Circumstellar Structure around Evolved Stars in the Cygnus-X Star Formation Region

We present observations of newly discovered 24 micron circumstellar structures detected with the Multiband Imaging Photometer for Spitzer (MIPS) around three evolved stars in the Cygnus-X star forming region. One of the objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a circular shell of 24 micron emission suggestive of either a limb-brightened shell or disk seen face-on. No diffuse emission was detected around either of these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC) bands. The third object is the luminous blue variable candidate G79.29+0.46. We resolved the previously known inner ring in all four IRAC bands. The 24 micron emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength emission, well beyond what can be attributed to the difference in resolutions between MIPS and IRAC. Additionally, we have discovered an outer ring of 24 micron emission, possibly due to an earlier episode of mass loss. For the two shell stars, we present the results of radiative transfer models, constraining the stellar and dust shell parameters. The shells are composed of amorphous carbon grains, plus polycyclic aromatic hydrocarbons in the case of G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA 4202-22 is unrelated to the Cygnus-X star formation region.Comment: Accepted by A

Filaments, ridges and the origin of high-mass stars and clusters in Cygnus X

Recent Herschel findings on filaments in nearby low-mass star-forming clouds clearly points to a new paradigm to explain the formation of high density gas in turbulent clouds leading to the protostellar collapse. These filaments are the locations for core fragmentation at roughly the local Jeans mass. The formation of massive stars and of rich stellar clusters in this new paradigm is however not yet understood. Massive elongated/filamentary structures, referred as ridges, are massive filaments observed in regions of high-mass stars formation which may host the formation of massive stars. They have large average densities and show large velocity dispersion, and are roughly as cold as their low-mass counterparts. This may indicate that a larger effective Jeans mass in these ridges due to additional turbulent support could explain a core fragmentation extending up to higher stellar masses. The level of turbulent support in ridges is however difficult to measure due a high level of dynamics (flows, rotation, infall) which may not represent well the level of true support (isotropic) for Jeans fragmentation. More generally the structure and properties of ridges/massive filaments is not well known and requires dedicated studies.I will present our most recent results obtained with Herschel and the IRAM 30m towards the DR21 ridge in Cygnus X. Several massive protostars are actually observed in the DR21 ridge confirming it is the birth place of massive stars. I will show that the whole large scale region is compatible with a global collapse of a 15 pc cloud of several 10s of thousands of solar masses. The most recent IRAM 30m observations show that the ridge is made of several sub-filaments which are all more massive than their counterparts in low-mass star forming regions. I will discuss the implications of these results in the context of the origin of massive stars

High-Mass Star and Massive Cluster Formation in the Milky Way

International audienceThis review examines the state-of-the-art knowledge of high-mass star and massive cluster formation, gained from ambitious observational surveys, which acknowledge the multi-scale characteristics of these processes. After a brief overview of theoretical models and main open issues, we present observational searches for the evolutionary phases of high-mass star formation, first among high-luminosity sources and more recently among young massive protostars and the elusive high-mass prestellar cores. We then introduce the most likely evolutionary scenario for high-mass star formation, which emphasizes the link of high-mass star formation to massive cloud and cluster formation. Finally, we introduce the first attempts to search for variations of the star formation activity and cluster formation in molecular cloud complexes, in the most extreme star-forming sites, and across the Milky Way. The combination of Galactic plane surveys and high-angular resolution images with submillimeter facilities such as Atacama Large Millimeter Array (ALMA) are prerequisites to make significant progresses in the forthcoming decade

High mass star and cluster formation: new insights from Herschel

The HOBYS key program essentially imaged with Herschel all of the regions forming OB-type stars at distances less than 3 kpc from the Sun. This survey allows to statistically study the formation of 10-20 Msun stars and confirms that high-mass pre-stellar cores do not exist. HOBYS also reveals mini-starburst cloud ridges, which have high mean densities and are the privileged sites for the formation of high-mass stars

HOBYS and W43, two more steps towards a Galaxy-wide understanding of star formation

The HOBYS key program allows to statistically study the formation of 10-20 Msun stars in molecular complexes from the first Galactic arm toward us. HOBYS therefore starts to measure the star formation rate of the Milky Way on 1-100 pc scales. The much more extreme W43 molecular complex is located at the tip of the long bar of the Milky Way and forms stars up to 50 Msun. It displays an intense star formation activity, which may originate from its dynamical cloud formation. The HOBYS and W43 surveys are necessary steps towards Galaxy-wide studies of high-mass star formation

