Yancy L. Shirley

Tracing the mass during low-mass star formation. II. Modeling the submillimeter emission from preprotostellar cores

We have modeled the emission from dust in preprotostellar cores, including a self-consistent calculation of the temperature distribution for each input density distribution. Model density distributions include Bonnor-Ebert spheres and power laws. The Bonnor-Ebert spheres fit the data well for all three cores that we have modeled. The dust temperatures decline to very low values (Td ~ 7 K) in the centers of these cores, strongly affecting the dust emission. Compared to earlier models that assume constant dust temperatures, our models indicate higher central densities and smaller regions of relatively constant density. Indeed, for L1544, a power-law density distribution, similar to that of a singular, isothermal sphere, cannot be ruled out. For the three sources modeled herein, there seems to be a sequence of increasing central condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and L1544, have spectroscopic evidence for contraction, suggesting an evolutionary sequence for preprotostellar cores.

The Physical conditions for massive star formation: Dust continuum maps and modeling

Fifty-one dense cores associated with water masers were mapped at 350 μm. These cores are very luminous, 103 < Lbol/L☉ < 106, indicative of the formation of massive stars. Dust continuum contour maps, radial intensity profiles, and photometry are presented for these sources. The submillimeter dust emission peak is, on average, nearly coincident with the water maser position. The spectral energy distributions and normalized radial profiles of dust continuum emission were modeled for 31 sources using a one-dimensional dust radiative transfer code, assuming a power-law density distribution in the envelope, n = nf(r/rf)-p. The best-fit density power-law exponent, p, ranged from 0.75 to 2.5 with p = 1.8 ± 0.4, similar to the mean value found recently by Beuther and coworkers in a large sample of massive star-forming regions. The mean value of p is also comparable to that found in regions forming only low-mass stars, but nf is over 2 orders of magnitude greater for the massive cores. The mean p is incompatible with a logatropic sphere (p = 1), but other star formation models cannot be ruled out. Different mass estimates are compared and mean masses of gas and dust are reported within a half-power radius determined from the dust emission, log M(< rdec) = 2.0 ± 0.6, and within a radius where the total density exceeds 104 cm-3, log M(< rn) = 2.5 ± 0.6. Evolutionary indicators commonly used for low-mass star formation, such as Tbol and Lbol/Lsmm, may have some utility for regions forming massive stars. Additionally, for comparison with extragalactic star formation studies, the luminosity-to-dust mass ratio is calculated for these sources, Lbol/MD = 1.4 × 104 L☉/M☉, with a method most parallel to that used in studies of distant galaxies. This ratio is similar to that seen in high-redshift starburst 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, with a lower cutoff L(IR) around 10(4.5) L(circle dot) 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.

Tracing the Mass during Low-Mass Star Formation. III. Models of the Submillimeter Dust Continuum Emission from Class 0 Protostars

Seven Class 0 sources mapped with SCUBA at 850 and 450 μm are modeled using a one-dimensional radiative transfer code. The modeling takes into account heating from an internal protostar, heating from the interstellar radiation field (ISRF), realistic beam effects, and chopping to model the normalized intensity profile and spectral energy distribution. Power-law density models, n(r) ∝ r-p, fit all of the sources; best-fit values are mostly p = 1.8 ± 0.1, but two sources with aspherical emission contours have lower values (p ~ 1.1). Including all sources, p = 1.63 ± 0.33. Based on studies of the sensitivity of the best-fit p to variations in other input parameters, uncertainties in p for an envelope model are Δp = ±0.2. If an unresolved source (e.g., a disk) contributes 70% of the flux at the peak, p is lowered in this extreme case and Δp = . The models allow a determination of the internal luminosity (Lint = 4.0 L☉) of the central protostar as well as a characteristic dust temperature for mass determination (Tiso = 13.8 ± 2.4 K). We find that heating from the ISRF strongly affects the shape of the dust temperature profile and the normalized intensity profile, but it does not contribute strongly to the overall bolometric luminosity of Class 0 sources. There is little evidence for variation in the dust opacity as a function of distance from the central source. The data are well fitted by dust opacities for coagulated dust grains with ice mantles (Ossenkopf & Henning). The density profile from an inside-out collapse model (Shu) does not fit the data well, unless the infall radius is set so small as to make the density nearly a power law.

A CS J = 5 → 4 Mapping Survey Toward High-Mass Star-forming Cores Associated with Water Masers

