Producing images from interferometer data requires accurate modeling of the sources in the field of view, which is typically done using the clean algorithm. Given the large number of degrees of ...freedom in interferometeric images, one constrains the possible model solutions for clean by masking regions that contain emission. Traditionally this process has largely been done by hand. This approach is not possible with today's large data volumes which require automated imaging pipelines. This paper describes an automated masking algorithm that operates within clean called auto-multithresh. This algorithm was developed and validated using a set of ∼1000 Atacama Large Millimeter/submillimeter Array (ALMA) images chosen to span a range of intrinsic morphology and data characteristics. It takes a top-down approach to producing masks: it uses the residual images to identify significant peaks and then expands the mask to include emission associated with these peaks down to lower signal-to-noise noise. The auto-multithresh algorithm has been implemented in CASA and has been used in production as part of the ALMA Imaging Pipeline starting with Cycle 5. It has been shown to be able to mask a wide range of emission ranging from simple point sources to complex extended emission with minimal tuning of the parameters based on the point-spread function of the data. Although the algorithm was developed for ALMA, it is general enough to have been used successfully with data from other interferometers with appropriate parameter tuning. Integrating the algorithm more deeply within the minor cycle could lead to future performance improvements.
We present a method to analyze the spectral energy distributions (SEDs) of young stellar objects (YSOs). Our approach is to fit data with precomputed two-dimensional (2D) radiation transfer models ...spanning a large region of parameter space. This allows us to determine not only a single set of physical parameter values but the entire range of values consistent with the multiwavelength observations of a given source. In this way we hope to avoid any overinterpretation when modeling a set of data. We have constructed spectral energy distributions from optical to submillimeter wavelengths, including new Spitzer IRAC and MIPS photometry, for 30 young and spatially resolved sources in the Taurus-Auriga star-forming region. We demonstrate fitting model SEDs to these sources and find that we correctly identify the evolutionary stage and physical parameters found from previous independent studies, such as disk mass, disk accretion rate, and stellar temperature. We also explore how fluxes at various wavelengths help to constrain physical parameters and show examples of degeneracies that can occur when fitting SEDs. A Web-based version of this tool is available to the community.
We have conducted ALMA CO isotopes and 1.3 mm continuum observations toward filamentary molecular clouds of the N159W-South region in the Large Magellanic Cloud with an angular resolution of ∼0 25 ...(∼0.07 pc). Although the previous lower-resolution (∼1″) ALMA observations revealed that there is a high-mass protostellar object at an intersection of two line-shaped filaments in 13CO with the length scale of ∼10 pc, the spatially resolved observations, in particular, toward the highest column density part traced by the 1.3 mm continuum emission, the N159W-South clump, show complicated hub-filamentary structures. We also discovered that there are multiple protostellar sources with bipolar outflows along the massive filament. The redshifted/blueshifted components of the 13CO emission around the massive filaments/protostars have complementary distributions, which is considered to be possible evidence for a cloud-cloud collision. We propose a new scenario in which the supersonically colliding gas flow triggers the formation of both the massive filament and protostars. This is a modification of the earlier scenario of cloud-cloud collision, by Fukui et al., that postulated the two filamentary clouds occur prior to the high-mass star formation. A recent theoretical study of the shock compression in colliding molecular flows by Inoue et al. demonstrates that the formation of filaments with hub structure is a usual outcome of the collision, lending support for the present scenario. The theory argues that the filaments are formed as dense parts in a shock compressed sheet-like layer, which resembles "an umbrella with pokes."