Multiplicity and clustering in Taurus star forming region: II. From ultra-wide pairs to dense NESTs

International audienceContext. Multiplicity and clustering of young pre-main sequence stars appear as critical clues to understand and constrain the star formation process. Taurus is the archetypical example of the most quiescent star forming regions that may still retain primeval signatures of star formation.Aims. This work identifies local overdense stellar structures as a critical scale between wide pairs and loose groups in Taurus.Methods. Using the density-based spatial clustering of applications with noise (dbscan) algorithm, and setting its free parameters based on the one-point correlation function and the k-nearest neighbor statistics, we have extracted reliably overdense structures from the sky-projected spatial distribution of stars.Results. Nearly half of the entire stellar population in Taurus is found to be concentrated in 20 very dense, tiny and prolate regions called NESTs (for Nested Elementary STructures). They are regularly spaced (≈2 pc) and mainly oriented along the principal gas filaments axes. Each NEST contains between four and 23 stars. Inside NESTs, the surface density of stars may be as high as 2500 pc−2 and the mean value is 340 pc−2. Nearly half (11) of these NESTs contain about 75% of the class 0 and I objects. The balance between Class I, II, and, III fraction within the NESTs suggests that they may be ordered as an evolutionary temporal scheme, some of them getting infertile with time, while other still giving birth to young stars. We have inferred that only 20% of stars in Taurus do not belong to any kind of stellar groups (either multiple system, ultra wide pairs or NESTs). The mass-size relation for stellar NESTs is very close to the Bonnor–Ebert expectation. The range in mass is about the same as that of dense molecular cores. The distribution in size is bimodal peaking at 12.5 and 50 kAU and the distribution of the number of YSOs in NESTs as a function of size exhibits two regimes.Conclusions. We propose that the NESTs in their two size regimes represent the spatial imprints of stellar distribution at birth as they may have emerged within few millions years from their natal cloud either from a single core or from a chain of cores. We have identified them as the preferred sites of star formation in Taurus. These NESTs are the regions of highest stellar density and intermediate spatial scale structures between ultra-wide pairs and loose groups

ALMA-IMF: I. Investigating the origin of stellar masses: Introduction to the Large Program and first results

