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Showing 1-8 of about 8 results.
Star-Gas Surface Density Correlations in 12 Nearby Molecular Clouds. I. Data Collection and Star-sampled AnalysisPokhrel, RiwajGutermuth, Robert A.Betti, Sarah K.Offner, Stella S. R.Myers, Philip C.Megeath, S. ThomasSokol, Alyssa D.Ali, BabarAllen, LoriAllen, Thomas S.Dunham, Michael M.Fischer, William J.Henning, ThomasHeyer, MarkHora, Joseph L.Pipher, Judith L.Tobin, John J.Wolk, Scott J.DOI: info:10.3847/1538-4357/ab92a2v. 89660
Pokhrel, Riwaj, Gutermuth, Robert A., Betti, Sarah K., Offner, Stella S. R., Myers, Philip C., Megeath, S. Thomas, Sokol, Alyssa D., Ali, Babar, Allen, Lori, Allen, Thomas S., Dunham, Michael M., Fischer, William J., Henning, Thomas, Heyer, Mark, Hora, Joseph L., Pipher, Judith L., Tobin, John J., and Wolk, Scott J. 2020. "Star-Gas Surface Density Correlations in 12 Nearby Molecular Clouds. I. Data Collection and Star-sampled Analysis." The Astrophysical Journal 896:60. https://doi.org/10.3847/1538-4357/ab92a2
ID: 157803
Type: article
Authors: Pokhrel, Riwaj; Gutermuth, Robert A.; Betti, Sarah K.; Offner, Stella S. R.; Myers, Philip C.; Megeath, S. Thomas; Sokol, Alyssa D.; Ali, Babar; Allen, Lori; Allen, Thomas S.; Dunham, Michael M.; Fischer, William J.; Henning, Thomas; Heyer, Mark; Hora, Joseph L.; Pipher, Judith L.; Tobin, John J.; Wolk, Scott J.
Abstract: We explore the relation between the stellar mass surface density and the mass surface density of molecular hydrogen gas in 12 nearby molecular clouds that are located at <1.5 kpc distance. The sample clouds span an order-of-magnitude range in mass, size, and star formation rates. We use thermal dust emission from Herschel maps to probe the gas surface density and the young stellar objects from the most recent Spitzer Extended Solar Neighborhood Archive catalog to probe the stellar surface density. Using a star-sampled nearest neighbor technique to probe the star-gas surface density correlations at the scale of a few parsecs, we find that the stellar mass surface density varies as a power law of the gas mass surface density, with a power-law index of ∼2 in all the clouds. The consistent power-law index implies that star formation efficiency is directly correlated with gas column density, and no gas column density threshold for star formation is observed. We compare the observed correlations with the predictions from an analytical model of thermal fragmentation and with the synthetic observations of a recent hydrodynamic simulation of a turbulent star-forming molecular cloud. We find that the observed correlations are consistent for some clouds with the thermal fragmentation model and can be reproduced using the hydrodynamic simulations.
The Mass Evolution of Protostellar Disks and Envelopes in the Perseus Molecular CloudAndersen, Bridget C.Stephens, Ian W.Dunham, Michael M.Pokhrel, RiwajJørgensen, Jes K.Frimann, SørenSegura-Cox, DominiqueMyers, Philip C.Bourke, Tyler L.Tobin, John J.Tychoniec, ŁukaszDOI: info:10.3847/1538-4357/ab05c7v. 87354
Andersen, Bridget C., Stephens, Ian W., Dunham, Michael M., Pokhrel, Riwaj, Jørgensen, Jes K., Frimann, Søren, Segura-Cox, Dominique, Myers, Philip C., Bourke, Tyler L., Tobin, John J., and Tychoniec, Łukasz. 2019. "The Mass Evolution of Protostellar Disks and Envelopes in the Perseus Molecular Cloud." The Astrophysical Journal 873:54. https://doi.org/10.3847/1538-4357/ab05c7
ID: 150899
Type: article
Authors: Andersen, Bridget C.; Stephens, Ian W.; Dunham, Michael M.; Pokhrel, Riwaj; Jørgensen, Jes K.; Frimann, Søren; Segura-Cox, Dominique; Myers, Philip C.; Bourke, Tyler L.; Tobin, John J.; Tychoniec, Łukasz
Abstract: In the standard picture for low-mass star formation, a dense molecular cloud undergoes gravitational collapse to form a protostellar system consisting of a new central star, a circumstellar disk, and a surrounding envelope of remaining material. The mass distribution of the system evolves as matter accretes from the large-scale envelope through the disk and onto the protostar. While this general picture is supported by simulations and indirect observational measurements, the specific timescales related to disk growth and envelope dissipation remain poorly constrained. In this paper we conduct a rigorous test of a method introduced by Jørgensen et al. to obtain mass measurements of disks and envelopes around embedded protostars with observations that do not resolve the disk (resolution of ˜1000 au). Using unresolved data from the recent Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey, we derive disk and envelope mass estimates for 59 protostellar systems in the Perseus molecular cloud. We compare our results to independent disk mass measurements from the VLA Nascent Disk and Multiplicity survey and find a strong linear correlation, suggesting that accurate disk masses can be measured from unresolved observations. Then, leveraging the size of the MASSES sample, we find no significant trend in protostellar mass distribution as a function of age, as approximated from bolometric temperatures. These results may indicate that the disk mass of a protostar is set near the onset of the Class 0 protostellar stage and remains roughly constant throughout the Class I protostellar stage.
