Dense gas and star formation in individual Giant Molecular Clouds in M31

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.9 pages, 6 figures, accepted for publication in MNRASStudies both of entire galaxies and of local Galactic star formation indicate a dependency of a molecular cloud's star formation rate (SFR) on its dense gas mass. In external galaxies, such measurements are derived from HCN(1-0) observations, usually encompassing many Giant Molecular Clouds (GMCs) at once. The Andromeda galaxy (M31) is a unique laboratory to study the relation of the SFR and HCN emission down to GMC scales at solar-like metallicities. In this work, we correlate our composite SFR determinations with archival HCN, HCO+, and CO observations, resulting in a sample of nine reasonably representative GMCs. We find that, at the scale of individual clouds, it is important to take into account both obscured and unobscured star formation to determine the SFR. When correlated against the dense-gas mass from HCN, we find that the SFR is low, in spite of these refinements. We nevertheless retrieve an SFR - dense-gas mass correlation, confirming that these SFR tracers are still meaningful on GMC scales. The correlation improves markedly when we consider the HCN/CO ratio instead of HCN by itself. This nominally indicates a dependency of the SFR on the dense-gas fraction, in contradiction to local studies. However, we hypothesize that this partly reflects the limited dynamic range in dense-gas mass, and partly that the ratio of single-pointing HCN and CO measurements may be less prone to systematics like sidelobes. In this case, the HCN/CO ratio would importantly be a better empirical measure of the dense-gas content itself.Peer reviewedFinal Published versio

A rotating disk around the very young massive star AFGL 490

We observed the embedded, young 8--10 Msun star AFGL 490 at subarcsecond resolution with the Plateau de Bure Interferometer in the C17O (2--1) transition and found convincing evidence that AFGL 490 is surrounded by a rotating disk. Using two-dimensional modeling of the physical and chemical disk structure coupled to line radiative transfer, we constrain its basic parameters. We obtain a relatively high disk mass of 1 Msun and a radius of ~ 1500 AU. A plausible explanation for the apparent asymmetry of the disk morphology is given.Comment: 4 pages, 5 figure

Application of direct inverse analogy method (DIVA) and viscous design optimization techniques

A direct-inverse approach to the transonic design problem was presented in its initial state at the First International Conference on Inverse Design Concepts and Optimization in Engineering Sciences (ICIDES-1). Further applications of the direct inverse analogy (DIVA) method to the design of airfoils and incremental wing improvements and experimental verification are reported. First results of a new viscous design code also from the residual correction type with semi-inverse boundary layer coupling are compared with DIVA which may enhance the accuracy of trailing edge design for highly loaded airfoils. Finally, the capabilities of an optimization routine coupled with the two viscous full potential solvers are investigated in comparison to the inverse method

First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy

© 2020 The American Astronomical Society.In our local Galactic neighborhood, molecular clouds are best studied using a combination of dust measurements, to determine robust masses, sizes, and internal structures of the clouds, and molecular-line observations to determine cloud kinematics and chemistry. We present here the first results of a program designed to extend such studies to nearby galaxies beyond the Magellanic Clouds. Utilizing the wideband upgrade of the Submillimeter Array (SMA) at 230 GHz, we have obtained the first continuum detections of the thermal dust emission on sub-GMC scales (∼15 pc) within the Andromeda galaxy (M31). These include the first resolved continuum detections of dust emission from individual giant molecular clouds (GMCs) beyond the Magellanic Clouds. Utilizing a powerful capability of the SMA, we simultaneously recorded CO(2-1) emission with identical (u, v) coverage, astrometry, and calibration, enabling the first measurements of the CO conversion factor, α CO(2-1), toward individual GMCs across an external galaxy. Our direct measurement yields an average CO-to-dust mass conversion factor of α' CO-dust = 0.042 ± 0.018 M o (K km s -1 pc 2) -1 for the J = 2-1 transition. This value does not appear to vary with galactocentric radius. Assuming a constant gas-to-dust ratio of 136, the resulting α CO = 5.7 ± 2.4 M o (K km s -1 pc 2) -1 for the 2-1 transition is in excellent agreement with that of GMCs in the Milky Way, given the uncertainties. Finally, using the same analysis techniques, we compare our results with observations of the local Orion molecular clouds, placed at the distance of M31 and simulated to appear as they would if observed by the SMA.Peer reviewedFinal Published versio

The Orion Protostellar Explosion and Runaway Stars Revisited : Stellar Masses, Disk Retention, and an Outflow from the Becklin-Neugebauer Object

© 2020 The American Astronomical Society. All rights reserved.The proper motions of the three stars ejected from Orion's OMC1 cloud core are combined with the requirement that their center of mass is gravitationally bound to OMC1 to show that radio source I (Src I) is likely to have a mass around 15 M o˙ consistent with recent measurements. Src I, the star with the smallest proper motion, is suspected to be either an astronomical-unit-scale binary or a protostellar merger remnant produced by a dynamic interaction ∼550 yr ago. Near-infrared 2.2 μm images spanning ∼21 yr confirm the ∼55 km s -1 motion of "source x" (Src x) away from the site of stellar ejection and point of origin of the explosive OMC1 protostellar outflow. The radial velocities and masses of the Becklin-Neugebauer (BN) object and Src I constrain the radial velocity of Src x to be. Several high proper-motion radio sources near BN, including Zapata 11 ([ZRK2004] 11) and a diffuse source near IRc 23, may trace a slow bipolar outflow from BN. The massive disk around Src I is likely the surviving portion of a disk that existed prior to the stellar ejection. Though highly perturbed, shocked, and reoriented by the N-body interaction, enough time has elapsed to allow the disk to relax with its spin axis roughly orthogonal to the proper motion.Peer reviewedFinal Published versio

