How Many Infrared Dark Clouds can form Massive Stars and Clusters?

We present a new assessment of the ability of Infrared Dark Clouds (IRDCs) to form massive stars and clusters. This is done by comparison with an empirical mass-size threshold for massive star formation (MSF). We establish m(r)>870M_sun(r/pc)^1.33 as a novel approximate MSF limit, based on clouds with and without MSF. Many IRDCs, if not most, fall short of this threshold. Without significant evolution, such clouds are unlikely MSF candidates. This provides a first quantitative assessment of the small number of IRDCs evolving towards MSF. IRDCs below this limit might still form stars and clusters of up to intermediate mass, though (like, e.g., the Ophiuchus and Perseus Molecular Clouds). Nevertheless, a major fraction of the mass contained in IRDCs might reside in few 10^2 clouds sustaining MSF.Comment: accepted to The Astrophysical Journal Letters; not yet including second set of referee comment

Electron Excitation of High Dipole Moment Molecules Reexamined

Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the "dense" gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H$_2$ molecules, $\simeq$10$^5$ for HCN, yields the requirements for electron excitation to be of practical importance if $n({\rm H}_2) \leq\ 10^{5.5} ~ \rm cm^{-3}$and$X({\rm e}^-) \geq\ 10^{-5}$, where the numerical factors reflect critical values$n_{\rm{}c}({\rm H_2})$and$X^*({\rm{}e}^-)$. This indicates that in regions where a large fraction of carbon is ionized,$X({\rm e}^-)$will be large enough to make electron excitation significant. The situation is in general similar for other "high density tracers", including HCO$^+$, CN, and CS. But there are significant differences in the critical electron fractional abundance,$X^*({\rm e}^-)$, defined by the value required for equal effect from collisions with H$_2$and e$^-$. Electron excitation is, for example, unimportant for CO and C$^+$. Electron excitation may be responsible for the surprisingly large spatial extent of the emission from dense gas tracers in some molecular clouds (Pety et al. 2017; Kauffmann, Goldsmith et al. 2017). The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO$^+$ ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.Comment: published in Ap

Low Virial Parameters in Molecular Clouds: Implications for High Mass Star Formation and Magnetic Fields

Whether or not molecular clouds and embedded cloud fragments are stable against collapse is of utmost importance for the study of the star formation process. Only "supercritical" cloud fragments are able to collapse and form stars. The virial parameter, alpha=M_vir/M, which compares the virial to the actual mass, provides one way to gauge stability against collapse. Supercritical cloud fragments are characterized by alpha<2, as indicated by a comprehensive stability analysis considering perturbations in pressure and density gradients. Past research has suggested that virial parameters alpha>2 prevail in clouds. This would suggest that collapse towards star formation is a gradual and relatively slow process, and that magnetic fields are not needed to explain the observed cloud structure. Here, we review a range of very recent observational studies that derive virial parameters <<2 and compile a catalogue of 1325 virial parameter estimates. Low values of alpha are in particular observed for regions of high mass star formation (HMSF). These observations may argue for a more rapid and violent evolution during collapse. This would enable "competitive accretion" in HMSF, constrain some models of "monolithic collapse", and might explain the absence of high--mass starless cores. Alternatively, the data could point at the presence of significant magnetic fields ~1 mG at high gas densities. We examine to what extent the derived observational properties might be biased by observational or theoretical uncertainties. For a wide range of reasonable parameters, our conclusions appear to be robust with respect to such biases.Comment: accepted to Ap