We have mapped 63 regions forming high-mass stars in CS J = 5 → 4 using the CSO. The CS peak position was observed in C34S J = 5 → 4 toward 57 cores and in 13CS J = 5 → 4 toward the nine brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The cores are located in the first and second Galactic quadrants with an average distance of 5.3 ± 3.7 kpc and were well detected with a median peak signal-to-noise ratio in the integrated intensity of 40. The integrated intensity of CS J = 5 → 4 correlates very well with the dust continuum emission at 350 μm. For 57 sufficiently isolated cores, a well-defined angular size (FWHM) was determined. The core radius (RCS), aspect ratio [(a/b)obs], virial mass (Mvir), surface density (Σ), and the luminosity in the CS J = 5 → 4 line (L(CS54)) are calculated. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed toward much larger values than the mean. The median values, μ1/2, are as follows: μ1/2 (RCS) = 0.32 pc, μ1/2 ((a/b)obs) = 1.20, μ1/2 (Mvir) = 920 M⊙, μ1/2 (Σ) = 0.60 g cm-2, μ1/2 (L(CS54)) = 1.9 × 10-2 L⊙, and μ1/2 (Lbol/Mvir) = 165 (L/M)⊙. We find a weak correlation between C34S line width and size, consistent with Δv ~ R0.3. The line widths are much higher than would be predicted by the usual relations between line width and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2-3 with mass estimates from dust emission at 350 μm after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 M⊙ can be approximated by a power law with a slope of about -0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCH II regions, and the pressures at small radii are similar to those in hypercompact H II regions (P/k ~ 1010 K cm-3). The filling factors for dense gas are substantial, and the median abundance of CS is about 10-9. The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter.

Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II.

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the Cores to Disks (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these data sets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M?). As in Perseus, the mid-infrared sources are located close to the center of the SCUBA cores and the narrowness of the spatial distribution of mid-infrared sources around the peaks of the SCUBA cores suggests that no significant dispersion of the newly formed YSOs has occurred. Embedded YSOs are found in 35% of the SCUBA cores?fewer than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion?including masses, star formation efficiencies, fraction of embedded sources, etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC 1333, and IC 348) are found to have higher star formation efficiencies than small groups such as B1, L1455, and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.

Molecular Star Formation Rate Indicators in Galaxies

We derive a physical model for the observed relations between star formation rate (SFR) and molecular line (CO and HCN) emission in galaxies and show how these observed relations are reflective of the underlying star formation law. We do this by combining 3D non-LTE radiative transfer calculations with hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We demonstrate that the observed SFR-molecular line relations are driven by the relationship between molecular line emission and gas density and anchored by the index of the underlying Schmidt law controlling the SFR in the galaxy. Lines with low critical densities (e.g., CO -->J = 1–0) are typically thermalized and trace the gas density faithfully. In these cases, the SFR will be related to line luminosity with an index similar to the Schmidt law index. Lines with high critical densities greater than the mean density of most of the emitting clouds in a galaxy (e.g., CO -->J = 3–2, HCN -->J = 1–0) will have only a small amount of thermalized gas and consequently a superlinear relationship between molecular line luminosity ( -->Lmol) and mean gas density (

Chemistry and Dynamics in Pre-protostellar Cores

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TRACING THE MASS DURING LOW-MASS STAR FORMATION. IV. OBSERVATIONS AND MODELING OF THE SUBMILLIMETER CONTINUUM EMISSION FROM CLASS I PROTOSTARS

-->). This results in an SFR-line luminosity index less than the Schmidt index for high critical density tracers. One observational consequence of this is a significant redistribution of light from the small pockets of dense, thermalized gas to diffuse gas along the line of sight, and prodigious emission from subthermally excited gas. At the highest star formation rates, the SFR- -->Lmol slope tends to the Schmidt index, regardless of the molecular transition. The fundamental relation is the Kennicutt-Schmidt law, rather than the relation between SFR and molecular line luminosity. Our model for SFR-molecular line relations quantitatively reproduces the slopes of the observed SFR-CO ( -->J = 1–0), CO ( -->J = 3–2), and HCN ( -->J = 1–0) relations when a Schmidt law with index of ~1.5 describes the SFR. We use these results to make imminently testable predictions for the SFR-molecular line relations of unobserved transitions.

ENHANCED DENSE GAS FRACTION IN ULTRALUMINOUS INFRARED GALAXIES

We have compared molecular-line emission to dust continuum emission and modeled molecular lines using Monte Carlo simulations in order to study the depletion of molecules and the ionization fraction in three pre-protostellar cores, L1512, L1544, and L1689B. L1512 is much less dense than L1544 and L1689B, which have similar density structures. L1689B has a different environment from those of L1512 and L1544. We used density and temperature profiles, calculated by modeling dust continuum emission in the submillimeter, for modeling molecular-line profiles. In addition, we have used molecular-line profiles and maps observed in several different molecules toward the three cores. We find a considerable diversity in chemical state among the three cores. The molecules include those sensitive to different timescales of chemical evolution such as CCS, the isotopes of CO and HCO+, DCO+, and N2H+. The CO molecule is significantly depleted in L1512 and L1544 but not in L1689B. CCS may be in the second enhancement of its abundance in L1512 and L1544 because of the significant depletion of CO molecules. N2H+ might already be starting to be depleted in L1512, but it traces very well the distribution of dust emission in L1544. On the other hand, L1689B may be so young that N2H+ has not reached its maximum yet. The ionization fraction has been calculated using H13CO+ and DCO+. The result shows that the ionization fraction is similar toward the centers of the three cores. This study suggests that chemical evolution depends on the absolute timescale during which a core stays in a given environment as well as its density structure.