We present an analysis of spectroscopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures toward the Orion Complex. By applying a hierarchical clustering algorithm to the ...six-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star-forming history within the Orion Complex and identify peculiar subclusters. With this method we reconstruct the older populations in the regions that are currently largely devoid of molecular gas, such as Orion C (which includes the Ori cluster) and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D group is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In λ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were halfway from the cluster center to their current positions. We also present a comparison between the parallax and proper-motion solutions obtained by Gaia DR2 and those obtained toward star-forming regions by the Very Long Baseline Array.
ABSTRACT We analyse the geometry of the Pipe Nebula, drawn by the distribution (Q spatial parameter) and hierarchy (Λ spatial segregation) of column density peaks previously detected and catalogued. ...By analysing the mass and volume density of the cores, we determine that both variables show spatial segregation with a high degree of substructure. In view of the early evolutionary state of the Pipe Nebula, our results suggest that segregation both by mass and by volume density may be primordial, in the sense of appearing early in the chain of physical processes that lead to cluster formation. We propose that volume density, and not mass, is the parameter that most clearly determines the initial spatial distribution of pre-stellar cores.
We present subarcsecond 1.3 mm continuum ALMA observations toward the Orion Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 au, which reveal a total of 31 continuum ...sources. We also present subarcsecond 7 mm continuum VLA observations of the same region, which allow further study of fragmentation down to a spatial resolution of 40 au. By applying a method of "mean surface density of companions" we find a characteristic spatial scale at ∼560 au, and we use this spatial scale to define the boundary of 19 "cores" in OMC-1S as groupings of millimeter sources. We find an additional characteristic spatial scale at ∼2800 au, which is the typical scale of the filaments in OMC-1S, suggesting a two-level fragmentation process. We measured the fragmentation level within each core and find a higher fragmentation toward the southern filament. In addition, the cores of the southern filament are also the densest cores (within 1100 au) in OMC-1S. This is fully consistent with previous studies of fragmentation at spatial scales one order of magnitude larger, and suggests that fragmentation down to 40 au seems to be governed by thermal Jeans processes in OMC-1S.
We present the first high spatial resolution X-ray study of NGC 2244, the 2 Myr old stellar cluster in the Rosette Nebula, using Chandra. Over 900 X-ray sources are detected; 77% have optical or ...FLAMINGOS NIR stellar counterparts and are mostly previously uncataloged young cluster members. The X-ray-selected population is estimated to be nearly complete between 0.5 and 3 M sub(image). A number of further results emerge from our analysis: (1) The X-ray LF and the associated K-band LF indicate a normal Salpeter IMF for NGC 2244. This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of <4 M sub(image) stars. By comparing the NGC 2244 and Orion Nebula Cluster XLFs, we estimate a total population of image2000 stars in NGC 2244. (2) The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster's stellar radial density profile shows two distinctive structures: a power-law cusp around HD 46150 that extends to image0.7 pc, surrounded by an isothermal sphere extending out to 4 pc with core radius 1.2 pc. This double structure, combined with the absence of mass segregation, indicates that this 2 Myr old cluster is not in dynamical equilibrium. (3) The fraction of X-ray-selected cluster members with K-band excesses caused by inner protoplanetary disks is 6%, slightly lower than the 10% disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample. (4) X- ray luminosities for 24 stars earlier than B4 confirm the long-standing image relation. The Rosette OB X-ray spectra are soft and consistent with the standard model of small-scale shocks in the inner wind of a single massive star.
We have recently witnessed the first multi-messenger detection of colliding neutron stars through gravitational waves (GWs) and electromagnetic (EM) waves (GW 170817) thanks to the joint efforts of ...LIGO/Virgo and Space/Ground-based telescopes. In this paper, we report on the RATIR follow-up observation strategies and show the results for the trigger G194575. This trigger is not of astrophysical interest; however, it is of great interest to the robust design of a follow-up engine to explore large sky-error regions. We discuss the development of an image-subtraction pipeline for the six-color, optical/NIR imaging camera RATIR. Considering a two-band (i and r) campaign in the fall of 2015, we find that the requirement of simultaneous detection in both bands leads to a factor ∼10 reduction in false alarm rate, which can be further reduced using additional bands. We also show that the performance of our proposed algorithm is robust to fluctuating observing conditions, maintaining a low false alarm rate with a modest decrease in system efficiency that can be overcome utilizing repeat visits. Expanding our pipeline to search for either optical or NIR detections (three or more bands), considering separately the optical riZ and NIR YJH bands, should result in a false alarm rate 1% and an efficiency 90%. RATIR's simultaneous optical/NIR observations are expected to yield about one candidate transient in the vast 100 deg2 LIGO error region for prioritized follow-up with larger aperture telescopes.