The State and Evolution of Isolated Dense Molecular Cores

This work presents studies of nearby ( The data from the dust emission survey is used to study the physical state and evolution of starless cores, “normal” protostars, and of the recently discovered Very Low Luminosity Objects (VeLLOs). This is the first study probing VeLLO dense core properties homogeneously for a larger sample of sources. Given that this survey covers both starless and protostellar cores, it is well suited to perform comparative studies. The aim is to understand how the mass distribution in dense cores controls the presence or absence of active star formation. As part of this effort I infer conditions that are necessary (but not sufficient) for active star formation to be possible. These can be understood as a consequence of the quasistatic evolution of a dense core, but do not conclusively imply the latter. Most VeLLO cores fulfil these conditions, questioning the notion that some VeLLOs form in cores that are not sufficiently evolved to form stars. I suggest a revision of the criteria used to identify “evolved” cores. Class 0 and class I protostars covered by my survey cannot be uniquely discriminated, suggesting also a revision of criteria used to assign infrared classes. Furthermore, I report the discovery of L1148-IRS, a candidate Very Low Luminosity Object (VeLLO; L If L1148-IRS is a VeLLO, then it is a very interesting one. Its present mass would be substellar, and its immediate envelope has a mass of only about 0.15 M_sun . Thus, L1148-IRS would be the first protostar to definitely have a significantly sub-solar final mass. The collapse of the natal dense core could not be understood in the framework of quasistatically evolving cores. This would make L1148 the first dense core in which non-quasistatic evolution plays a significant role.Die Struktur und Entwicklung isolierter Molekülwolkenkerne Diese Arbeit präsentiert Studien nahestehender ( Die Daten der Staubstrahlungs-Durchmusterung werden genutzt um den physikalischen Zustand und die Entwicklung von sternlosen Kernen, "normalen" Protosternen, als auch von erst kürzlich entdeckten sternartigen Objekten extrem geringer Leuchtkraft (Very Low Luminosity Objects, sog. VeLLOs) zu untersuchen. Damit ist diese Studie die erste welche die Eigenschaften von VeLLO-Kernen in gleicher Weise für eine grössere Anzahl von Objekten ermittelt und vergleicht. Die Durchmusterung überdeckt sternlose und protostellare Kerne, was in idealer Weise Vergleiche zwischen diesen Objekttypen ermöglicht. Das Ziel ist es zu verstehen, wie die Masseverteilung in Kernen die An- oder Abwesenheit aktiver Sternentstehung kontrolliert. Dies führt unter Anderem auf notwendige (aber nicht hinreichende) Bedingungen die für die Entstehung von Sternen erfüllt sein müssen. Diese können als Konsequenz einer quasistatischen Entwicklung von Molekülwolken-Kernen verstanden werden, beweisen diese aber nicht abschliessend. Fast alle VeLLOs erfüllen diese Kriterien, was die Auffassung in Frage stellt dass manche VeLLO-Kerne nicht aureichend entwickelt sind um Sterne zu bilden. Ich lege deshalb eine Überarbeitung jener Kriterien nahe die genutzt werden um "entwickelte" Wolken-Kerne zu finden. Protosterne der Klassen 0 und I welche die Durchmusterung überdeckt können nicht eindeutig unterschieden werden, was weiterhin die Überarbeitung von Kriterien für die Zuordnung von Infrarot-Klassen anregt. Weiterhin berichte ich von der Entdeckung von L1148-IRS, einem Kandidaten für ein Very Low Luminosity Object (VeLLO; L Wenn L1148-IRS ein VeLLO ist, dann ist es ein aussergewöhnlicher Vertreter dieser Klasse. Die gegenwärtige Masse währe substellar, und die das Objekt direkt umschliessende Hülle hätte eine Masse von nur etwa 0.15 M_Sonne. Damit währe L1148-IRS der erste Protostern mit einer Endmasse die definitiv deutlich unter einer Sonnenmasse liegt. Zudem währe der Kollaps des umgebenden Molekülwolken-Kerns nicht im Rahmen aktueller Modelle von quasistatischer Kern-Entwicklung zu verstehen. Damit währe L1148 der erste bekannte Kern in dem nicht-quasistatische Entwicklung eine bedeutende Rolle spielen muss

The Effect of Noise on the Dust Temperature - Spectral Index Correlation

We investigate how uncertainties in flux measurements affect the results from modified blackbody SED fits. We show that an inverse correlation between the dust temperature T and spectral index (beta) naturally arises from least squares fits due to the uncertainties, even for sources with a single T and beta. Fitting SEDs to noisy fluxes solely in the Rayleigh-Jeans regime produces unreliable T and beta estimates. Thus, for long wavelength observations (lambda >~ 200 micron), or for warm sources (T >~ 60 K), it becomes difficult to distinguish sources with different temperatures. We assess the role of noise in recent observational results that indicate an inverse and continuously varying T - beta relation. Though an inverse and continuous T - beta correlation may be a physical property of dust in the ISM, we find that the observed inverse correlation may be primarily due to noise.Comment: 14 pages, including 5 Figures; Accepted for publication in Ap

The Galactic Center Cloud G0.253+0.016: A Massive Dense Cloud with low Star Formation Potential

We present the first interferometric molecular line and dust emission maps for the Galactic Center (GC) cloud G0.253+0.016, observed using CARMA and the SMA. This cloud is very dense, and concentrates a mass exceeding the Orion Molecular Cloud Complex (2 × 10^5 M_☉) into a radius of only 3 pc, but it is essentially starless. G0.253+0.016 therefore violates "star formation laws" presently used to explain trends in galactic and extragalactic star formation by a factor ~45. Our observations show a lack of dense cores of significant mass and density, thus explaining the low star formation activity. Instead, cores with low densities and line widths ≾1 km s^(–1)—probably the narrowest lines reported for the GC region to date—are found. Evolution over several 10^5 yr is needed before more massive cores, and possibly an Arches-like stellar cluster, could form. Given the disruptive dynamics of the GC region, and the potentially unbound nature of G0.253+0.016, it is not clear that this evolution will happen