Improved Measurements of Molecular Cloud Distances Based on Global Search

Abstract The principle of the background-eliminated extinction-parallax (BEEP) method is examining the extinction difference between on- and off-cloud regions to reveal the extinction jump caused by molecular clouds, thereby revealing the distance in complex dust environments. The BEEP method requires high-quality images of molecular clouds and high-precision stellar parallaxes and extinction data, which can be provided by the Milky Way Imaging Scroll Painting (MWISP) CO survey and the Gaia DR2 catalog, as well as supplementary A V extinction data. In this work, the BEEP method is further improved (BEEP-II) to measure molecular cloud distances in a global search manner. Applying the BEEP-II method to three regions mapped by the MWISP CO survey, we collectively measured 238 distances for 234 molecular clouds. Compared with previous BEEP results, the BEEP-II method measures distances efficiently, particularly for those molecular clouds with large angular size or in complicated environments, making it suitable for distance measurements of molecular clouds in large samples.

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A CS survey of massive stars embedded in molecular clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900239351 ◽

1991 ◽

Vol 147 ◽

pp. 25-28

Author(s):

L. Bronfman ◽

J. May ◽

L. A. Nyman ◽

P. Thaddeus

Keyword(s):

Radial Distribution ◽

Molecular Cloud ◽

Molecular Clouds ◽

Milky Way ◽

Massive Stars ◽

Molecular Line ◽

Stellar Objects ◽

Thin Line

the CS J=2 →1 molecular line at 98 GHz, a normally optically thin line requiring high densities to be excited, has been detected with SEST (Swedish ESO Submillimeter Telescope) toward 294 IRAS pointlike sources having the characteristic FIR colors of embedded stellar objects and apparently associated with the largest molecular cloud complexes in the southern Milky Way. We present here their Galactocentric radial distribution and a correlation between their FIR and CS luminosities.

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The Supershell-Molecular Cloud Connection in the Milky Way and Beyond

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313000525 ◽

2012 ◽

Vol 8 (S292) ◽

pp. 83-86

Author(s):

J. R. Dawson ◽

N. M. McClure-Griffiths ◽

Y. Fukui ◽

J. Dickey ◽

T. Wong ◽

...

Keyword(s):

Molecular Cloud ◽

Molecular Clouds ◽

Large Scale ◽

Milky Way ◽

Observational Evidence ◽

Cloud Formation ◽

Stellar Feedback ◽

The Relationship ◽

Global Contribution

AbstractThe role of large-scale stellar feedback in the formation of molecular clouds has been investigated observationally by examining the relationship between Hi and 12CO(J = 1−0) in supershells. Detailed parsec-resolution case studies of two Milky Way supershells demonstrate an enhanced level of molecularisation over both objects, and hence provide the first quantitative observational evidence of increased molecular cloud production in volumes of space affected by supershell activity. Recent results on supergiant shells in the LMC suggest that while they do indeed help to organise the ISM into over-dense structures, their global contribution to molecular cloud formation is of the order of only ∼ 10%.

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Interstellar Objects Follow the Collapse of Molecular Clouds

The Astrophysical Journal ◽

10.3847/1538-4357/ac0c10 ◽

2021 ◽

Vol 921 (2) ◽

pp. 168

Author(s):

Susanne Pfalzner ◽

Dylan Paterson ◽

Michele T. Bannister ◽

Simon Portegies Zwart

Keyword(s):