We present ALMA observations of CO isotopes and 1.3 mm continuum emission toward the N159E-Papillon Nebula in the Large Magellanic Cloud (LMC). The spatial resolution is 0 25-0 28 (0.06-0.07 pc), ...which is a factor of 3 higher than previous ALMA observations in this region. The high resolution allowed us to resolve highly filamentary CO distributions with typical widths of ∼0.1 pc (full width half maximum) and line masses of a few 100 M pc−1. The filaments (more than ten in number) show an outstanding hub-filament structure emanating from the nebular center toward the north. We identified for the first time two massive protostellar outflows of ∼104 yr dynamical age along one of the most massive filaments. The observations also revealed several pillar-like CO features around the Nebula. The H ii region and the pillars have a complementary spatial distribution and the column density of the pillars is an order of magnitude higher than that of the pillars in the Eagle nebula (M16) in the Galaxy, suggesting an early stage of pillar formation with an age younger than ∼105 yr. We suggest that a cloud-cloud collision triggered the formation of the filaments and protostar within the last ∼2 Myr. It is possible that the collision is more recent, as part of the kpc-scale H i flows come from the tidal interaction resulting from the close encounter between the LMC and SMC ∼200 Myr ago as suggested for R136 by Fukui et al.
We present high angular resolution (∼80 mas) ALMA continuum images of the SN 1987A system, together with CO J = 2 1, J = 6 5, and SiO J = 5 4 to J = 7 6 images, which clearly resolve the ejecta (dust ...continuum and molecules) and ring (synchrotron continuum) components. Dust in the ejecta is asymmetric and clumpy, and overall the dust fills the spatial void seen in H images, filling that region with material from heavier elements. The dust clumps generally fill the space where CO J = 6 5 is fainter, tentatively indicating that these dust clumps and CO are locationally and chemically linked. In these regions, carbonaceous dust grains might have formed after dissociation of CO. The dust grains would have cooled by radiation, and subsequent collisions of grains with gas would also cool the gas, suppressing the CO J = 6 5 intensity. The data show a dust peak spatially coincident with the molecular hole seen in previous ALMA CO J = 2 1 and SiO J = 5 4 images. That dust peak, combined with CO and SiO line spectra, suggests that the dust and gas could be at higher temperatures than the surrounding material, though higher density cannot be totally excluded. One of the possibilities is that a compact source provides additional heat at that location. Fits to the far-infrared-millimeter spectral energy distribution give ejecta dust temperatures of 18-23 K. We revise the ejecta dust mass to Mdust = 0.2-0.4 for carbon or silicate grains, or a maximum of <0.7 for a mixture of grain species, using the predicted nucleosynthesis yields as an upper limit.
ABSTRACT We investigate Wolf-Rayet (WR) stars as a source of feedback contributing to the removal of natal material in the early evolution of massive star clusters. Despite previous work suggesting ...that massive star clusters clear out their natal material before the massive stars evolve into the WR phase, WR stars have been detected in several emerging massive star clusters. These detections suggest that the timescale for clusters to emerge can be at least as long as the time required to produce WR stars (a few million years), and could also indicate that WR stars may be providing the tipping point in the combined feedback processes that drive a massive star cluster to emerge. We explore the potential overlap between the emerging phase and the WR phase with an observational survey to search for WR stars in emerging massive star clusters hosting WR stars. We select candidate emerging massive star clusters from known radio continuum sources with thermal emission and obtain optical spectra with the 4 m Mayall Telescope at Kitt Peak National Observatory and the 6.5 m MMT.4 We identify 21 sources with significantly detected WR signatures, which we term "emerging WR clusters." WR features are detected in ∼50% of the radio-selected sample, and thus we find that WR stars are commonly present in currently emerging massive star clusters. The observed extinctions and ages suggest that clusters without WR detections remain embedded for longer periods of time, and may indicate that WR stars can aid, and therefore accelerate, the emergence process.