Catalog of High Protostellar Surface Density Regions in Nearby Embedded ClustersLi, JuanMyers, Philip C.Kirk, HelenGutermuth, Robert A.Dunham, Michael M.Pokhrel, RiwajDOI: info:10.3847/1538-4357/aaf652v. 871163
Li, Juan, Myers, Philip C., Kirk, Helen, Gutermuth, Robert A., Dunham, Michael M., and Pokhrel, Riwaj. 2019. "Catalog of High Protostellar Surface Density Regions in Nearby Embedded Clusters." The Astrophysical Journal 871:163. https://doi.org/10.3847/1538-4357/aaf652
ID: 150506
Type: article
Authors: Li, Juan; Myers, Philip C.; Kirk, Helen; Gutermuth, Robert A.; Dunham, Michael M.; Pokhrel, Riwaj
Abstract: We analyze high-quality stellar catalogs for 24 young and nearby (within 1 kpc) embedded clusters and present a catalog of 32 groups which have a high concentration of protostars. The median effective radius of these groups is 0.17 pc. The median protostellar and pre-main-sequence star surface densities are 46 M pc‑2 and 11 M pc‑2, respectively. We estimate the age of these groups using a model of constant birthrate and random accretion stopping and find a median value of 0.25 Myr. Some groups in Aquila, Serpens, Corona Australia, and Ophichus L1688 show high protostellar surface density and high molecular gas surface density, and seem to be undergoing vigorous star formation. These groups provide an excellent opportunity to study the initial conditions of clustered star formation. Comparisons of protostellar and pre-main-sequence stellar surface densities reveal continuous low-mass star formation of these groups over several Myr in some clouds. For groups with typical protostellar separations of less than 0.4 pc, we find that these separations agree well with the thermal Jeans fragmentation scale. On the other hand, for groups with typical protostellar separations larger than 0.4 pc, these separations are always larger than the associated Jeans length.
Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES)-Full Data ReleaseStephens, Ian W.Bourke, Tyler L.Dunham, Michael M.Myers, Philip C.Pokhrel, RiwajTobin, John J.Arce, Héctor G.Sadavoy, Sarah I.Vorobyov, Eduard I.Pineda, Jaime E.Offner, Stella S. R.Lee, Katherine I.Kristensen, Lars E.Jørgensen, Jes K.Gurwell, Mark A.Goodman, Alyssa A.DOI: info:10.3847/1538-4365/ab5181v. 24521
Stephens, Ian W., Bourke, Tyler L., Dunham, Michael M., Myers, Philip C., Pokhrel, Riwaj, Tobin, John J., Arce, Héctor G., Sadavoy, Sarah I., Vorobyov, Eduard I., Pineda, Jaime E., Offner, Stella S. R., Lee, Katherine I., Kristensen, Lars E., Jørgensen, Jes K., Gurwell, Mark A., and Goodman, Alyssa A. 2019. "Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES)-Full Data Release." The Astrophysical Journal Supplement Series 245:21. https://doi.org/10.3847/1538-4365/ab5181
ID: 154516
Type: article
Authors: Stephens, Ian W.; Bourke, Tyler L.; Dunham, Michael M.; Myers, Philip C.; Pokhrel, Riwaj; Tobin, John J.; Arce, Héctor G.; Sadavoy, Sarah I.; Vorobyov, Eduard I.; Pineda, Jaime E.; Offner, Stella S. R.; Lee, Katherine I.; Kristensen, Lars E.; Jørgensen, Jes K.; Gurwell, Mark A.; Goodman, Alyssa A.