Interstellar Medium ◽

Molecular Cloud ◽

Gas Dynamics ◽

Molecular Clouds ◽

Milky Way ◽

Solar Neighborhood ◽

Observational Constraints ◽

Parent Population ◽

Test Particles ◽

Relevant Component

Abstract Interstellar objects (ISOs), the parent population of 1i/‘Oumuamua and 2i/Borisov, are abundant in the interstellar medium of the Milky Way. This means that the interstellar medium, including molecular-cloud regions, has three components: gas, dust, and ISOs. From observational constraints of the field density of ISOs drifting in the solar neighborhood, we infer that a typical molecular cloud of 10 pc diameter contains some 1018 ISOs. At typical sizes ranging from hundreds of meters to tens of kilometers, ISOs are entirely decoupled from the gas dynamics in these molecular clouds. Here we address the question of whether ISOs can follow the collapse of molecular clouds. We perform low-resolution simulations of the collapse of molecular clouds containing initially static ISO populations toward the point where stars form. In this proof-of-principle study, we find that the interstellar objects definitely follow the collapse of the gas—and many become bound to the new-forming numerical approximations to future stars (sinks). At minimum, 40% of all sinks have one or more ISO test particles gravitationally bound to them for the initial ISO distributions tested here. This value corresponds to at least 1010 actual ISOs being bound after three initial freefall times. Thus, ISOs are a relevant component of star formation. We find that more massive sinks bind disproportionately large fractions of the initial ISO population, implying competitive capture of ISOs. Sinks can also be solitary, as their ISOs can become unbound again—particularly if sinks are ejected from the system. Emerging planetary systems will thus develop in remarkably varied environments, ranging from solitary to richly populated with bound ISOs.

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Fermi-LAT Detection of Extended Gamma-Ray Emission in the Vicinity of SNR G045.7-00.4: Evidence of Escaping Cosmic Rays Interacting with the Surrounding Molecular Clouds

The Astrophysical Journal ◽

10.3847/1538-4357/ac36c6 ◽

2021 ◽

Vol 923 (1) ◽

pp. 106

Author(s):

Hai-Ming Zhang ◽

Ruo-Yu Liu ◽

Yang Su ◽

Hui Zhu ◽

Shao-Qiang Xi ◽

...

Keyword(s):

Cosmic Rays ◽

Molecular Cloud ◽

Molecular Clouds ◽

Supernova Remnant ◽

Milky Way ◽

Gamma Ray ◽

Molecular Gas ◽

Western Boundary ◽

Large Area ◽

Gamma Ray Emission

Abstract We present an analysis of Fermi Large Area Telescope data of the gamma-ray emission in the vicinity of a radio supernova remnant (SNR), G045.7-00.4. To study the origin of the gamma-ray emission, we also make use of the CO survey data of Milky Way Imaging Scroll Painting to study the massive molecular gas complex that surrounds the SNR. The whole size of the gigaelectronvolt emission is significantly larger than that of the radio morphology. Above 3 GeV, the gigaelectronvolt emission is resolved into two sources: one is spatially consistent with the position of the SNR with a size comparable to that of the radio emission, and the other is located outside of the western boundary of the SNR and spatially coincident with the densest region of the surrounding molecular cloud. We suggest that the gigaelectronvolt emission of the western source may arise from cosmic rays (CRs) that have escaped the SNR and illuminated the surrounding molecular cloud. We find that the gamma-ray spectra of the western source can be consistently explained by this scenario with a total energy of ∼1050 erg in escaping CRs assuming the escape is isotropic.

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A CS survey of massive stars embedded in molecular clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900198730 ◽

1991 ◽

Vol 147 ◽

pp. 25-28

Author(s):

L. Bronfman ◽

J. May ◽

L. A. Nyman ◽

P. Thaddeus

Keyword(s):

Radial Distribution ◽

Molecular Cloud ◽

Molecular Clouds ◽

Milky Way ◽

Massive Stars ◽

Molecular Line ◽

Stellar Objects ◽

Thin Line

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Distances to molecular clouds at high galactic latitudes based on Gaia DR2

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834337 ◽

2019 ◽

Vol 624 ◽

pp. A6 ◽

Cited By ~ 14

Author(s):

Qing-Zeng Yan ◽

Bo Zhang ◽

Ye Xu ◽

Sufen Guo ◽

Jean-Pierre Macquart ◽

...

