In this Letter we investigate the shape of the probability distribution of column densities (PDF) in molecular clouds. Through the use of low-noise, extinction-calibrated Herschel/Planck emission ...data for eight molecular clouds, we demonstrate that, contrary to common belief, the PDFs of molecular clouds are not described well by log-normal functions, but are instead power laws with exponents close to two and with breaks between AK ≃ 0.1 and 0.2 mag, so close to the CO self-shielding limit and not far from the transition between molecular and atomic gas. Additionally, we argue that the intrinsic functional form of the PDF cannot be securely determined below AK ≃ 0.1 mag, limiting our ability to investigate more complex models for the shape of the cloud PDF.
We have conducted a 12CO(2-1) survey of several molecular gas complexes in the vicinity of H ii regions within the spiral galaxy NGC 300 using the Atacama Large Millimeter Array (ALMA). Our ...observations attain a resolution of 10 pc and 1 , sufficient to fully resolve giant molecular clouds (GMCs) and the highest obtained to date beyond the Local Group. We use the CPROPS algorithm to identify and characterize 250 GMCs across the observed regions. GMCs in NGC 300 appear qualitatively and quantitatively similar to those in the Milky Way disk: they show an identical scaling relationship between size R and linewidth ΔV (ΔV ∝ R0.48 0.05), appear to be mostly in virial equilibrium, and are consistent with having a constant surface density of about 60 pc−2. The GMC mass spectrum is similar to those in the inner disks of spiral galaxies (including the Milky Way). Our results suggest that global galactic properties such as total stellar mass, morphology, and average metallicity may not play a major role in setting GMC properties, at least within the disks of galaxies on the star-forming main sequence. Instead, GMC properties may be more strongly influenced by local environmental factors such as the midplane disk pressure. In particular, in the inner disk of NGC 300, we find this pressure to be similar to that in the local Milky Way but markedly lower than that in the disk of M51, where GMCs are characterized by systematically higher surface densities and a higher coefficient for the size-linewidth relation.
We present high-resolution, high dynamic range column-density and color-temperature maps of the Orion complex using a combination of Planck dust-emission maps, Herschel dust-emission maps, and 2MASS ...NIR dust-extinction maps. The column-density maps combine the robustness of the 2MASS NIR extinction maps with the resolution and coverage of the Herschel and Planck dust-emission maps and constitute the highest dynamic range column-density maps ever constructed for the entire Orion complex, covering 0.01 mag < A sub(K) < 30 mag, or 2 x 10 super(20) cm-2 < N < 2 x 10 super(23) cm-2. We determined the ratio of the 2.2 mu m extinction coefficient to the 850 mu m opacity and found that the values obtained for both Orion A and B are significantly lower than the predictions of standard dust models, but agree with newer models that incorporate icy silicate-graphite conglomerates for the grain population. We show that the cloud projected probability distribution function, over a large range of column densities, can be well fitted by a simple power law. Moreover, we considered the local Schmidt-law for star formation, and confirm earlier results, showing that the protostar surface density capital sigma * follows a simple law capital sigma * proportional, variant capital sigma sub(gas) super( beta ) with beta ~2.
We use two existing molecular cloud catalogs derived from the same CO survey and two catalogs derived from local dust extinction surveys to investigate the nature of the giant molecular cloud (GMC) ...mass-size relation in the Galaxy. We find that the four surveys are well described by MGMC ∼ R2, implying a constant mean surface density, , for the cataloged clouds. However, the scaling coefficients and scatter differ significantly between the CO- and extinction-derived relations. We find that the additional scatter seen in the CO relations is due to a systematic variation in with Galactic radius that is unobservable in the local extinction data. We decompose this radial variation of into two components, a linear negative gradient with Galactic radius and a broad peak coincident with the molecular ring and superposed on the linear gradient. We show that the former may be due to a radial dependence of XCO on metallicity, while the latter likely results from a combination of increased surface densities of individual GMCs and a systematic upward bias in the measurements of due to cloud blending in the molecular ring. We attribute the difference in scaling coefficients between the CO and extinction data to an underestimate of XCO. We recalibrate the CO observations of nearby GMCs using extinction measurements to find that locally XCO = 3.6 0.3 × 1020 cm−2 (K km s−1)−1. We conclude that outside the molecular ring, the GMC population of the Galaxy can be described to relatively good precision by a constant of 35 pc−2.
On the Star Formation Rates in Molecular Clouds Lada, Charles J; Lombardi, Marco; Alves, João F
Astrophysical journal/The Astrophysical journal,
11/2010, Letnik:
724, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In this paper, we investigate the level of star formation activity within nearby molecular clouds. We employ a uniform set of infrared extinction maps to provide accurate assessments of cloud mass ...and structure and compare these with inventories of young stellar objects within the clouds. We present evidence indicating that both the yield and rate of star formation can vary considerably in local clouds, independent of their mass and size. We find that the surface density structure of such clouds appears to be important in controlling both these factors. In particular, we find that the star formation rate (SFR) in molecular clouds is linearly proportional to the cloud mass (M 0.8) above an extinction threshold of A K 0.8 mag, corresponding to a gas surface density threshold of Delta *Sgas 116 M pc2. We argue that this surface density threshold corresponds to a gas volume density threshold which we estimate to be n(H2) 104 cm--3. Specifically, we find SFR (M yr--1) = 4.6 ? 2.6 X 10--8 M 0.8 (M ) for the clouds in our sample. This relation between the rate of star formation and the amount of dense gas in molecular clouds appears to be in excellent agreement with previous observations of both galactic and extragalactic star-forming activity. It is likely the underlying physical relationship or empirical law that most directly connects star formation activity with interstellar gas over many spatial scales within and between individual galaxies. These results suggest that the key to obtaining a predictive understanding of the SFRs in molecular clouds and galaxies is to understand those physical factors which give rise to the dense components of these clouds.