Abstract: We present and release the full data set for the Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey. This survey used the Submillimeter Array (SMA) to image the 74 known protostars within the Perseus molecular cloud. The SMA was used in two array configurations to capture outflows for scales >30″ (>9000 au) and to probe scales down to ∼1″ (∼300 au). The protostars were observed with the 1.3 mm and 850 μm receivers simultaneously to detect continuum at both wavelengths and molecular line emission from CO(2─1), 13CO(2─1), C18O(2─1), N2D+(3─2), CO(3─2), HCO+(4─3), and H13CO+(4─3). Some of the observations also used the SMA's recently upgraded correlator, SWARM, whose broader bandwidth allowed for several more spectral lines to be observed (e.g., SO, H2CO, DCO+, DCN, CS, CN). Of the main continuum and spectral tracers observed, 84% of the images and cubes had emission detected. The median C18O(2─1) line width is ∼1.0 km s−1, which is slightly higher than those measured with single-dish telescopes at scales of 3000─20,000 au. Of the 74 targets, six are suggested to be first hydrostatic core candidates, and we suggest that L1451-mm is the best candidate. We question a previous continuum detection toward L1448 IRS2E. In the SVS 13 system, SVS 13A certainly appears to be the most evolved source, while SVS 13C appears to be hotter and more evolved than SVS 13B. The MASSES survey is the largest publicly available interferometric continuum and spectral line protostellar survey to date, and is largely unbiased as it only targets protostars in Perseus. All visibility (uv) data and imaged data are publicly available at https://dataverse.harvard.edu/dataverse/full_MASSES/.
Hierarchical Fragmentation in the Perseus Molecular Cloud: From the Cloud Scale to Protostellar ObjectsPokhrel, RiwajMyers, Philip C.Dunham, Michael M.Stephens, Ian W.Sadavoy, Sarah I.Zhang, QizhouBourke, Tyler L.Tobin, John J.Lee, Katherine I.Gutermuth, Robert A.Offner, Stella S. R.DOI: info:10.3847/1538-4357/aaa240v. 8535
Pokhrel, Riwaj, Myers, Philip C., Dunham, Michael M., Stephens, Ian W., Sadavoy, Sarah I., Zhang, Qizhou, Bourke, Tyler L., Tobin, John J., Lee, Katherine I., Gutermuth, Robert A., and Offner, Stella S. R. 2018. "Hierarchical Fragmentation in the Perseus Molecular Cloud: From the Cloud Scale to Protostellar Objects." The Astrophysical Journal 853:5. https://doi.org/10.3847/1538-4357/aaa240
ID: 145756
Type: article
Authors: Pokhrel, Riwaj; Myers, Philip C.; Dunham, Michael M.; Stephens, Ian W.; Sadavoy, Sarah I.; Zhang, Qizhou; Bourke, Tyler L.; Tobin, John J.; Lee, Katherine I.; Gutermuth, Robert A.; Offner, Stella S. R.
Abstract: We present a study of hierarchical structure in the Perseus molecular cloud, from the scale of the entire cloud (&gsim; 10 pc) to smaller clumps (~1 pc), cores (~0.05--0.1 pc), envelopes (~300--3000 au), and protostellar objects (~15 au). We use new observations from the Submillimeter Array (SMA) large project ``Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES)'' to probe the envelopes, and recent single-dish and interferometric observations from the literature for the remaining scales. This is the first study to analyze hierarchical structure over five scales in the same cloud complex. We compare the number of fragments with the number of Jeans masses in each scale to calculate the Jeans efficiency, or the ratio of observed to expected number of fragments. The velocity dispersion is assumed to arise either from purely thermal motions or from combined thermal and non-thermal motions inferred from observed spectral line widths. For each scale, thermal Jeans fragmentation predicts more fragments than observed, corresponding to inefficient thermal Jeans fragmentation. For the smallest scale, thermal plus non-thermal Jeans fragmentation also predicts too many protostellar objects. However, at each of the larger scales thermal plus non-thermal Jeans fragmentation predicts fewer than one fragment, corresponding to no fragmentation into envelopes, cores, and clumps. Over all scales, the results are inconsistent with complete Jeans fragmentation based on either thermal or thermal plus non-thermal motions. They are more nearly consistent with inefficient thermal Jeans fragmentation, where the thermal Jeans efficiency increases from the largest to the smallest scale.
Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES)---1.3 mm Subcompact Data ReleaseStephens, Ian W.Dunham, Michael M.Myers, Philip C.Pokhrel, RiwajBourke, Tyler L.Vorobyov, Eduard I.Tobin, John J.Sadavoy, Sarah I.Pineda, Jaime E.Offner, Stella S. R.Lee, Katherine I.Kristensen, Lars E.Jørgensen, Jes K.Goodman, Alyssa A.Arce, Héctor G.Gurwell, MarkDOI: info:10.3847/1538-4365/aacda9v. 23722
Stephens, Ian W., Dunham, Michael M., Myers, Philip C., Pokhrel, Riwaj, Bourke, Tyler L., Vorobyov, Eduard I., Tobin, John J., Sadavoy, Sarah I., Pineda, Jaime E., Offner, Stella S. R., Lee, Katherine I., Kristensen, Lars E., Jørgensen, Jes K., Goodman, Alyssa A., Arce, Héctor G., and Gurwell, Mark. 2018. "Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES)---1.3 mm Subcompact Data Release." The Astrophysical Journal Supplement Series 237:22. https://doi.org/10.3847/1538-4365/aacda9
ID: 148980
Type: article
Authors: Stephens, Ian W.; Dunham, Michael M.; Myers, Philip C.; Pokhrel, Riwaj; Bourke, Tyler L.; Vorobyov, Eduard I.; Tobin, John J.; Sadavoy, Sarah I.; Pineda, Jaime E.; Offner, Stella S. R.; Lee, Katherine I.; Kristensen, Lars E.; Jørgensen, Jes K.; Goodman, Alyssa A.; Arce, Héctor G.; Gurwell, Mark
Abstract: We present the Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey, which uses the Submillimeter Array (SMA) interferometer to map the continuum and molecular lines for all 74 known Class 0/I protostellar systems in the Perseus molecular cloud. The primary goal of the survey is to observe an unbiased sample of young protostars in a single molecular cloud so that we can characterize the evolution of protostars. This paper releases the MASSES 1.3 mm data from the subcompact configuration (&tilde;4" or &tilde;1000 au resolution), which is the SMA's most compact array configuration. We release both uv visibility data and imaged data for the spectral lines CO(2-1), 13CO(2-1), C18O(2-1), and N2D+(3-2), as well as for the 1.3 mm continuum. We identify the tracers that are detected toward each source. We also show example images of continuum and CO(2-1) outflows, analyze C18O(2-1) spectra, and present data from the SVS 13 star-forming region. The calculated envelope masses from the continuum show a decreasing trend with bolometric temperature (a proxy for age). Typical C18O(2-1) line widths are 1.45 km s-1, which is higher than the C18O line widths detected toward Perseus filaments and cores. We find that N2D+(3-2) is significantly more likely to be detected toward younger protostars. We show that the protostars in SVS 13 are contained within filamentary structures as traced by C18O(2-1) and N2D+(3-2). We also present the locations of SVS 13A's high-velocity (absolute line-of-sight velocities >150 km s-1) red and blue outflow components. Data can be downloaded from https://dataverse.harvard.edu/dataverse/MASSES.
ALMA Observations of Dust Polarization and Molecular Line Emission from the Class 0 Protostellar Source Serpens SMM1Hull, Charles L. H.Girart, Josep M.Tychoniec, LukaszRao, RamprasadCortés, Paulo C.Pokhrel, RiwajZhang, QizhouHoude, MartinDunham, Michael M.Kristensen, Lars E.Lai, Shih-PingLi, Zhi-YunPlambeck, Richard L.DOI: info:10.3847/1538-4357/aa7fe9v. 84792
Hull, Charles L. H., Girart, Josep M., Tychoniec, Lukasz, Rao, Ramprasad, Cortés, Paulo C., Pokhrel, Riwaj, Zhang, Qizhou, Houde, Martin, Dunham, Michael M., Kristensen, Lars E., Lai, Shih-Ping, Li, Zhi-Yun, and Plambeck, Richard L. 2017. "ALMA Observations of Dust Polarization and Molecular Line Emission from the Class 0 Protostellar Source Serpens SMM1." The Astrophysical Journal 847:92. https://doi.org/10.3847/1538-4357/aa7fe9
ID: 144776
Type: article
Authors: Hull, Charles L. H.; Girart, Josep M.; Tychoniec, Lukasz; Rao, Ramprasad; Cortés, Paulo C.; Pokhrel, Riwaj; Zhang, Qizhou; Houde, Martin; Dunham, Michael M.; Kristensen, Lars E.; Lai, Shih-Ping; Li, Zhi-Yun; Plambeck, Richard L.