F. Motte et al.[Aims] Thanks to the high angular resolution, sensitivity, image fidelity, and frequency coverage of ALMA, we aim to improve our understanding of star formation. One of the breakthroughs expected from ALMA, which is the basis of our Cycle 5 ALMA-IMF Large Program, is the question of the origin of the initial mass function (IMF) of stars. Here we present the ALMA-IMF protocluster selection, first results, and scientific prospects.[Methods] ALMA-IMF imaged a total noncontiguous area of ~53 pc2, covering extreme, nearby protoclusters of the Milky Way. We observed 15 massive (2.5 −33 × 103 M⊙), nearby (2−5.5 kpc) protoclusters that were selected to span relevant early protocluster evolutionary stages. Our 1.3 and 3 mm observations provide continuum images that are homogeneously sensitive to point-like cores with masses of ~0.2 M⊙ and ~0.6 M⊙, respectively, with a matched spatial resolution of ~2000 au across the sample at both wavelengths. Moreover, with the broad spectral coverage provided by ALMA, we detect lines that probe the ionized and molecular gas, as well as complex molecules. Taken together, these data probe the protocluster structure, kinematics, chemistry, and feedback over scales from clouds to filaments to cores.[Results] We classify ALMA-IMF protoclusters as Young (six protoclusters), Intermediate (five protoclusters), or Evolved (four proto-clusters) based on the amount of dense gas in the cloud that has potentially been impacted by H II region(s). The ALMA-IMF catalog contains ~700 cores that span a mass range of ~0.15 M⊙ to ~250 M⊙ at a typical size of ~2100 au. We show that this core sample has no significant distance bias and can be used to build core mass functions (CMFs) at similar physical scales. Significant gas motions, which we highlight here in the G353.41 region, are traced down to core scales and can be used to look for inflowing gas streamers and to quantify the impact of the possible associated core mass growth on the shape of the CMF with time. Our first analysis does not reveal any significant evolution of the matter concentration from clouds to cores (i.e., from 1 pc to 0.01 pc scales) or from the youngest to more evolved protoclusters, indicating that cloud dynamical evolution and stellar feedback have for the moment only had a slight effect on the structure of high-density gas in our sample. Furthermore, the first-look analysis of the line richness toward bright cores indicates that the survey encompasses several tens of hot cores, of which we highlight the most massive in the G351.77 cloud. Their homogeneous characterization can be used to constrain the emerging molecular complexity in protostars of high to intermediate masses.[Conclusions] The ALMA-IMF Large Program is uniquely designed to transform our understanding of the IMF origin, taking the effects of cloud characteristics and evolution into account. It will provide the community with an unprecedented database with a high legacy value for protocluster clouds, filaments, cores, hot cores, outflows, inflows, and stellar clusters studies.This paper makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.01355.L, #2013.1.01365.S, and #2015.1.01273.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This project has received funding from the European Research Council (ERC) via the ERC Synergy Grant ECOGAL (grant 855130), from the French Agence Nationale de la Recherche (ANR) through the project COSMHIC (ANR-20-CE31-0009), and the French Programme National de Physique Stellaire and Physique et Chimie du Milieu Interstellaire (PNPS and PCMI) of CNRS/INSU (with INC/INP/IN2P3). S.B. acknowledges support from the French Agence Nationale de la Recherche (ANR) through the project GENESIS (ANR-16-CE92-0035-01). T.C. and M.B. have received financial support from the French State in the framework of the IdEx Università de Bordeaux Investments for the future Program. Y.P., A.L.S., G.B., and B.L. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, for the Project “The Dawn of Organic Chemistry” (DOC), grant agreement No. 741002. F.L. acknowledges the support of the Marie Curie Action of the European Union (project MagiKStar, grant agreement number 841276). A.S. gratefully acknowledges funding support through Fondecyt Regular (project code 1180350) and from the Chilean Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA) Basal grant AFB-170002. R.G.M. acknowledges support from UNAM-PAPIIT project IN104319 and from CONACyT Ciencia de Frontera project ID 86372. Part of this work was performed at the high-performance computers at IRyA-UNAM. We acknowledge the investment over the years from CONACyT and UNAM, as well as the work from the IT staff at this institute. A.G. acknowledges support from the National Science Foundation under grant No. 2008101. G.B. also acknowledge funding from the State Agency for Research (AEI) of the Spanish MCIU through the AYA2017-84390-C2-2-R grant. P.S. and B.W. were supported by a Grant-in-Aid for Scientific Research (KAKENHI Number 18H01259) of the Japan Society for the Promotion of Science (JSPS). P.S. and H.-L.L. gratefully acknowledge the support from the NAOJ Visiting Fellow Program to visit the National Astronomical Observatory of Japan in 2019, February. R.A.G. gratefully acknowledges support from ANID Beca Doctorado Nacional 21200897. T.B. acknowledges the support from S. N. Bose National Centre for Basic Sciences under the Department of Science and Technology, Govt. of India. G.B. also acknowledges funding from the State Agency for Research (AEI) of the Spanish MCIU through the AYA2017-84390-C2-2-R grant. C.B. gratefully acknowledges support from the National Science Foundation under Award No. 1816715. L.B. acknowledges support from ANID BASAL grant AFB-170002. D.W. gratefully acknowledges support from the National Science Foundation under Award No. 1816715.Peer reviewe

COnnecting LOcal to GLObal star formation via MIni-starburst

To connect local to global star formation, we compare the mass, size, line width, and star formation rate (SFR) density of cloud structures ranging from Galactic clouds ($\sim$1--10~pc in size) to galaxies (with $\sim$1--10~kpc sizes).Our focus is on molecular cloud structures, called massive complexes, which have intermediate sizes of $\sim$100~pc and masses larger than 10^6~\msun.We propose using a starburstiness quantity $\zeta$ to quantify the star formation intensity of a molecular cloud complex, or how much its SFR deviates from the star formation relations such as Kennicutt and Lada ones. For molecular cloud complex that has $\zeta>3$ , we call it mini-starburst complex, otherwise normal or main-sequence complex. This $\zeta>3$ threshold leads to a SFR threshold. Most complexes can be classified as ministarburst as having $\zeta>3$. We propose that mini-starburst events represent an enhancement of the star formation activity, following dynamic events such as compression, colliding flows, or agglomeration which compress materials within a cloud complex

Nature of high-mass dense cores in NGC6334/6357

Proposa