Jan Forbrich

The distance to the Orion Nebula

We have used the Very Long Baseline Array to measure the trigonometric parallax of several member stars of the Orion Nebula Cluster showing non-thermal radio emission. We have determined the distance to the cluster to be 414 ± 7 pc. Our distance determination allows for an improved calibration of luminosities and ages of young stars. We have also measured the proper motions of four cluster stars which, when accurate radial velocities are measured, will put strong constraints on the origin of the cluster.

YSOVAR: THE FIRST SENSITIVE, WIDE-AREA, MID-INFRARED PHOTOMETRIC MONITORING OF THE ORION NEBULA CLUSTER

We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

A Spitzer Census of Star Formation Activity in the Pipe Nebula

The Pipe Nebula, a large nearby molecular cloud, lacks obvious signposts of star formation in all but one of more than 130 dust extinction cores that have been identified within it. In order to quantitatively determine the current level of star formation activity in the Pipe Nebula, we analyzed 13 deg2 of sensitive mid-infrared maps of the entire cloud, obtained with the Multiband Imaging Photometer for Spitzer at wavelengths of 24 μm and 70 μm, to search for candidate young stellar objects (YSOs) in the high-extinction regions. We argue that our search is complete for class I and typical class II YSOs with luminosities of L bol ~ 0.2 L ☉ and greater. We find only 18 candidate YSOs in the high-extinction regions of the entire Pipe cloud. Twelve of these sources are previously known members of a small cluster associated with Barnard 59, the largest and most massive dense core in the cloud. With only six candidate class I and class II YSOs detected toward extinction cores outside of this cluster, our findings emphatically confirm the notion of an extremely low level of star formation activity in the Pipe Nebula. The resulting star formation efficiency for the entire cloud mass is only ~0.06%.

A millimeter continuum survey for massive protoclusters in the outer galaxy

Our search for the earliest stages of massive star formation turned up 12 massive pre-protocluster candidates plus a few protoclusters. For this search, we selected 47 FIR-bright IRAS sources in the outer Galaxy. We mapped regions of several square arcminutes around the IRAS source in the millimeter continuum in order to find massive cold cloud cores possibly being in a very early stage of massive star formation. Masses and densities are derived for the 128 molecular cloud cores found in the obtained maps. We present these maps together with near-infrared, mid-infrared, and radio data collected from the 2MASS, MSX, and NVSS catalogs. Further data from the literature on detections of high-density tracers, outflows, and masers are added. The multiwavelength data sets are used to characterize each observed region. The massive cloud cores (M > 100 M☉) are placed in a tentative evolutionary sequence depending on their emission at the investigated wavelengths. Candidates for the youngest stages of massive star formation are identified by the lack of detections in the above-mentioned near-infrared, mid-infrared, and radio surveys. Twelve massive cores prominent in the millimeter continuum fulfill this requirement. Since neither FIR nor radio emission have been detected from these cloud cores, massive protostars must be very deeply embedded in these cores. Some of these objects may actually be pre-protocluster cores: an up to now rare object class, where the initial conditions of massive star formation can be studied.

Radio and X-ray variability of young stellar objects in the Coronet cluster

The Coronet cluster in the nearby R CrA dark cloud offers the rare opportunity to study at least four class I protostellar sources as well as one candidate class 0 source, a Herbig Ae star, and a candidate brown dwarf within a few square arcminutes - most of them detected at radio- and X-ray wavelengths. These sources were observed with the Very Large Array (VLA) at A = 3.5 cm on nine occasions in 1998, spread over nearly four months. The source IRS 5, shown earlier to emit circularly polarized radio emission, was observed to undergo a flux increase accompanied by changes in its polarization properties. Comparison with VLA measurements taken in January 1997 allows analysis of longer-term variability. In addition to this radio monitoring, we analyze archival Chandra and XMM-Newton X-ray data of these sources. Three class I protostars are bright enough for X-ray spectroscopy, and we perform a variability analysis for these sources, covering a total of 154 ks spread over more than two and a half years. Also in X-rays, IRS 5 shows the most pronounced variability, whilst the other two class I protostars IRS 1 and IRS 2 have more stable emission. X-ray data is also analyzed for the recently identified candidate class 0 source IRS 7E, which shows strong variability as well as for the Herbig Ae star R CrA for which we find extremely hot X-ray-emitting plasma. For IRS 1, 2 and 5, the hydrogen column densities derived from the X-ray spectra are at about half the values derived with near-infrared techniques, a situation similar to what has been observed towards some other young stellar objects.

