Most low-mass protostars form in clusters, in particular high-mass clusters; however, how low-mass stars form in high-mass clusters and what the mass distribution is, are still open questions both in our own Galaxy and elsewhere. To access the popula
tion of forming embedded low-mass protostars observationally, we propose to use molecular outflows as tracers. Because the outflow emission scales with mass, the effective contrast between low-mass protostars and their high-mass cousins is greatly lowered. In particular, maps of methanol emission at 338.4 GHz (J=7_0 - 6_0 A+) in low-mass clusters illustrate that this transition is an excellent probe of the low-mass population. We here present a model of a forming cluster where methanol emission is assigned to every embedded low-mass protostar. The resulting model image of methanol emission is compared to recent ALMA observations toward a high-mass cluster and the similarity is striking: the toy model reproduces observations to better than a factor of two and suggests that approximately 50% of the total flux originates in low-mass outflows. Future fine-tuning of the model will eventually make it a tool for interpreting the embedded low-mass population of distant regions within our own Galaxy and ultimately higher-redshift starburst galaxies, not just for methanol emission but also water and high-J CO.
Context; Our understanding of the star formation process has traditionally been confined to certain mass or luminosity boundaries because most studies focus only on low-, intermediate- or high-mass star-forming regions. As part of the Water In Star-f
orming regions with Herschel (WISH) key program, water and other important molecules, such as CO and OH, have been observed in 51 embedded young stellar objects (YSOs). The studied sample covers a range of luminosities from <1 to >10^5 L_sol. Aims; We analyse the CO line emission towards a large sample of protostars in terms of both line intensities and profiles. Methods; Herschel-HIFI spectra of the 12CO 10-9, 13CO 10-9 and C18O 5-4, 9-8 and 10-9 lines are analysed for a sample of 51 YSOs. In addition, JCMT spectra of 12CO 3-2 and C18O 3-2 extend this analysis to cooler gas components. Results; All observed CO and isotopologue spectra show a strong linear correlation between the logarithms of the line and bolometric luminosities across six orders of magnitude on both axes. This suggests that the high-J CO lines primarily trace the amount of dense gas associated with YSOs. This relation can be extended to larger (extragalactic) scales. The majority of the detected 12CO line profiles can be decomposed into a broad and a narrow Gaussian component, while the C18O spectra are mainly fitted with a single Gaussian. A broadening of the line profile is also observed from pre-stellar cores to embedded protostars, which is due mostly to non-thermal motions (turbulence/infall). The widths of the broad 12CO 3-2 and 10-9 velocity components correlate with those of the narrow C18O 9-8 profiles, suggesting that the entrained outflowing gas and envelope motions are related independent of the mass of the protostar. These results indicate that physical processes in protostellar envelopes have similar characteristics across the studied luminosity range.
Observations of higher-excited transitions of abundant molecules such as CO are important for determining where energy in the form of shocks is fed back into the parental envelope of forming stars. The nearby prototypical and protobinary low-mass hot
core, IRAS16293-2422 (I16293) is ideal for such a study. The source was targeted with ALMA for science verification purposes in band 9, which includes CO J=6-5 (E_up/k_B ~ 116 K), at an unprecedented spatial resolution (~0.2, 25 AU). I16293 itself is composed of two sources, A and B, with a projected distance of 5. CO J=6-5 emission is detected throughout the region, particularly in small, arcsecond-sized hotspots, where the outflow interacts with the envelope. The observations only recover a fraction of the emission in the line wings when compared to data from single-dish telescopes, with a higher fraction of emission recovered at higher velocities. The very high angular resolution of these new data reveal that a bow shock from source A coincides, in the plane of the sky, with the position of source B. Source B, on the other hand, does not show current outflow activity. In this region, outflow entrainment takes place over large spatial scales, >~ 100 AU, and in small discrete knots. This unique dataset shows that the combination of a high-temperature tracer (e.g., CO J=6-5) and very high angular resolution observations is crucial for interpreting the structure of the warm inner environment of low-mass protostars.
