Luis A. Zapata

Orion KL: The hot core that is not a "Hot Core"

We present sensitive high angular resolution submillimeter and millimeter observations of torsionally/vibrationally highly excited lines of the CH3OH, HC3N, SO2 ,a nd CH 3CN molecules and of the continuum emission at 870 and 1300 μm from the Orion KL region, made with the Submillimeter Array (SMA). These observations and SMA CO J = 3− 2a ndJ = 2−1 imaging of the explosive flow originating in this region suggest that the molecular Orion “hot core” is a pre-existing density enhancement heated from the outside by the explosive event. Unlike in other hot cores, we do not find any self-luminous submillimeter, radio, or infrared source embedded in the hot molecular gas, nor observe filamentary CO flow structures or “fingers” in the shadow of the hot core pointing away from the explosion center. The low-excitation CH3CN emission shows the typical molecular heart-shaped structure, traditionally named the hot core, and is centered close to the dynamical origin of the explosion. The highest excitation CH3CN lines all originate from the northeast lobe of the heart-shaped structure, i.e. from the densest and most highly obscured parts of the extended ridge. The torsionally excited CH3OH and vibrationally excited HC3N lines appear to form a shell around the strongest submillimeter continuum source. All of these observations suggest that the southeast and southwest sectors of the explosive flow have impinged on a pre-existing very dense part of the extended ridge, thus creating the bright Orion KL hot core. However, additional theoretical and observational studies are required to test this new heating scenario.

A RING/DISK/OUTFLOW SYSTEM ASSOCIATED WITH W51 NORTH: A VERY MASSIVE STAR IN THE MAKING

Sensitive and high angular resolution (~04) SO2[222,20 → 221,21] and SiO[5 → 4] line and 1.3 and 7 mm continuum observations made with the Submillimeter Array (SMA) and the Very Large Array (VLA) toward the young massive cluster W51 IRS2 are presented. We report the presence of a large (of about 3000 AU) and massive (40 M ☉) dusty circumstellar disk and a hot gas molecular ring around a high-mass protostar or a compact small stellar system associated with W51 North. The simultaneous observations of the silicon monoxide molecule, an outflow gas tracer, further revealed a massive (200 M ☉) and collimated (~14°) outflow nearly perpendicular to the dusty and molecular structures suggesting thus the presence of a single very massive protostar with a bolometric luminosity on the order of 105 L ☉. A molecular hybrid local thermodynamic equilibrium model of a Keplerian and infalling disk with an inner cavity and a central stellar mass of more than 60 M ☉ agrees well with the SO2[222,20 → 221,21] line observations. Finally, these results suggest that mechanisms, such as mergers of low- and intermediate-mass stars, might not be necessary for forming very massive stars.

AN EXTENSIVE, SENSITIVE SEARCH FOR SiO MASERS IN HIGH- AND INTERMEDIATE-MASS STAR-FORMING REGIONS

We present sensitive Very Large Array observations with an angular resolution of a few arcseconds of the J = 1-0 line of SiO in the v = 1 and 2 vibrationally excited states toward a sample of 60 Galactic regions in which stars of high or intermediate mass are currently forming and/or have recently formed. We report the detection of SiO maser emission in both vibrationally excited transitions toward only three very luminous regions: Orion-KL, W51N, and Sgr B2(M). Toward all three, SiO maser emission had previously been reported, in Orion-KL in both lines, in W51N only in the v = 2 line and in Sgr B2(M) only in the v = 1 line. Our work confirms that SiO maser emission in star-forming regions is a rare phenomenon, indeed, that requires special, probably extreme, physical and chemical conditions not commonly found. In addition to this SiO maser survey, we also present images of the simultaneously observed 7 mm continuum emission from a subset of our sample of star-forming regions where such emission was detected. This is, in most cases, likely to be free-free emission from compact and ultracompact H II regions.