Abstract: We present high angular resolution dust polarization and molecular line observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the Class 0 protostar Serpens SMM1. By complementing these observations with new polarization observations from the Submillimeter Array (SMA) and archival data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the James Clerk Maxwell Telescopes (JCMT), we can compare the magnetic field orientations at different spatial scales. We find major changes in the magnetic field orientation between large (~0.1 pc) scales-where the magnetic field is oriented E-W, perpendicular to the major axis of the dusty filament where SMM1 is embedded-and the intermediate and small scales probed by CARMA (~1000 au resolution), the SMA (~350 au resolution), and ALMA (~140 au resolution). The ALMA maps reveal that the redshifted lobe of the bipolar outflow is shaping the magnetic field in SMM1 on the southeast side of the source; however, on the northwestern side and elsewhere in the source, low-velocity shocks may be causing the observed chaotic magnetic field pattern. High-spatial-resolution continuum and spectral-line observations also reveal a tight (~130 au) protobinary system in SMM1-b, the eastern component of which is launching an extremely high-velocity, one-sided jet visible in both {CO}(J=2\to 1) and {SiO}(J=5\to 4); however, that jet does not appear to be shaping the magnetic field. These observations show that with the sensitivity and resolution of ALMA, we can now begin to understand the role that feedback (e.g., from protostellar outflows) plays in shaping the magnetic field in very young, star-forming sources like SMM1.
Alignment between Protostellar Outflows and Filamentary StructureStephens, Ian W.Dunham, Michael M.Myers, Philip C.Pokhrel, RiwajSadavoy, Sarah I.Vorobyov, Eduard I.Tobin, John J.Pineda, Jaime E.Offner, Stella S. R.Lee, Katherine I.Kristensen, Lars E.Jørgensen, Jes K.Goodman, Alyssa A.Bourke, Tyler L.Arce, Héctor G.Plunkett, Adele L.DOI: info:10.3847/1538-4357/aa8262v. 84616
Stephens, Ian W., Dunham, Michael M., Myers, Philip C., Pokhrel, Riwaj, Sadavoy, Sarah I., Vorobyov, Eduard I., Tobin, John J., Pineda, Jaime E., Offner, Stella S. R., Lee, Katherine I., Kristensen, Lars E., Jørgensen, Jes K., Goodman, Alyssa A., Bourke, Tyler L., Arce, Héctor G., and Plunkett, Adele L. 2017. "Alignment between Protostellar Outflows and Filamentary Structure." The Astrophysical Journal 846:16. https://doi.org/10.3847/1538-4357/aa8262
ID: 144730
Type: article
Authors: Stephens, Ian W.; Dunham, Michael M.; Myers, Philip C.; Pokhrel, Riwaj; Sadavoy, Sarah I.; Vorobyov, Eduard I.; Tobin, John J.; Pineda, Jaime E.; Offner, Stella S. R.; Lee, Katherine I.; Kristensen, Lars E.; Jørgensen, Jes K.; Goodman, Alyssa A.; Bourke, Tyler L.; Arce, Héctor G.; Plunkett, Adele L.
Abstract: We present new Submillimeter Array (SMA) observations of CO(2-1) outflows toward young, embedded protostars in the Perseus molecular cloud as part of the Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey. For 57 Perseus protostars, we characterize the orientation of the outflow angles and compare them with the orientation of the local filaments as derived from Herschel observations. We find that the relative angles between outflows and filaments are inconsistent with purely parallel or purely perpendicular distributions. Instead, the observed distribution of outflow-filament angles are more consistent with either randomly aligned angles or a mix of projected parallel and perpendicular angles. A mix of parallel and perpendicular angles requires perpendicular alignment to be more common by a factor of ~3. Our results show that the observed distributions probably hold regardless of the protostar's multiplicity, age, or the host core's opacity. These observations indicate that the angular momentum axis of a protostar may be independent of the large-scale structure. We discuss the significance of independent protostellar rotation axes in the general picture of filament-based star formation.