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.

/pdf/theory-of-star-formation-sol2jzngnw.pdf

Theory of Star Formation

The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes. Note: Accepted by AA 18 A4 pages, 11 figures;

/pdf/a-computer-program-for-fast-non-lte-analysis-of-interstellar-17wsxkwvpj.pdf

A computer program for fast non-LTE analysis of interstellar line spectra

Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, in- cluding energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to E/k ∼ 1000 K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/∼moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calcu- lations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

/pdf/an-atomic-and-molecular-database-for-analysis-of-4xud6qp5de.pdf

An atomic and molecular database for analysis of submillimetre line observations

Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

/pdf/complex-organic-interstellar-molecules-sz3r2nq94z.pdf

Complex Organic Interstellar Molecules

Flattened, rotating disks of cool dust and gas extending for tens to hundreds of astronomical units are found around almost all low-mass stars shortly after their birth. These disks generally persist for several million years, during which time some material accretes onto the star, some is lost through outflows and photoevaporation, and some condenses into centimeter- and larger-sized bodies or planetesimals. Through observations mainly at IR through millimeter wavelengths, we can determine how common disks are at different ages; measure basic properties including mass, size, structure, and composition; and follow their varied evolutionary pathways. In this way, we see the first steps toward exoplanet formation and learn about the origins of the Solar System. This review addresses observations of the outer parts, beyond 1 AU, of protoplanetary disks with a focus on recent IR and (sub)millimeter results and an eye to the promise of new facilities in the immediate future.

http://www.ifa.hawaii.edu/users/cieza/WC2011_ARAA.pdf

Protoplanetary Disks and Their Evolution

We present 1D radiative transfer modelling of the envelopes of a sample of 18 low-mass protostars and pre-stellar cores with the aim of setting up realistic physical models, for use in a chemical description of the sources. The density and temperature proles of the envelopes are constrained from their radial proles obtained from SCUBA maps at 450 and 850 m and from measurements of the source fluxes ranging from 60 mt o 1.3 mm. The densities of the envelopes within 10 000 AU can be described by single power-laws / r for the class 0 and I sources with ranging from 1.3 to 1.9, with typical uncertainties of0.2. Four sources have flatter proles, either due to asymmetries or to the presence of an outer constant density region. No signicant dierence is found between class 0 and I sources. The power-law ts fail for the pre-stellar cores, supporting recent results that such cores do not have a central source of heating. The derived physical models are used as input for Monte Carlo modelling of submillimeter C 18 Oa nd C 17 O emission. It is found that class I objects typically show CO abundances close to those found in local molecular clouds, but that class 0 sources and pre-stellar cores show lower abundances by almost an order of magnitude implying that signicant depletion occurs for the early phases of star formation. While the 2{1 and 3{2 isotopic lines can be tted using a constant fractional CO abundance throughout the envelope, the 1{0 lines are signicantly underestimated, possibly due to contribution of ambient molecular cloud material to the observed emission. The dierence between the class 0 and I objects may be related to the properties of the CO ices.

https://www.aanda.org/articles/aa/pdf/2002/27/aa2205.pdf

Physical structure and CO abundance of low-mass protostellar envelopes

A detailed radiative transfer analysis of the observed continuum and molecular line emission toward the deeply embedded young stellar object IRAS 16293-2422 is performed. Accurate molecular abundances and abundance changes with radius are presented. The continuum modelling is used to constrain the temperature and density distributions in the envelope, enabling quantitative estimates of various molecular abundances. The density structure is well described by a single power-law falling off as r^(-1.7), i.e., in the range of values predicted by infall models. A detailed analysis of the molecular line emission strengthens the adopted physical model and lends further support that parts of the circumstellar surroundings of IRAS are in a state of collapse. The molecular excitation analysis reveals that the emission from some molecular species is well reproduced assuming a constant fractional abundance throughout the envelope. The abundances and isotope ratios are generally close to typical values found in cold molecular clouds in these cases, and there is a high degree of deuterium fractionation. There are, however, a number of notable exceptions. Lines covering a wide range of excitation conditions indicate for some molecules, e.g., H_2CO, CH_3OH, SO, SO_2 and OCS, a drastic increase in their abundances in the warm and dense inner region of the circumstellar envelope. The location at which this increase occurs is consistent with the radius at which ices are expected to thermally evaporate off the grains. In all, there is strong evidence for the presence of a "hot core" close to the protostar, whose physical properties are similar to those detected towards most high mass protostars except for a scaling factor. However, the small scale of the hot gas and the infalling nature of the envelope lead to very different chemical time scales between low mass and high mass hot cores, such that only very rapidly produced second-generation complex molecules can be formed in IRAS 16293-2422 . Alternatively, the ices may be liberated due to grain-grain collisions in turbulent shear zones where the outflow interacts with the envelope. Higher angular resolution observations are needed to pinpoint the origin of the abundance enhancements and distinguish these two scenarios. The accurate molecular abundances presented for this low-mass protostar serve as a reference for comparison with other objects, in particular circumstellar disks and comets.

