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At least 523 records · Page 29
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DOE OSTI2016

Protostars at low extinction in Orion A

In the list of young stellar objects (YSOs) compiled by Megeath et al. for the Orion A molecular cloud, only 44 out of 1208 sources found projected onto low extinction (A{sub K}<0.8 mag) gas are identified as protostars. These objects are puzzling because protostars are not typically expected to be associated with extended low extinction material. Here, we use high resolution extinction maps generated from Herschel data, optical/infrared and Spitzer Space Telescope photometry and spectroscopy of the low extinction protostellar candidate sources to determine if they are likely true protostellar sources or contaminants. Out of 44 candidate objects, we determine that 10 sources are likely protostars, with the rest being more evolved YSOs (18), galaxies (4), false detections of nebulosity and cloud edges (9), or real sources for which more data are required to ascertain their nature (3). We find none of the confirmed protostars to be associated with recognizable dense cores and we briefly discuss possible origins for these orphaned objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

( Herschel Herschel)/HIFI spectral mapping of C{sup +}, CH{sup +}, and CH in Orion BN/KL: the prevailing role of ultraviolet irradiation in CH{sup +} formation

The CH{sup +} ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH{sup +} J = 1−0, 2−1 rotational transitions, three sets of CH Λ-doubled triplet lines, {sup 12}C{sup +} and {sup 13}C{sup +} {sup 2}P{sub 3/2}--{sup 2}P{sub 1/2}, and CH{sub 3}OH 835 GHz E-symmetry Q-branch transitions, obtained with Herschel/HIFI over a region of ≈12 arcmin{sup 2} centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and {sup 12}C{sup +} optical depths. Emission from all of C{sup +}, CH{sup +}, and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH{sup +} in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H{sub 2} S(1) emission tracing shocked gas in the outflow. We propose that C{sup +} is being consumed by rapid production of CO to explain the lack of both C{sup +} and CH{sup +} in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH{sup +} formation and excitation is incomplete without including the important—possibly dominant—role of UV irradiation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A VLA SURVEY FOR FAINT COMPACT RADIO SOURCES IN THE ORION NEBULA CLUSTER

We present Karl G. Jansky Very Large Array 1.3, 3.6, and 6 cm continuum maps of compact radio sources in the Orion Nebular Cluster (ONC). We mosaicked 34 arcmin{sup 2} at 1.3 cm, 70 arcmin{sup 2} at 3.6 cm and 109 arcmin{sup 2} at 6 cm, containing 778 near-infrared detected young stellar objects and 190 Hubble Space Telescope -identified proplyds (with significant overlap between those characterizations). We detected radio emission from 175 compact radio sources in the ONC, including 26 sources that were detected for the first time at these wavelengths. For each detected source, we fitted a simple free–free and dust emission model to characterize the radio emission. We extrapolate the free–free emission spectrum model for each source to ALMA bands to illustrate how these measurements could be used to correctly measure protoplanetary disk dust masses from submillimeter flux measurements. Finally, we compare the fluxes measured in this survey with previously measured fluxes for our targets, as well as four separate epochs of 1.3 cm data, to search for and quantify the variability of our sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA Observations of Asymmetric Molecular Gas Emission from a Protoplanetary Disk in the Orion Nebula

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of molecular line emission from d216-0939, one of the largest and most massive protoplanetary disks in the Orion Nebula Cluster. We model the spectrally resolved HCO{sup +} (4–3), CO (3–2), and HCN (4–3) lines observed at 0.&prime;5 resolution to fit the temperature and density structure of the disk. We also weakly detect and spectrally resolve the CS (7–6) line but do not model it. The abundances we derive for CO and HCO{sup +} are generally consistent with expected values from chemical modeling of protoplanetary disks, while the HCN abundance is higher than expected. We dynamically measure the mass of the central star to be 2.17±0.07 M{sub ⊙}, which is inconsistent with the previously determined spectral type of K5. We also report the detection of a spatially unresolved high-velocity blueshifted excess emission feature with a measurable positional offset from the central star, consistent with a Keplerian orbit at 60 ± 20 au. Using the integrated flux of the feature in HCO{sup +} (4–3), we estimate the total H{sub 2} gas mass of this feature to be at least 1.8--8 M{sub Jupiter}, depending on the assumed temperature. The feature is due to a local temperature and/or density enhancement consistent with either a hydrodynamic vortex or the expected signature of the envelope of a forming protoplanet within the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