We present a grid of radiation transfer models of axisymmetric young stellar objects (YSOs), covering a wide range of stellar masses (from 0.1 to 50 M ) and evolutionary stages (from the early ...envelope infall stage to the late disk-only stage). The grid consists of 20,000 YSO models, with spectral energy distributions (SEDs) and polarization spectra computed at 10 viewing angles for each model, resulting in a total of 200,000 SEDs. We have made a careful assessment of the theoretical and observational constraints on the physical conditions of disks and envelopes in YSOs and have attempted to fully span the corresponding regions in parameter space. These models are publicly available on a dedicated Web server. In this paper we summarize the main features of our models, as well as the range of parameters explored. Having a large grid covering reasonable regions of parameter space allows us to shed light on many trends in near- and mid-IR observations of YSOs (such as changes in the spectral indices and colors of their SEDs), linking them with physical parameters (such as disk and infalling envelope parameters). In particular, we examine the dependence of the spectral indices of the model SEDs on envelope accretion rate and disk mass. In addition, we show variations of spectral indices with stellar temperature, disk inner radius, and disk flaring power for a subset of disk-only models. We also examine how changing the wavelength range of data used to calculate spectral indices affects their values. We show sample color-color plots of the entire grid as well as simulated clusters at various distances with typical Spitzer sensitivities. We find that young embedded sources generally occupy a large region of color-color space due to inclination and stellar temperature effects. Disk sources occupy a smaller region of color-color space but overlap substantially with the region occupied by embedded sources, especially in the near- and mid-IR. We identify regions in color-color space where our models indicate that only sources at a given evolutionary stage should lie. We find that, while near-IR (such as JHK) and mid-IR (such as IRAC) fluxes are useful in discriminating between stars and YSOs, and are useful for identifying very young sources, the addition of longer wavelength data such as MIPS 24 km is extremely valuable for determining the evolutionary stage of YSOs.
Abstract As the closest major galaxy merger and home to thousands of super star clusters (SSCs), the Antennae Galaxies (NGC 4038 and NGC 4039) are an important location to study the molecular clouds ...at sites of vigorous star formation. We cataloged giant molecular clouds (GMCs) in the region where the two galaxies overlap using high-resolution (∼0.″1–10 pc) Atacama Large Millimeter/submillimeter Array observations of the 12 CO(2−1) and 13 CO(2−1) emission lines. Of the 72 individual GMCs identified in the overlap region, 17 are within uncertainties of having the necessary mass, pressure, and size needed to form SSCs. Of those 17 GMCs, only one has significant ionizing radiation, indicating that the birth environments are likely still intact in the 16 other GMCs. We compared the physical properties calculated from 12 CO(2−1) GMC data with observations of 10 other galaxies obtained using the same emission line and similar resolution. Compared to other sources in this sample, the GMCs from the Antennae, as well as in other starbursts and in the centers of galaxies, have the highest luminosities, surface densities, and turbulent pressures. The GMCs in starbursts and at the centers of galaxies also have large line widths, although the line widths in the Antennae are among the widest. These comparative results also indicate that the Antennae GMCs have the highest virial parameters despite their high densities.
Abstract
Stars form within molecular clouds, so characterizing the physical states of molecular clouds is key to understanding the process of star formation. Cloud structure and stability are ...frequently assessed using metrics including the virial parameter and Larson scaling relationships between cloud radius, velocity dispersion, and surface density. Departures from the typical Galactic relationships between these quantities have been observed in low-metallicity environments. The amount of H
2
gas in cloud envelopes without corresponding CO emission is expected to be high under these conditions; therefore, this CO-dark gas could plausibly be responsible for the observed variations in cloud properties. We derive simple corrections that can be applied to empirical clump properties (mass, radius, velocity dispersion, surface density, and virial parameter) to account for CO-dark gas in clumps following power-law and Plummer mass density profiles. We find that CO-dark gas is not likely to be the cause of departures from Larson’s relationships in low-metallicity regions, but that virial parameters may be systematically overestimated. We demonstrate that correcting for CO-dark gas is critical for accurately comparing the dynamical state and evolution of molecular clouds across diverse environments.
The CO-dark molecular gas mass in 30 Doradus Chevance, Mélanie; Madden, Suzanne C; Fischer, Christian ...
Monthly Notices of the Royal Astronomical Society,
06/2020, Volume:
494, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most ...abundant molecule in the Universe, H2 is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H2 conversion factor (XCO) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine-structure lines from the ionized and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas is responsible for a large reservoir of ‘CO-dark’ molecular gas, leaving a large fraction of the total H2 gas (≳75 per cent) undetected when adopting a standard XCO factor in this massive star-forming region.
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