/pdf/does-iras-16293-2422-have-a-hot-core-chemical-inventory-and-31pi1i7bir.pdf

Does IRAS 16293–2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment

This paper presents the first substantial study of the chemistry of the envelopes around a sample of 18 low-mass pre- and protostellar objects for which physical properties have previously been derived from radiative transfer modeling of their dust continuum emission. Single-dish line observations of 24 transitions of 9 molecular species (not counting isotopes) including HCO + , N 2 H + , CS, SO, SO 2 , HCN, HNC, HC 3 N and CN are reported. The line intensities are used to constrain the molecular abundances by comparison to Monte Carlo radiative transfer modeling of the line strengths. In general the nitrogen-bearing species together with HCO + and CO cannot be fitted by a constant fractional abundance when the lowest excitation transitions are included, but require radial dependences of their chemistry since the intensity of the lowest excitation lines are systematically underestimated in such models. A scenario is suggested in which these species are depleted in a specific region of the envelope where the density is high enough that the freeze-out timescale is shorter than the dynamical timescale and the temperature low enough that the molecule is not evaporated from the icy grain mantles. This can be simulated by a “drop” abundance profile with standard (undepleted) abundances in the inner- and outermost regions and a drop in abundance in between where the molecule freezes out. An empirical chemical network is constructed on the basis of correlations between the abundances of various species. For example, it is seen that the HCO + and CO abundances are linearly correlated, both increasing with decreasing envelope mass. This is shown to be the case if the main formation route of HCO + is through reactions between CO and H$_3^+$, and if the CO abundance still is low enough that reactions between H$_3^+$ and N 2 are the main mechanism responsible for the removal of H$_3^+$. Species such as CS, SO and HCN show no trend with envelope mass. In particular no trend is seen between “evolutionary stage” of the objects and the abundances of the main sulfur- or nitrogen-containing species. Among the nitrogen-bearing species abundances of CN, HNC and HC 3 N are found to be closely correlated, which can be understood from considerations of the chemical network. The CS/SO abundance ratio is found to correlate with the abundances of CN and HC 3 N, which may reflect a dependence on the atomic carbon abundance. An anti-correlation is found between the deuteration of HCO + and HCN, reflecting different temperature dependences for gas-phase deuteration mechanisms. The abundances are compared to other protostellar environments. In particular it is found that the abundances in the cold outer envelope of the previously studied class 0 protostar IRAS 16293-2422 are in good agreement with the average abundances for the presented sample of class 0 objects.