High-energy processes in low-mass protostars – an X-ray to radio multi-wavelength perspective

High-energy processes in protostars remain poorly understood. Only after the recently finished Chandra Orion Ultra-deep Project (COUP), statistically significant information on X-ray emission from Young Stellar Objects (YSOs) has been obtained. For an understanding of the mechanisms responsible for the X-ray emission, multi-wavelength correlations of flares, especially in the radio regime, are necessary and have become an active field of research. Magnetic fields cause several high-energy phenomena in the coronae of YSOs mainly due to magnetic reconnection which are observable in a wide wavelength range from nonthermal centimetric radio emission to X-rays. In this work, these processes were probed using a variety of very different methods which can be grouped into three major topics: X-ray to radio multi-wavelength variability of Young Stellar Objects The Coronet cluster in the nearby R~CrA dark cloud offers the rare opportunity to study a compact cluster of several very young protostars which are detected at radio- and X-ray wavelengths. Initially, a study focusing separately on X-ray and radio variability of these sources was conducted. Subsequently, in August 2005, the same region was studied in the first simultaneous X-ray, radio, near-infrared, and optical monitoring campaign of YSOs. Several observatories were observing simultaneously, namely the Chandra X-ray Observatory, the VLA, as well as telescopes with optical and near-infrared detectors in Chile and South Africa. Remarkable intra-band variability but no clearly correlated variability was found. This most importantly suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. Combining the Chandra X-ray data collected in the course of the multi-wavelength campaign with previous archival data, one of the deepest X-ray datasets ever obtained of a star-forming region is obtained and discussed.Radio emission from protostars Looking for compact nonthermal centimetric radio emission, high-sensitivity Very-Long-Baseline Interferometry (VLBI) observations of four nearby protostars were carried out, yielding the currently most sensitive data of such sources. Weak compact emission was found in the VLBI data of the class~0/I binary YLW15 VLA2, constraining the size of its corona to sub-AU scales. Since this source is part of a binary system with observed orbital motion, further VLBI observations will allow to quickly determine the orbit very accurately. The observed sources apparently were showing quiescent radio emission on the larger scales probed by the Very Large Array (VLA). Until now, only very few radio flares of YSOs have been observed in detail. In further work, two such examples are presented and analyzed: a flaring, deeply embedded protostar in Orion and a flaring binary T~Tauri system whose activity is due to inter-binary coronal interaction.The earliest stages: Magnetic fields in molecular clouds While mapping molecular clouds in polarized dust continuum emission has become a standard technique, the potentially more powerful technique using the "Goldreich-Kylafis" effect has been only rarely used until now. This effect predicts weakly linearly polarized molecular line emission under certain circumstances. By choosing different transitions, it is possible to probe the magnetic field direction in different regions in a molecular cloud core, and additionally one gets information along the line of sight for optically thin emission lines. The XPOL correlation polarimeter at the IRAM 30m telescope was used in a search for such linearly polarized emission in several bright molecular transition lines towards prominent star-forming regions. The combined effects of instrumental polarization and extended emission were simulated for a thorough interpretation of the results. In one case, the observed polarization exceeds the simulated instrumental value

Marsh-atmosphere CO2 exchange in a New England salt marsh

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 120 (2015): 1825–1838, doi:10.1002/2015JG003044.We studied marsh-atmosphere exchange of carbon dioxide in a high marsh dominated salt marsh during the months of May to October in 2012–2014. Tidal inundation at the site occurred only during biweekly spring tides, during which we observed a reduction in fluxes during day and night. We estimated net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (Reco) using a modified PLIRTLE model, which requires photosynthetically active radiation, temperature, and normalized difference vegetation index (NDVI) as control variables. NDVI decreased during inundation, when the marsh canopy was submerged. Two-time series of NDVI, including and excluding effects of tidal inundation, allowed us to quantify the flux reduction during inundation. The effect of the flux reduction was small (2–4%) at our site, but is likely higher for marshes at a lower elevation. From May to October, GPP averaged −863 g C m−2, Reco averaged 591 g C m−2, and NEE averaged −291 g C m−2. In 2012, which was an exceptionally warm year, we observed an early start of net carbon uptake but higher respiration than in 2013 and 2014 due to higher-air temperature in August. This resulted in the lowest NEE during the study period (−255.9±6.9 g C m−2). The highest seasonal net uptake (−336.5±6.3 g C m−2) was observed in 2013, which was linked to higher rainfall and temperature in July. Mean sea level was very similar during all 3 years which allowed us to isolate the importance of climatic factors.NSF grants OCE-1058747 and OCE-12382122019-03-2

Nothing to hide: An X-ray survey for young stellar objects in the Pipe Nebula

We have previously analyzed sensitive mid-infrared observations to establish that the Pipe Nebula has a very low star-formation efficiency. That study focused on YSOs with excess infrared emission (i.e, protostars and pre-main sequence stars with disks), however, and could have missed a population of more evolved pre-main sequence stars or Class III objects (i.e., young stars with dissipated disks that no longer show excess infrared emission). Evolved pre-main sequence stars are X-ray bright, so we have used ROSAT All-Sky Survey data to search for diskless pre-main sequence stars throughout the Pipe Nebula. We have also analyzed archival XMM-Newton observations of three prominent areas within the Pipe: Barnard 59, containing a known cluster of young stellar objects; Barnard 68, a dense core that has yet to form stars; and the Pipe molecular ring, a high-extinction region in the bowl of the Pipe. We additionally characterize the X-ray properties of YSOs in Barnard 59. The ROSAT and XMM-Newton data provide no indication of a significant population of more evolved pre-main sequence stars within the Pipe, reinforcing our previous measurement of the Pipe's very low star formation efficiency.Comment: Accepted for publication in Ap