IC 348-SMM2E: a Class 0 proto-brown dwarf candidate forming as a scaled-down version of low-mass stars

We report on Submillimeter Array observations of the 870 micron continuum and CO(3-2), 13CO(2-1) and C18O(2-1) line emission of a faint object, SMM2E, near the driving source of the HH797 outflow in the IC348 cluster. The continuum emission shows an unresolved source for which we estimate a mass of gas and dust of 30 Mjup, and the CO(3-2) line reveals a compact bipolar outflow centred on SMM2E, and barely seen also in 13CO(2-1). In addition, C18O(2-1) emission reveals hints of a possible rotating envelope/disk perpendicular to the outflow, for which we infer a dynamical mass of ~16 Mjup. In order to further constrain the accreted mass of the object, we gathered data from Spitzer, Herschel, and new and archive submillimetre observations, and built the Spectral Energy Distribution (SED). The SED can be fitted with one single modified black-body from 70 micron down to 2.1 cm, using a dust temperature of ~24 K, a dust emissivity index of 0.8, and an envelope mass of ~35 Mjup. The bolometric luminosity is 0.10 Lsun, and the bolometric temperature is 35 K. Thus, SMM2E is comparable to the known Class 0 objects in the stellar domain. An estimate of the final mass indicates that SMM2E will most likely remain substellar, and the SMM2E outflow force matches the trend with luminosity known for young stellar objects. Thus, SMM2E constitutes an excellent example of a Class 0 proto-brown dwarf candidate which forms as a scaled-down version of low-mass stars. Finally, SMM2E seems to be part of a wide (~2400 AU) multiple system of Class 0 sources.

The high-mass star-forming region IRAS 18182 1433

Aims. We present mm line and continuum observations at high spatial resolution characterizing the physical and chemical properties of the young massive star-forming region IRAS 18182−1433. Methods. The region was observed with the Submillimeter Array in the 1.3 mm band. The data are complemented with short-spacing information from single-dish CO(2‐1) observations. SiO(1‐0) data from the VLA are added to the analysis. Results. Multiple massive outflows emanate from the mm continuum peak. The CO(2‐1) data reveal a quadrupolar outflow system consis ting of two outflows inclined by ∼90 ◦ . One outflow exhibits a cone-like red-shifted morphology wi th a jet-like blue-shifted counterpart where a blue counter-cone can only be tentatively identified. The SiO(1‐ 0) data suggest the presence of a third outflow. Analyzing the 12 CO/ 13 CO line ratios indicates decreasing CO line opacities with increasing velocities. Although we observe a multiple outflow system, the m m continuum peak remains single-peaked at the given spatial resolution (∼13500 AU). The other seven detected molecular species ‐ also high-density tracers like CH3CN, CH3OH, HCOOCH3 ‐ are all∼1-2 ′′ offset from the mm continuum peak, but spatially associated with a strong molecular outflow peak and a cm emission feature indicative of a thermal jet. This spatial displacement between the molecular lines and the mm continuum emission could be either due to an unresolved sub-source at the position of the cm feature, or the outflow /jet itself alters the chemistry of the core enhancing the molecular abundances toward that region. A temperature estimate based on the CH3CN(12k− 11k) lines suggests temperatures of the order 150 K. A velocity analysis of the high-density tracing molecules reveals that at the given spatial resolutio n none of them shows any coherent velocity structure which would be consistent with a rotating disk. We discuss this lack of rotation signatu res and attribute it to intrinsic diffi culties to observationally isolate massive accretion disk s from the surrounding dense gas envelopes and the molecular outflows.

Radio continuum emission from knots in the DG Tauri jet

Context. HH 158, the jet from the young star DG Tau, is one of the few sources of its type where jet knots have been detected at optical and X-ray wavelengths. Aims. We aim to search for radio knots to compare them with the optical and X-ray knots. We also aim to model the emission from the radio knots. Methods. We analyzed archive data and also obtained new Very Large Array observations of this source, as well as an optical image to measure the present position of the knots. We furthermore modeled the radio emission from the knots in terms of shocks in a jet with intrinsically time-dependent ejection velocities. Results. We detected radio knots in the 1996.98 and 2009.62 VLA data. These radio knots are, within error, coincident with optical knots. We also modeled satisfactorily the observed radio flux densities as shock features from a jet with intrinsic variability. All observed radio, optical, and X-ray knot positions can be intepreted as four successive knots, ejected with a period of 4.80 years and traveling away from the source with a velocity of 198 km s −1 in the plane of the sky. Conclusions. The radio and optical knots are spatially correlated and our model can explain the observed radio flux densities. However, the X-ray knots do not appear to have optical or radio counterparts and their nature remains poorly understood.