VLBA Observations of Strong Anisotripic Radio Scattering Toward the Orion Nebula

We present observations of VLBA 20, a radio source found toward the edge of the Orion Nebula Cluster (ONC). Nonthermal emission dominates the spectral energy distribution of this object from the radio to mid-infrared regime, suggesting that VLBA 20 is extragalactic. This source is heavily scattered in the radio regime. Very Long Baseline Array observations resolve it to ∼34 × 19 mas at 5 GHz, and the wavelength dependence of the scattering disk is consistent with ν {sup −2} at other frequencies. The origin of the scattering is most likely the ionized X-ray emitting gas from the winds of the most massive stars of the ONC. The scattering is highly anisotropic, with the axis ratio of 2:1, higher than what is typically observed toward other sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ALMA observations of Orion Source I at 350 and 660 GHz

Orion Source I (“SrcI”) is the protostar at the center of the Kleinmann-Low Nebula. ALMA observations of SrcI with 0.″2 angular resolution were made at 350 and 660 GHz to search for the H26α and H21α hydrogen recombination lines and to measure the continuum flux densities. The recombination lines were not detected, ruling out the possibility that SrcI is a hypercompact H ii region. The deconvolved size of the continuum source is approximately 0.″23 × 0.″07 (∼100 × 30 au); it is interpreted as a disk viewed almost edge-on. Optically thick thermal emission from ∼500 K dust is the most plausible source of the continuum, even at frequencies as low as 43 GHz; the disk mass is most likely in the range 0.02–0.2 M{sub ⊙}. A rich spectrum of molecular lines is detected, mostly from sulfur- and silicon-rich molecules like SO, SO{sub 2}, and SiS, but also including vibrationally excited CO and several unidentified transitions. Lines with upper energy levels E{sub U}>500 K appear in emission and are symmetric about the source’s LSR velocity of 5 km s{sup −1}, while lines with E{sub U}<500 K appear as blueshifted absorption features against the continuum, indicating that they originate in outflowing gas. The emission lines exhibit a velocity gradient along the major axis of the disk that is consistent with rotation around a 5–7 M{sub ⊙} central object. The relatively low mass of SrcI and the existence of a 100 au disk around it are difficult to reconcile with the model in which SrcI and the nearby Becklin–Neugebauer Object were ejected from a multiple system 500 years ago.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

THE PROPER MOTIONS OF THE DOUBLE RADIO SOURCE n IN THE ORION BN/KL REGION

We have extended the time baseline for observations of the proper motions of radio sources in the Orion BN/KL region from 14.7 to 22.5 years. We present improved determinations for the sources BN and I. In addition, we address the proper motions of the double radio source n, that have been questioned in the literature. We confirm that all three sources are moving away at transverse velocities of tens of kilometers per second from a region in-between them, where they were located about 500 years ago. Source n exhibits a new component that we interpret as due to a one-sided ejection of free–free emitting plasma that took place after 2006.36. We used the highly accurate relative proper motions between sources BN and I to determine that their closest separation took place in the year 1475 ± 6, when they were within ∼100 au or less from each other in the plane of the sky.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

IN-SYNC. V. Stellar Kinematics and Dynamics in the Orion A Molecular Cloud

The kinematics and dynamics of young stellar populations enable us to test theories of star formation. With this aim, we continue our analysis of the SDSS-III/APOGEE IN-SYNC survey, a high-resolution near-infrared spectroscopic survey of young clusters. We focus on the Orion A star-forming region, for which IN-SYNC obtained spectra of ∼2700 stars. In Paper IV we used these data to study the young stellar population. Here we study the kinematic properties through radial velocities (v {sub r}). The young stellar population remains kinematically associated with the molecular gas, following a ∼10 km s{sup −1} gradient along the filament. However, near the center of the region, the v {sub r} distribution is slightly blueshifted and asymmetric; we suggest that this population, which is older, is slightly in the foreground. We find evidence for kinematic subclustering, detecting statistically significant groupings of colocated stars with coherent motions. These are mostly in the lower-density regions of the cloud, while the ONC radial velocities are smoothly distributed, consistent with it being an older, more dynamically evolved cluster. The velocity dispersion σ{sub v} varies along the filament. The ONC appears virialized, or just slightly supervirial, consistent with an old dynamical age. Here there is also some evidence for ongoing expansion, from a v {sub r}-extinction correlation. In the southern filament, σ{sub v} is ∼2–3 times larger than virial in the L1641N region, where we infer a superposition along the line of sight of stellar subpopulations, detached from the gas. In contrast, σ{sub v} decreases toward L1641S, where the population is again in agreement with a virial state.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Extreme Radio Flares and Associated X-Ray Variability from Young Stellar Objects in the Orion Nebula Cluster