Keyword(s):

Systematic Error ◽

Molecular Cloud ◽

Molecular Clouds ◽

Nearby Star ◽

Bayesian Analyses ◽

Star Forming ◽

Star Forming Regions ◽

Data Release ◽

First Time ◽

Gaia Dr2

We report the distances of molecular clouds at high Galactic latitudes (|b| > 10°) derived from parallax and G-band extinction (AG) measurements in the second Gaia data release, Gaia DR2. Aided by Bayesian analyses, we determined distances by identifying the breakpoint in the extinction AG toward molecular clouds and using the extinction AG of Gaia stars around molecular clouds to confirm the breakpoint. We used nearby star-forming regions, such as Orion, Taurus, Cepheus, and Perseus, whose distances are well known to examine the reliability of our method. By comparing with previous results, we found that the molecular cloud distances derived from this method are reliable. The systematic error in the distances is approximately 5%. In total, 52 molecular clouds have well-determined distances, most of which are at high Galactic latitudes, and we provide reliable distances for 13 molecular clouds for the first time.

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The Ballistic Particle Model

Symposium - International Astronomical Union ◽

10.1017/s0074180900032629 ◽

1983 ◽

Vol 100 ◽

pp. 133-134

Author(s):

Frank N. Bash

Keyword(s):

Radial Velocity ◽

Gravitational Potential ◽

Molecular Clouds ◽

Milky Way ◽

Terminal Velocity ◽

Particle Model ◽

The Sun ◽

Giant Molecular Clouds ◽

Spiral Pattern ◽

Armed Spiral

Bash and Peters (1976) suggested that giant molecular clouds (GMC's) can be viewed as ballistic particles launched from the two-armed spiral-shock (TASS) wave with orbits influenced only by the overall galactic gravitational potential perturbed by the spiral gravitational potential in the arms. For GMC's in the Milky Way, the model predicts that the radial velocity observed from the Sun increases with age (time since launch). We showed that the terminal velocity of CO observed from l ≃ 30° to l ≃ 60° can be understood if all GMC's are born in the spiral pattern given by Yuan (1969) and live 30 × 106 yrs. Older GMC's were predicted to have radial velocities which exceed observed terminal velocities.

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A Gaia DR2 view of the open cluster population in the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833476 ◽

2018 ◽

Vol 618 ◽

pp. A93 ◽

Cited By ~ 180

Author(s):

T. Cantat-Gaudin ◽

C. Jordi ◽

A. Vallenari ◽

A. Bragaglia ◽

L. Balaguer-Núñez ◽

...

Keyword(s):

Stellar Evolution ◽

Milky Way ◽

Galactic Plane ◽

Galactic Disk ◽

Open Cluster ◽

Open Clusters ◽

Number Of Clusters ◽

History Of ◽

Gaia Dr2

Context. Open clusters are convenient probes of the structure and history of the Galactic disk. They are also fundamental to stellar evolution studies. The second Gaia data release contains precise astrometry at the submilliarcsecond level and homogeneous photometry at the mmag level, that can be used to characterise a large number of clusters over the entire sky. Aims. In this study we aim to establish a list of members and derive mean parameters, in particular distances, for as many clusters as possible, making use of Gaia data alone. Methods. We compiled a list of thousands of known or putative clusters from the literature. We then applied an unsupervised membership assignment code, UPMASK, to the Gaia DR2 data contained within the fields of those clusters. Results. We obtained a list of members and cluster parameters for 1229 clusters. As expected, the youngest clusters are seen to be tightly distributed near the Galactic plane and to trace the spiral arms of the Milky Way, while older objects are more uniformly distributed, deviate further from the plane, and tend to be located at larger Galactocentric distances. Thanks to the quality of Gaia DR2 astrometry, the fully homogeneous parameters derived in this study are the most precise to date. Furthermore, we report on the serendipitous discovery of 60 new open clusters in the fields analysed during this study.