Abstract
We present a survey of the Perseus molecular cloud in the
J
= 1 → 0 transition of HCN, a widely used tracer of dense molecular gas. The survey was conducted with the CfA 1.2 m telescope, ...which at 89 GHz has a beamwidth of 11′ and a spectral resolution of 0.85 km s
−1
. A total of 8.1 deg
2
was surveyed on a uniform 10′ grid to a sensitivity of 14 mK per channel. We compared the survey with similar surveys of CO and dust in order to study and calibrate the HCN line as a dense-gas tracer. We find the HCN emission to extend over a considerable fraction of the cloud. We show that the HCN intensity remains linear with H
2
column density well into the regime where the CO line saturates. We use radiative-transfer modeling to show that this likely results from subthermal excitation of HCN in a cloud where the column and volume densities of H
2
are positively correlated. To match our HCN observations the model requires an exponential decrease in HCN abundance with increasing extinction, consistent with HCN depletion onto grains. The modeling also reveals that the mean volume density of H
2
in the HCN-emitting regions is ∼10
4
cm
−3
, well below the HCN critical density. For the first time, we obtain a direct measurement of the ratio of dense-gas mass to HCN luminosity for an entire nearby molecular cloud:
α
(HCN) = 92
M
⊙
/(K km s
−1
pc
2
).
In this Letter I compare recent findings suggesting a low binary star fraction for late-type stars with knowledge concerning the forms of the stellar initial and present-day mass functions for masses ...down to the hydrogen-burning limit. This comparison indicates that most stellar systems formed in the Galaxy are likely single and not binary, as has been often asserted. Indeed, in the current epoch two-thirds of all main-sequence stellar systems in the Galactic disk are composed of single stars. Some implications of this realization for understanding the star and planet formation process are briefly mentioned.
3D shape of Orion A from Gaia DR2 Großschedl, Josefa E.; Alves, João; Meingast, Stefan ...
Astronomy & astrophysics,
11/2018, Letnik:
619
Journal Article
Recenzirano
Odprti dostop
We use the Gaia DR2 distances of about 700 mid-infrared selected young stellar objects in the benchmark giant molecular cloud Orion A to infer its 3D shape and orientation. We find that Orion A is ...not the fairly straight filamentary cloud that we see in (2D) projection, but instead a cometary-like cloud oriented toward the Galactic plane, with two distinct components: a denser and enhanced star-forming (bent) Head, and a lower density and star-formation quieter ∼75 pc long Tail. The true extent of Orion A is not the projected ∼40 pc but ∼90 pc, making it by far the largest molecular cloud in the local neighborhood. Its aspect ratio (∼30:1) and high column-density fraction (∼45%) make it similar to large-scale Milky Way filaments (“bones”), despite its distance to the galactic mid-plane being an order of magnitude larger than typically found for these structures.
The physics and modes of star cluster formation: observations Lada, Charles J.
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
02/2010, Letnik:
368, Številka:
1913
Journal Article
Recenzirano
Odprti dostop
Stellar clusters are born in cold and dusty molecular clouds and the youngest clusters are embedded to various degrees in a dusty dark molecular material. Such embedded clusters can be considered ...protocluster systems. The most deeply buried examples are so heavily obscured by dust that they are only visible at infrared wavelengths. These embedded protoclusters constitute the nearest laboratories for a direct astronomical investigation of the physical processes of cluster formation and early evolution. I review the present state of empirical knowledge concerning embedded-cluster systems and discuss the implications for understanding their formation and subsequent evolution to produce bound stellar clusters.
The Molecular Clouds of M31 Lada, Charles J.; Forbrich, Jan; Petitpas, Glen ...
Astrophysical journal/The Astrophysical journal,
05/2024, Letnik:
966, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract Deep interferometric observations of CO and dust continuum emission are obtained with the Submillimeter Array at 230 GHz to investigate the physical nature of the giant molecular cloud (GMC) ...population in the Andromeda galaxy (M31). We use J = 2 − 1 12 CO and 13 CO emission to derive the masses, sizes, and velocity dispersions of 162 spatially resolved GMCs. We perform a detailed study of a subset of 117 GMCs that exhibit simple, single-component line profile shapes. Examining the Larson scaling relations for these GMCs, we find (1) a highly correlated mass–size relation in both 12 CO and 13 CO emission; (2) a weakly correlated 12 CO line width–size (LWS) relation along with a weaker, almost nonexistent, 13 CO LWS relation, suggesting a possible dependence of the LWS relation on spatial scale; and (3) that only 43% of these GMCs are gravitationally bound. We identify two classes of GMCs based on the strength and extent of their 13 CO emission. Examination of the Larson relations finds that both classes are individually characterized by strong 12 CO mass–size relations and much weaker 12 CO and 13 CO LWS relations. The majority (73%) of strong 13 CO-emitting GMCs are found to be gravitationally bound. However, only 25% of the weak 13 CO-emitting GMCs are bound. The resulting breakdown in the Larson relations in the weak 13 CO-emitting population decouples the mass–size and LWS relations, demonstrating that independent physical causes are required to understand the origin of each. Finally, in nearly every aspect, the physical properties of the M31 GMCs are found to be very similar to those of local Milky Way clouds.
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