Abstract
We present the core mass function (CMF) of the massive star-forming clump G33.92 + 0.11 using 1.3 mm observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). With ...a resolution of 1000 au, this is one of the highest resolution CMF measurements to date. The CMF is corrected by flux and number incompleteness to obtain a sample that is complete for gas masses
M
≳ 2.0
M
⊙
. The resulting CMF is well represented by a power-law function (
dN
/
d
log
M
∝
M
Γ
), whose slope is determined using two different approaches: (i) by least-squares fitting of power-law functions to the flux- and number-corrected CMF, and (ii) by comparing the observed CMF to simulated samples with similar incompleteness. We provide a prescription to quantify and correct a flattening bias affecting the slope fits in the first approach, which is caused by small-sample or edge effects when the data are represented by either classical histograms or a kernel density estimate, respectively. The resulting slopes from both approaches are in good agreement each other, with
Γ
=
−
1.11
−
0.11
+
0.12
being our adopted value. Although this slope appears to be slightly flatter than the Salpeter slope Γ = −1.35 for the stellar initial mass function (IMF), we find from Monte Carlo simulations that the CMF in G33.92 + 0.11 is statistically indistinguishable from the Salpeter representation of the stellar IMF. Our results are consistent with the idea that the form of the IMF is inherited from the CMF, at least at high masses and when the latter is observed at high enough resolution.
We report new ∼1000 au spatial resolution observations of 225 GHz dust continuum emission toward the OB cluster-forming molecular clump G33.92+0.11. On parsec scales, this molecular clump presents a ...morphology with several arm-like dense gas structures surrounding the two central massive ( 100 M ) cores. From the new higher resolution observations, we identified 28 localized, spatially compact dust continuum emission sources, which may be candidates of young stellar objects (YSOs). Only one of them is not embedded within known arm-like (or elongated) dense gas structures. The spatial separations of these compact sources can be very well explained by Jeans lengths. We found that G33.92+0.11 may be consistently described by a marginally centrifugally supported, Toomre unstable accretion flow that is approximately in a face-on projection. The arm-like overdensities are a natural consequence of the Toomre instability, which can fragment to form YSOs in shorter timescales than the timescale of the global clump contraction. On our resolved spatial scales, there is no evidence yet that the fragmentation is halted by turbulence, the magnetic field, or stellar feedback.
ABSTRACT
We present a comparative study of the physical properties and the spatial distribution of column density peaks in two giant molecular clouds (GMCs), the Pipe Nebula and Orion A, which ...exemplify opposite cases of star cluster formation stages. The density peaks were extracted from dust extinction maps constructed from Herschel/SPIRE far-infrared images. We compare the distribution functions for dust temperature, mass, equivalent radius, and mean volume density of peaks in both clouds, and made a more fair comparison by isolating the less active Tail region in Orion A and by convolving the Pipe Nebula map to simulate placing it at a distance similar to that of the Orion Complex. The peak mass distributions for Orion A, the Tail, and the convolved Pipe have similar ranges, sharing a maximum near 5 M⊙ and a similar power-law drop above 10 M⊙. Despite the clearly distinct evolutive stage of the clouds, there are very important similarities in the physical and spatial distribution properties of the column density peaks, pointing to a scenario where they form as a result of uniform fragmentation of filamentary structures across the various scales of the cloud, with density being the parameter leading the fragmentation, and with clustering being a direct result of thermal fragmentation at different spatial scales. Our work strongly supports the idea that the formation of clusters in GMC could be the result of the primordial organization of pre-stellar material.
We present a study of dense structures in the L 1495 filament in the Taurus Molecular Cloud and examine its star-forming properties. In particular, we construct a dust extinction map of the filament ...using deep near-infrared observations, exposing its small-scale structure in unprecedented detail. The filament shows highly fragmented substructures and a high mass-per-length value of M line = 17 M pc--1, reflecting star-forming potential in all parts of it. However, a part of the filament, namely B 211, is remarkably devoid of young stellar objects. We argue that in this region the initial filament collapse and fragmentation is still taking place and star formation is yet to occur. In the star-forming part of the filament, we identify 39 cores with masses from 0.4 to 10 M and preferred separations in agreement with the local Jeans length. Most of these cores exceed the Bonnor-Ebert critical mass, and are therefore likely to collapse and form stars. The dense core mass function follows a power law with exponent Delta *G = 1.2 ? 0.2, a form commonly observed in star-forming regions.
In this paper, we describe the results obtained in our study of Galactic Wolf-Rayet stars of the WN type, using the APOGEE2 spectrographs. The APOGEE2 WN star near-infrared spectra presented here ...were selected through a careful examination of the APOGEE2 DR16 sample, being compound of at least one exemplar of each Wolf-Rayet star from WN4 to WN11 types. Besides the APOGEE2 WN sample, we also identified three new Galactic exemplars of the WN type. The first is WR 147-1, a narrow-lined Wolf-Rayet star of the WN4-5 type located in the direction of the Cygnus-X North complex. The two remaining objects, WR 48-11 and WR 48-12, are found at projected angular distances of several arcminutes southwest from the Danks 1 cluster. WR 48-12 has an estimated heliocentric distance of kpc, which is not compatible (within the quoted uncertainties) with that of about 3.8 0.6 kpc for the G305 complex. On the other hand, WR 48-11 has a probable heliocentric distance of kpc, which is about three times that of the former, implying that it is probably placed well behind the G305 complex, perhaps at the outermost portion of the Perseus arm.
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