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Gaia DR2 proper motions of dwarf galaxies within 420 kpc

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833343 ◽

2018 ◽

Vol 619 ◽

pp. A103 ◽

Cited By ~ 94

Author(s):

T. K. Fritz ◽

G. Battaglia ◽

M. S. Pawlowski ◽

N. Kallivayalil ◽

R. van der Marel ◽

...

Keyword(s):

Milky Way ◽

Dwarf Galaxies ◽

High Mass ◽

Proper Motions ◽

Proper Understanding ◽

Satellite Problem ◽

Cosmological Context ◽

Good Consistency ◽

Gaia Dr2 ◽

Limited Accuracy

A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological context requires knowledge of their 3D velocities and orbits. However, proper motion (PM) measurements have generally been of limited accuracy and are available only for more massive dwarfs. We therefore present a new study of the kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that have been spectroscopically observed in the literature. We derive systemic PMs for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good consistency for the subset with measurements available from other studies. We derive the implied Galactocentric velocities, and calculate orbits in canonical MW halo potentials of low (0.8 × 1012 M⊙) and high mass (1.6 × 1012 M⊙). Comparison of the distributions of orbital apocenters and 3D velocities to the halo virial radius and escape velocity, respectively, suggests that the satellite kinematics are best explained in the high-mass halo. Tuc III, Crater II, and additional candidates have orbital pericenters small enough to imply significant tidal influences. Relevant to the missing satellite problem, the fact that fewer galaxies are observed to be near apocenter than near pericenter implies that there must be a population of distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital poles that do not align with the MW plane of satellites (given reasonable assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to establish whether they align; and 17 satellites align, of which 11 are co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are counter-orbiting. Group infall might have contributed to this, but no definitive association is found for the members of the Crater-Leo group.

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Understanding biases in measurements of molecular cloud kinematics using line emission

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2432 ◽

2020 ◽

Vol 498 (2) ◽

pp. 2440-2455

Author(s):

Yuxuan (宇轩) Yuan (原) ◽

Mark R Krumholz ◽

Blakesley Burkhart

Keyword(s):

Magnetic Field ◽

Molecular Cloud ◽

Molecular Clouds ◽

Velocity Dispersion ◽

Signal To Noise Ratio ◽

Line Emission ◽

Molecular Line ◽

Velocity Dispersions ◽

Cloud Kinematics ◽

Measurement Biases

ABSTRACT Molecular line observations using a variety of tracers are often used to investigate the kinematic structure of molecular clouds. However, measurements of cloud velocity dispersions with different lines, even in the same region, often yield inconsistent results. The reasons for this disagreement are not entirely clear, since molecular line observations are subject to a number of biases. In this paper, we untangle and investigate various factors that drive linewidth measurement biases by constructing synthetic position–position–velocity cubes for a variety of tracers from a suite of self-gravitating magnetohydrodynamic simulations of molecular clouds. We compare linewidths derived from synthetic observations of these data cubes to the true values in the simulations. We find that differences in linewidth as measured by different tracers are driven by a combination of density-dependent excitation, whereby tracers that are sensitive to higher densities sample smaller regions with smaller velocity dispersions, opacity broadening, especially for highly optically thick tracers such as CO, and finite resolution and sensitivity, which suppress the wings of emission lines. We find that, at fixed signal-to-noise ratio, three commonly used tracers, the J = 4 → 3 line of CO, the J = 1 → 0 line of C18O, and the (1,1) inversion transition of NH3, generally offer the best compromise between these competing biases, and produce estimates of the velocity dispersion that reflect the true kinematics of a molecular cloud to an accuracy of $\approx 10{{\ \rm per\ cent}}$ regardless of the cloud magnetic field strengths, evolutionary state, or orientations of the line of sight relative to the magnetic field. Tracers excited primarily in gas denser than that traced by NH3 tend to underestimate the true velocity dispersion by $\approx 20{{\ \rm per\ cent}}$ on average, while low-density tracers that are highly optically thick tend to have biases of comparable size in the opposite direction.

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