We present Herschel SPIRE FTS spectroscopy of the nearby luminous infrared galaxy NGC 6240. In total 20 lines are detected, including CO J=4-3 through J=13-12, 6 H2O rotational lines, and [CI] and [NII] fine-structure lines. The CO to continuum lumin
osity ratio is 10 times higher in NGC 6240 than Mrk 231. Although the CO ladders of NGC 6240 and Mrk 231 are very similar, UV and/or X-ray irradiation are unlikely to be responsible for the excitation of the gas in NGC 6240. We applied both C and J shock models to the H2 v=1-0 S(1) and v=2-1 S(1) lines and the CO rotational ladder. The CO ladder is best reproduced by a model with shock velocity v_s=10 km s^-1 and a pre-shock density n_H=5 * 10^4 cm^-3. We find that the solution best fitting the H2 lines is degenerate: The shock velocities and number densities range between v_s = 17 - 47 km s^-1 and n_H=10^7 - 5 * 10^4 cm^-3, respectively. The H2 lines thus need a much more powerful shock than the CO lines. We deduce that most of the gas is currently moderately stirred up by slow (10 km s^-1) shocks while only a small fraction (< 1 percent) of the ISM is exposed to the high velocity shocks. This implies that the gas is rapidly loosing its highly turbulent motions. We argue that a high CO line-to-continuum ratio is a key diagnostic for the presence of shocks.
Aims. Young stars interact vigorously with their surroundings, as evident from the highly rotationally excited CO (up to Eup=4000 K) and H2O emission (up to 600 K) detected by the Herschel Space Observatory in embedded low-mass protostars. Our aim is
to construct a model that reproduces the observations quantitatively, to investigate the origin of the emission, and to use the lines as probes of the various heating mechanisms. Methods. The model consists of a spherical envelope with a bipolar outflow cavity. Three heating mechanisms are considered: passive heating by the protostellar luminosity, UV irradiation of the outflow cavity walls, and C-type shocks along the cavity walls. Line fluxes are calculated for CO and H2O and compared to Herschel data and complementary ground-based data for the protostars NGC1333 IRAS2A, HH 46 and DK Cha. The three sources are selected to span a range of evolutionary phases and physical characteristics. Results. The passively heated gas in the envelope accounts for 3-10% of the CO luminosity summed over all rotational lines up to J=40-39; it is best probed by low-J CO isotopologue lines such as C18O 2-1 and 3-2. The UV-heated gas and the C-type shocks, probed by 12CO 10-9 and higher-J lines, contribute 20-80% each. The model fits show a tentative evolutionary trend: the CO emission is dominated by shocks in the youngest source and by UV-heated gas in the oldest one. This trend is mainly driven by the lower envelope density in more evolved sources. The total H2O line luminosity in all cases is dominated by shocks (>99%). The exact percentages for both species are uncertain by at least a factor of 2 due to uncertainties in the gas temperature as function of the incident UV flux. However, on a qualitative level, both UV-heated gas and C-type shocks are needed to reproduce the emission in far-infrared rotational lines of CO and H2O.
OH is an important molecule in the H2O chemistry and the cooling budget of star-forming regions. The goal of the Herschel key program `Water in Star-forming regions with Herschel (WISH) is to study H2O and related species during protostellar evolutio
n. Our aim in this letter is to assess the origin of the OH emission from star-forming regions and constrain the properties of the emitting gas. High-resolution observations of the OH 2Pi1/2 J = 3/2-1/2 triplet at 1837.8 GHz (163.1 micron) towards the high-mass star-forming region W3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel reveal the first hyperfine velocity-resolved OH far-infrared spectrum of a star-forming region. The line profile of the OH emission shows two components: a narrow component (FWHM approx. 4-5 km/s) with partially resolved hyperfine structure resides on top of a broad (FWHM approx. 30 km/s) component. The narrow emission agrees well with results from radiative transfer calculations of a spherical envelope model for W3 IRS 5 with a constant OH abundance of approx. 8e-9. Comparison with H2O yields OH/H2O abundance ratios of around 1e-3 for T > 100 K and around unity for T < 100K, consistent with the current picture of the dense cloud chemistry with freeze-out and photodesorption. The broad component is attributed to outflow emission. An abundance ratio of OH/H2O > 0.028 in the outflow is derived from comparison with results of water line modeling. This ratio can be explained by a fast J-type shock or a slower UV-irradiated C-type shock.