/pdf/molecular-inventories-and-chemical-evolution-of-low-mass-hh13jxf6wp.pdf

Molecular inventories and chemical evolution of low-mass protostellar envelopes

An extensive radiative transfer analysis of circumstellar SiO "thermal" radio line emission from a large sample of M-type AGB stars has been performed. The sample contains 18 irregulars of type Lb (IRV), 7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12 Miras. New observational data, which contain spectra of several ground vibrational state SiO rotational lines, are presented. The detection rate was about 60% (44% for the IRVs, and 68% for the SRVs). SiO fractional abundances have been determined through radiative transfer modelling. The abundance distribution of the IRV/SRV sample has a median value of 6 × 10 −6 , and a minimum of 2 × 10 −6 and a maximum of 5 × 10 −5 . The high mass-loss rate Miras have a much lower median abundance, <10 −6 . The derived SiO abundances are in all cases well below the abundance expected from stellar atmosphere equilibrium chemistry, on average by a factor of ten. In addition, there is a trend of decreasing SiO abundance with increasing mass-loss rate. This is interpreted in terms of depletion of SiO molecules by the formation of silicate grains in the circumstellar envelopes, with an efficiency which is high already at low mass-loss rates and which increases with the mass-loss rate. The high mass-loss rate Miras appear to have a bimodal SiO abundance distribution, a low abundance group (on average 4 × 10 −7 ) and a high abundance group (on average 5 × 10 −6 ). The estimated SiO envelope sizes agree well with the estimated SiO photodissociation radii using an unshielded photodissociation rate of 2.5 × 10 −10 s −1 . The SiO and CO radio line profiles differ in shape. In general, the SiO line profiles are narrower than the CO line profiles, but they have low-intensity wings which cover the full velocity range of the CO line profile. This is interpreted as partly an effect of selfabsorption in the SiO lines, and partly (as has been done also by others) as due to the influence of gas acceleration in the region which produces a significant fraction of the SiO line emission. Finally, a number of sources which have peculiar CO line profiles are discussed from the point of view of their SiO line properties.

/pdf/thermal-sio-radio-line-emission-towards-m-type-agb-stars-a-5grgba35wf.pdf

"Thermal" SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics

We have determined mass loss rates and gas expansion velocities for a sample of 69 M-type irregular (IRV; 22 objects) and semiregular (SRV; 47 objects) AGB-variables using a radiative transfer code to model their circumstellar CO radio line emission. We believe that this sample is representative for the mass losing stars of this type. The (molecular hydrogen) mass loss rate distribution has a median value of 2:0 10 7 M yr 1 , and a minimum of 2:0 10 8 M yr 1 and a maximum of 8 10 7 M yr 1 . M-type IRVs and SRVs with a mass loss rate in excess of 5 10 7 M yr 1 must be very rare, and among these mass losing stars the number of sources with mass loss rates below a few 10 8 M yr 1 must be small. We find no significant dierence between the IRVs and the SRVs in terms of their mass loss characteristics. Among the SRVs the mass loss rate shows no dependence on the period. Likewise the mass loss rate shows no correlation with the stellar temperature. The gas expansion velocity distribution has a median of 7.0 km s 1 , and a minimum of 2.2 km s 1 and a maximum of 14.4 km s 1 . No doubt, these objects sample the low gas expansion velocity end of AGB winds. The fraction of objects with low gas expansion velocities is very high, about 30% have velocities lower than 5 km s 1 , and there are objects with velocities lower than 3 km s 1 : V584 Aql, T Ari, BI Car, RX Lac, and L 2 Pup. The mass loss rate and the gas expansion velocity correlate well, a result in line with theoretical predictions for an optically thin, dust-driven wind. In general, the model produces line profiles which acceptably fit the observed ones. An exceptional case is R Dor, where the high-quality, observed line profiles are essentially flat-topped, while the model ones are sharply double-peaked. The sample contains four sources with distinctly double-component CO line profiles, i.e., a narrow feature centered on a broader feature: EP Aqr, RV Boo, X Her, and SV Psc. We have modelled the two components separately for each star and derive mass loss rates and gas expansion velocities. We have compared the results of this M-star sample with a similar C-star sample analysed in the same way. The mass loss rate characteristics are very similar for the two samples. On the contrary, the gas expansion velocity distributions are clearly dierent. In particular, the number of low-velocity sources is much higher in the M-star sample. We found no example of the sharply double-peaked CO line profile, which is evidence of a large, detached CO-shell, among the M-stars. About 10% of the C-stars show this phenomenon.

/pdf/mass-loss-rates-of-a-sample-of-irregular-and-semiregular-m-57o3k6ofq9.pdf

F. L. Schöier

Papers

Physical structure and CO abundance of low-mass protostellar envelopes

Does IRAS 16293–2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment

Molecular inventories and chemical evolution of low-mass protostellar envelopes

"Thermal" SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics

Mass loss rates of a sample of irregular and semiregular M-type AGB-variables