A Rotating Disk around the Very Young Massive Star AFGL 490

We observed the embedded, young 8-10 M☉ star AFGL 490 at subarcsecond resolution with the Plateau de Bure Interferometer (PdBI) 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 M☉ and a radius of ~1500 AU. A plausible explanation for the apparent asymmetry of the disk morphology is given.

Synchrotron emission from the T Tauri binary system V773 Tauri A

The pre-main sequence binary system V773 Tau A shows remarkable flaring activity around periastron passage. Here, we present the observation of such a flare at a wavelengt h of 3 mm (90 GHz) performed with the Plateau de Bure Interferometer. ⋆ We examine different possible causes for the energy losses responsible for the e-folding time of 2.31± 0.19 hours of that flare. We exclude synchrotron, collisional, an d inverse Compton losses because they are not consistent with observational constraints, and we propose that the fading of the emission is due to the leakage of electrons themselves at each reflection between the two mirror points of the magnetic stru cture partially trapping them. The magnetic structure compatible with both our leakage model and previous observations is that of a helmet streamer that, as in the solar case, can occur at t he top of the X-ray-emitting, stellar-sized coronal loops of one o f the stars. The streamer may extend up to∼ 20 R∗ and interact with the corona of the other star at periastron passage, causing r ecurring flares. The inferred magnetic field strength at the t wo mirror points of the helmet streamer is in the range 0.12 - 125 G, and the corresponding Lorentz factor,γ, of the partially trapped electrons is in the range 20<γ< 632. We therefore rule out that the emission could be of gyro-synchrotron nature: the derived high Lorentz factor proves that the nature of the emission at 90 GHz from this pre-main binary system is synchrotron radiation.

VISION − Vienna survey in Orion - I. VISTA Orion A Survey

Stefan Meingast, et al., “VISION – Vienna survey in Orion”, Astronomy & Astrophysics, Vol. 587, March 2016. This version of record is available online at:https://www.aanda.org/articles/aa/abs/2016/02/aa26100-15/aa26100-15.html

SIMULTANEOUS X-RAY AND RADIO OBSERVATIONS OF YOUNG STELLAR OBJECTS IN NGC 1333 AND IC 348

Young stellar objects (YSOs) and in particular protostars are known to show a variety of high-energy processes. Observations in the X-ray and centimetric radio wavelength ranges are thought to constrain some of these processes, e.g., coronal-type magnetic activity. There is a well-known empirical correlation of radio and X-ray luminosities in active stars, the so-called Gudel-Benz (GB) relation. Previous evidence of whether YSOs are compatible with this relation remains inconclusive for the earliest evolutionary stages. The main difficulty is that due to the extreme variability of these sources, simultaneous observations are essential. Until now, only a few YSOs and only a handful of protostars have been observed simultaneously in the X-ray and radio range. To expand the sample, we have obtained such observations of two young clusters rich in protostars, NGC 1333 and IC 348. While the absolute sensitivity is lower for these regions than for more nearby clusters like CrA, we find that even in deep continuum observations carried out with the NRAO Very Large Array (VLA), the radio detection fraction for protostars in these clusters is much lower than the X-ray detection fraction. Very few YSOs are detected in both bands, and we find the radio and X-ray populations among YSOs to be largely distinct. We combine these new results with previous simultaneous Chandra and VLA observations of star-forming regions and find that YSOs with detections in both bands appear to be offset toward higher radio luminosities for given X-ray luminosities when compared with the GB relation, although even in this sensitive data set most sources are too weak for the radio detections to provide information on the emission processes. The considerably improved sensitivity of the Expanded VLA will provide a better census of the YSO radio population as well as better constraints on the emission mechanisms.

A 1.3 cm wavelength radio flare from a deeply embedded source in the Orion BN/KL region

Aims. Our aim was to measure and characterize the short-wavelength radio emission from young stellar objects (YSOs) in the Orion Nebula Cluster and the BN/KL star-forming region. Methods. We used the NRAO Very Large Array at a wavelength of A = 1.3 cm and we studied archival X-ray, infrared, and radio data. Results. During our observation, a strong outburst (flux increasing > 10 fold) occurred in one of the 16 sources detected at A = 1.3 cm, while the others remained (nearly) constant. This source does not have an infrared counterpart, but has subsequently been observed to flare in X-rays. Curiously, a very weak variable double radio source was found at other epochs near this position, one of whose components is coincident with it. A very high extinction derived from modeling the X-ray emission and the absence of an infrared counterpart both suggest that this source is very deeply embedded.