ALMA and VLA observations of the outflows in IRAS 16293-2422

We present Atacama Large Millimeter/submillimeter Array (ALMA) and Very Large Array (VLA) observations of the molecular and ionized gas at 0.1–0.3 arcsec resolution in the Class 0 protostellar system IRAS 16293−2422. These data clarify the origins of the protostellar outflows from the deeply embedded sources in this complex region. Source A2 is confirmed to be at the origin of the well-known large-scale north-east–south-west flow. The most recent VLA observations reveal a new ejection from that protostar, demonstrating that it drives an episodic jet. The central compact part of the other known large-scale flow in the system, oriented roughly east–west, is well delineated by the CO(6-5) emission imaged with ALMA and is confirmed to be driven from within component A. Finally, a one-sided blueshifted bubble-like outflow structure is detected here for the first time from source B to the north-west of the system. Its very short dynamical time-scale (∼200 yr), low velocity and moderate collimation support the idea that source B is the youngest object in the system, and possibly one of the youngest protostars known.

A rotating molecular jet in Orion

We present CO(2−1), 13 CO(2−1), CO(6−5), CO(7−6), and SO(65−54) line observations made with the IRAM 30 m and Atacama Pathfinder Experiment (APEX) radiotelescopes and the Submillimeter Array (SMA) toward the highly collimated (11 ◦ ) and extended (∼2 � ) southwest lobe of the bipolar outflow Ori-S6 located in the Orion South region. We report for all these lines, the detection of velocity asymmetries about the flow axis with velocity differences roughly on the order of 1 km s −1 over distances of about 5000 AU, 4k m s −1 over distances of about 2000 AU, and close to the source of between 7 and 11 km s −1 over smaller scales of about 1000 AU. The redshifted gas velocities are located to the southeast of the outflow’s axis, the blueshifted ones to the northwest. We interpret these velocity differences as a signature of rotation, but also discuss some alternatives which we recognize as unlikely in view of the asymmetries’ large downstream continuation. In particular, any straightforward interpretation by an ambient velocity gradient does not seem viable. This rotation across the Ori-S6 outflow is observed out to (projected) distances beyond 2.5 × 10 4 AU from the flow’s presumed origin. Comparison of our large-scale (single dish) and small-scale (SMA) observations suggests the rotational velocity to decline not faster than 1/R with distance R from the axis; in the innermost few arcsecs an increase of rotational velocity with R is even indicated. The magnetic field lines threading the inner rotating CO shell may well be anchored in a disk of a radius of ∼50 AU; the field lines further out need a more extended rotating base. Our high angular resolution SMA observations also suggest this outflow to be energized by the compact millimeter radio source 139-409, a circumbinary flattened ring that is located in a small cluster of very young stars associated with the extended and bright source FIR4.

In Search of Circumstellar Disks around Young Massive Stars

We present 7 mm, 1.3 cm, and 3.6 cm continuum observations made with the Very Large Array toward a sample of 10 luminous IRAS sources that are believed to be regions of massive star formation. We detect compact 7 mm emission in four of these objects: IRAS 18089-1732(1), IRAS 18182-1433, IRAS 18264-1152, and IRAS 18308-0841, and for the first time find that these IRAS sources are associated with double or triple radio sources separated by a few arcseconds. We discuss the characteristics of these sources based mostly on their spectral indices and find that their natures are diverse. Some features indicate that the 7 mm emission is dominated by dust from disks or envelopes. Toward other components, the 7 mm emission appears to be dominated by free-free radiation, both from ionized outflows or from optically thick H II regions. Furthermore, there is evidence of synchrotron contamination in some of these sources. Finally, we find that the sources associated with ionized outflows or thermal jets are correlated with CH3OH masers. The precise determination of the nature of these objects requires additional multifrequency observations at high angular resolution. The 3.6 cm continuum observations also revealed seven ultracompact (UC) H II regions in the vicinity of the sources IRAS 18089-1732(1), and two more in the source IRAS 18182-1433. We show that the small photoionized nebulae of these UC H II regions are produced by early B-type stars.

A ROTATING MOLECULAR JET FROM A PERSEUS PROTOSTAR

We present the {sup 12}CO(2-1) line and 1.4 mm continuum archival observations, made with the Submillimeter Array, of the outflow HH 797 located in the IC 348 cluster in Perseus. The continuum emission is associated with a circumstellar disk surrounding the class 0 object IC 348-MMS/SMM2, a very young solar analog. The line emission, on the other hand, delineates a collimated outflow and reveals velocity asymmetries about the flow axis over the entire length of the flow. The amplitude of velocity differences is of the order of 2 km s{sup -1} over distances of about 1000 AU, and we interpret them as evidence for jet rotation-although we also discuss alternative possibilities. A comparison with theoretical models suggests that the magnetic field lines threading the protostellar jet might be anchored to the disk of a radius of about 20 AU.