and is well represented also by a lognormal form with a characteristic mass slightly larger than for the disk, M , ,e xcluding as ignif icant population of m!0.2-0.3 c brown dwarfs in globular clusters and in the spheroid. The IMF characteristic of early star formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below!1M , .T hese results suggest a characteristic mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present-day conditions.Theseconclusions,however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations. These IMFs allow a reasonably robust determination of the Galactic present-day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate mass-to-light ratio determinations. The mass-to-light ratios obtained with the disk and the spheroid IMF yield values 1.8-1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans-type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super-Alfvenic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large-scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans mass is replaced by a distribution of local Jeans masses, representative of the lognormal probability density function of the turbulent gas. Objects below the mean thermal Jeans mass still have a possibility to collapse, although with a decreasing probability.

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Galactic stellar and substellar initial mass function

We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Star Formation in the Milky Way and Nearby Galaxies

We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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Theory of Star Formation

New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth ( °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth or ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| 5°), X shows little systematic variation with latitude from a mean value of (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

https://iopscience.iop.org/article/10.1086/318388/pdf

The Milky Way in Molecular Clouds: A New Complete CO Survey

CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ≈ 1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

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The CO-to-H2 Conversion Factor

We report the first detection of the western bow shock component from IRAS 2 in NGC 1333 along with observations of previously detected shocks and outflow winds from this source and those from IRAS 4. We compare the shock and outflow distributions from these two young stellar objects (YSOs), and the locations of other YSOs, with the overall distribution of the dense molecular gas in the star-forming core using high spatial resolution observations of CS (J = 2 → 1, 3 → 2, and 5 → 4) emission made with the Institut de Radio Astronomie Millimetrique 30 m antenna. These comparisons provide a new picture of the morphology and dynamics of the star-forming core of NGC 1333. The CS maps show (1) a large cavity with many YSOs just at the inner edge of the cavity, (2) a dense, compressed shell at 8 km s-1; and (3) a gas layer at 7 km s-1 probably located inside the cavity. We find that the IRAS 2 and IRAS 4 outflows impact different gas layers as indicated by the spatial association of the red- and blueshifted lobes, and that IRAS 2 is located near the front edge of the CS shell. The burst of star formation that has shaped NGC 1333 is occurring in the compressed shell traced by CS and now appears to be in a late evolutionary stage.

The IRAS 2 and IRAS 4 Outflows and Star Formation in NGC 1333

Context. The Galactic central molecular zone (CMZ) is a region containing massive and dense molecular clouds, with dynamics driven by a variety of energy sources including a massive black hole. It is thus the nearest template for understanding physical processes in extragalactic nuclei. The CMZ’s neutral interstellar gas has been mapped spectrally in many neutral atomic and molecular gas tracers, but the ionized and CO-dark H 2 regions are less well traced spectroscopically. Aims. We identify features of the UV irradiated neutral gas, photon dominated regions (PDRs) and CO-dark H 2 , and highly ionized gas in the CMZ as traced by the fine structure line of C + at 158 μ m, [C ii], and characterize their properties. Methods. We observed the [C ii] 158 μ m fine structure line with high spectral resolution using Herschel HIFI with two perpendicular on-the-fly strip scans, along l = −0.8 to +0.8 and b = −0.8 to +0.8, both centered on ( l , b ) = (0°, 0°). We analyze the spatial-velocity distribution of the [C ii] data, compare them to those of [C i] and CO, and to dust continuum maps, in order to determine the properties and distribution of the ionized and neutral gas and its dynamics within the CMZ. Results. The longitude- and latitude-velocity maps of [C ii] trace portions of the orbiting open gas streams of dense molecular clouds, the cloud G0.253+0.016, also known as the Brick, the arched filaments, the ionized gas within several pc of Sgr A and Sgr B2, and the warm dust bubble. We use the [C ii] data to determine the physical and dynamical properties of these CMZ features. Conclusions. The bright far-IR 158 μ m C + line, [C ii], when observed with high spatial and spectral resolution, traces a wide range of emission features in the CMZ. The [C ii]  emission arises primarily from dense PDRs and highly ionized gas, and is an important tracer of the kinematics and physical conditions of this gas.

