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

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

Schmidt's Conjecture and Star Formation in Molecular Clouds

We investigate Schmidt's conjecture (i.e., that the star formation rate scales in a power-law fashion with the gas density) for four well-studied local molecular clouds (GMCs). Using the Bayesian methodology we show that a local Schmidt scaling relation of the form Sigma*(A_K) = kappa x (A_K)^{beta} (protostars pc^{-2}) exists within (but not between) GMCs. Further we find that the Schmidt scaling law, by itself, does not provide an adequate description of star formation activity in GMCs. Because the total number of protostars produced by a cloud is given by the product of Sigma*(A_K) and S'(> A_K), the differential surface area distribution function, integrated over the entire cloud, the cloud's structure plays a fundamental role in setting the level of its star formation activity. For clouds with similar functional forms of Sigma*(A_K), observed differences in their total SFRs are primarily due to the differences in S'(> A_K) between the clouds. The coupling of Sigma*(A_K) with the measured S'(> A_K) in these clouds also produces a steep jump in the SFR and protostellar production above A_K ~ 0.8 magnitudes. Finally, we show that there is no global Schmidt law that relates the star formation rate and gas mass surface densities between GMCs. Consequently, the observed Kennicutt-Schmidt scaling relation for disk galaxies is likely an artifact of unresolved measurements of GMCs and not a result of any underlying physical law of star formation characterizing the molecular gas.Comment: 34 pages, 8 figures, and 2 tables; accepted for publication in ApJ on Sept 23, 201

The HP2 Survey - IV. The Pipe nebula : Effective dust temperatures in dense cores

14 pages, 22 figures. Accepted for publication in Astronomy & Astrophysics Reproduced with permission from Astronomy & Astrophysics. © 2018 ESOMulti-wavelength observations in the sub-millimeter regime provide information on the distribution of both the dust column density and the effective dust temperature in molecular clouds. In this study, we created high-resolution and high-dynamic-range maps of the Pipe nebula region and explored the value of dust-temperature measurements in particular towards the dense cores embedded in the cloud. The maps are based on data from the Herschel and Planck satellites, and calibrated with a near-infrared extinction map based on 2MASS observations. We have considered a sample of previously defined cores and found that the majority of core regions contain at least one local temperature minimum. Moreover, we observed an anti-correlation between column density and temperature. The slope of this anti-correlation is dependent on the region boundaries and can be used as a metric to distinguish dense from diffuse areas in the cloud if systematic effects are addressed appropriately. Employing dust-temperature data thus allows us to draw conclusions on the thermodynamically dominant processes in this sample of cores: External heating by the interstellar radiation field and shielding by the surrounding medium. In addition, we have taken a first step towards a physically motivated core definition by recognising that the column-densityerature anti-correlation is sensitive to the core boundaries. Dust-temperature maps therefore clearly contain valuable information about the physical state of the observed medium.Peer reviewe

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

The First Vlbi Detection of an Ultracool Dwarf: Implications for the Detectability of Sub-Stellar Companions

We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM 513–46546 (M8.5) and 2MASS J00361617+1821104 (L3.5) in an attempt to detect sub-stellar companions via direct imaging or reflex motion. Both objects are known radio emitters with strong evidence for periodic emission on timescales of about 2 and 3 hours, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM 513–46546 on a scale of 2.5 mas (∼ 50 stellar radii), leading to a direct limit on the radio emission brightness temperature of TB & 4 × 105 K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally and asymmetrically resolved at a low S/N ratio, possibly indicating that TVLM 513–46546 is a binary with a projected separation of ∼ 1 mas (∼ 20 stellar radii). Using the 7-hr baseline of our observation we find no astrometric shift in the position of TVLM 513–46546, with a 3σ limit of about 0.6 mas. This is about 3 times larger than expected for an equal mass companion with a few-hour orbital period. Future monitoring of its position on a range of timescales will provide the required astrometric sensitivity to detect a planetary companion with a mass of ∼ 10 MJ in a & 15 d (& 0.06 AU) orbit, or with a mass of ∼ 2 MJ in an orbit of & 0.5 yr (& 0.3 AU).Astronom