https://www.aanda.org/articles/aa/pdf/2017/03/aa29497-16.pdf

Kinematics and properties of the central molecular zone as probed with [C ii]

We have investigated the physical and chemical status of the pre-protostellar core B68. A previous extinction study suggested that the density profile of B68 is remarkably consistent with a Bonnor-Ebert sphere with 2.1 $M_{\odot}$ at 16 K. We mapped B68 in C3H2, CCS, and NH3 with the Deep Space Network (DSN) 70m telescope at Goldstone. Our results show that the B68 chemical structure is consistent with that seen in other pre-protostellar cores (L1498, L1544) and is explained by time dependent chemical models that include depletion. We measured a kinetic temperature of 11K with NH3 (1,1) and (2,2) spectra. We also show that B68 is thermally dominated with little contribution from turbulence support (< 10%). We consider a modified Bonnor-Ebert sphere to include effects of turbulence and magnetic fields and use it to constrain the uncertainties in its distance determination. We conclude that the distance to B68 is $\sim$ 95pc with a corresponding mass of $\sim$ 1.0 $M_{\odot}$. If some magnetic field is present it can be further away (beyond $\sim$ 100pc) and still satisfy the density structure of a Bonnor-Ebert sphere. Our observations do not preclude any instability such as the onset of collapse, or slow contraction, occurring in the center of the core, which cannot be resolved with our beam size (45").

https://arxiv.org/pdf/astro-ph/0303642.pdf

The Physical and Chemical Status of Pre-protostellar Core B68

Two rotational transitions of CCD, N = 1-2 at 144 GHz and 2-3 at 216 GHz, were detected in a laboratory glow discharge through deuterated acetylene and helium, after which one, N = 2-1, was detected toward the rich molecular cloud behind the Orion Nebula. The 144 GHz transition is a well-resolved spin doublet split by 55 MHz, the components of which contain hyperfine structure of the order of 1 MHz, so far only partially resolved. From observations toward two positions in Orion, at and near the Kleinmann-Low nebula, the column density of CCD is determined to be 1.8 x 10 to the 13th/sq cm and the isotopic ratio CCD/CCH = 0.05.CCD was not detected at two positions in TMC-1. 18 references.

Laboratory and Astronomical Detection of the Deuterated Ethynyl Radical CCD

Context. The distribution of various interstellar gas components and the pressure in the interstellar medium (ISM) is a result of the interplay of different dynamical mechanisms and energy sources on the gas in the Milky Way. The scale heights of the different gas tracers, such as H i and CO, are a measure of these processes. The scale height of [C ii] emission in the Galactic plane is important for understanding those ISM components not traced by CO or H i. Aims. We determine the average distribution of [C ii] perpendicular to the plane in the inner Galactic disk and compare it to the distributions of other key gas tracers, such as CO and H i. Methods. We calculated the vertical, z, distribution of [C ii] in the inner Galactic disk by adopting a model for the emission that combines the latitudinal, b, spectrally unresolved BICE survey, with the spectrally resolved Herschel Galactic plane survey of [C ii] at b = 0 ◦ . Our model assumed a Gaussian emissivity distribution vertical to the plane, and related the distribution in z to that of the latitude b using the spectrally resolved [C ii] Herschel survey as the boundary solution for the emissivity at b = 0 ◦ . Results. We find that the distribution of [C ii] perpendicular to the plane has a full-width half-maximum of 172 pc, larger than that of CO, which averages ∼110 pc in the inner Galaxy, but smaller than that of H i, ∼230 pc, and is offset by −28 pc. Conclusions. We explain the difference in distributions of [C ii], CO, and H i as due to [C ii] tracing a mix of ISM components. Models of hydrostatic equilibrium of clouds in the disk predict different scale heights, for the same interstellar pressure. The diffuse molecular clouds with [C ii] but no CO emission likely have a scale height intermediate between the low density atomic hydrogen H i clouds and the dense CO molecular clouds.

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William D. Langer

Papers

The IRAS 2 and IRAS 4 Outflows and Star Formation in NGC 1333

Kinematics and properties of the central molecular zone as probed with [C ii]

The Physical and Chemical Status of Pre-protostellar Core B68

Laboratory and Astronomical Detection of the Deuterated Ethynyl Radical CCD

The scale height of gas traced by [C ii] in the Galactic plane