Young stellar objects are known to exhibit strong radio variability on timescales of weeks to months, and a few reports have documented extreme radio flares with at least an order of magnitude change in flux density on timescales of hours to days. However, there have been few constraints on the occurrence rate of such radio flares or on the correlation with pre-main sequence X-ray flares, although such correlations are known for the Sun and nearby active stars. Here we report simultaneous deep VLA radio and Chandra X-ray observations of the Orion Nebula Cluster, targeting hundreds of sources to look for the occurrence rate of extreme radio variability and potential correlation with the most extreme X-ray variability. We identify 13 radio sources with extreme radio variability, with some showing an order of magnitude change in flux density in less than 30 minutes. All of these sources show X-ray emission and variability, but we find clear correlations with extreme radio flaring only on timescales <1 hr. Strong X-ray variability does not predict the extreme radio sources and vice versa. Radio flares thus provide us with a new perspective on high-energy processes in YSOs and the irradiation of their protoplanetary disks. Finally, our results highlight implications for interferometric imaging of sources violating the constant-sky assumption.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Kinematics of the Optically Visible YSOs toward the Orion B Molecular Cloud

We present results from high-resolution optical spectra toward 66 young stars in the Orion B molecular cloud to study their kinematics and other properties. Observations of the H α and Li i 6707 Å lines are used to check membership and accretion properties. While the stellar radial velocities of NGC 2068 and L1622 are in good agreement with that of the molecular gas, many of the stars in NGC 2024 show a considerable offset. This could be a signature of either the expansion of the cluster, the high degree of the ejection of the stars from the cluster through dynamical interaction, or the acceleration of the gas due to stellar feedback.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Green Bank Ammonia Survey: Dense Cores under Pressure in Orion A

We use data on gas temperature and velocity dispersion from the Green Bank Ammonia Survey and core masses and sizes from the James Clerk Maxwell Telescope Gould Belt Survey to estimate the virial states of dense cores within the Orion A molecular cloud. Surprisingly, we find that almost none of the dense cores are sufficiently massive to be bound when considering only the balance between self-gravity and the thermal and non-thermal motions present in the dense gas. Including the additional pressure binding imposed by the weight of the ambient molecular cloud material and additional smaller pressure terms, however, suggests that most of the dense cores are pressure-confined.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Stellar Rotation of T Tauri Stars in the Orion Star-forming Complex

We present a large-scale study of stellar rotation for T Tauri stars in the Orion star-forming complex. We use the projected rotational velocity ($v\sin (i)$) estimations reported by the APOGEE-2 collaboration as well as individual masses and ages derived from the position of the stars in the HR diagram, considering Gaia-EDR3 parallaxes and photometry plus diverse evolutionary models. We find an empirical trend for $v\sin (i)$ decreasing with age for low-mass stars (0.4M ⊙ < M $\star$ < 1.2M ⊙ ). Our results support the existence of a mechanism linking $v\sin (i)$ to the presence of accreting protoplanetary disks, responsible for regulating stellar rotation on timescales of about 6 Myr, which is the timescale in which most of the T Tauri stars lose their inner disk. Our results provide important constraints to models of rotation in the early phases of evolution of young stars and their disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The 3D Kinematics of the Orion Nebula Cluster: NIRSPEC-AO Radial Velocities of the Core Population

The kinematics and dynamics of stellar and substellar populations within young, still-forming clusters provide valuable information for constraining theories of formation mechanisms. Using Keck II NIRSPEC+AO data, we have measured radial velocities for 56 low-mass sources within 4' of the core of the Orion Nebula Cluster (ONC). We also remeasure radial velocities for 172 sources observed with SDSS/APOGEE. These data are combined with proper motions measured using HST ACS/WFPC2/WFC3IR and Keck II NIRC2, creating a sample of 135 sources with all three velocity components. The velocities measured are consistent with a normal distribution in all three components. We measure intrinsic velocity dispersions of (${\sigma }_{{v}_{\alpha }}$, ${\sigma }_{{v}_{\delta }}$, ${\sigma }_{{v}_{r}}$) = (1.64 ± 0.12, 2.03 ± 0.13, ${2.56}_{-0.17}^{+0.16}$) km s –1 . Our computed intrinsic velocity dispersion profiles are consistent with the dynamical equilibrium models from Da Rio et al. (2014) in the tangential direction but not in the line-of-sight direction, possibly indicating that the core of the ONC is not yet virialized, and may require a nonspherical potential to explain the observed velocity dispersion profiles. We also observe a slight elongation along the north–south direction following the filament, which has been well studied in previous literature, and an elongation in the line-of-sight to tangential velocity direction. These 3D kinematics will help in the development of realistic models of the formation and early evolution of massive clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A CANDIDATE PLANETARY-MASS OBJECT WITH A PHOTOEVAPORATING DISK IN ORION

In this work, we report the discovery of a candidate planetary-mass object with a photoevaporating protoplanetary disk, Proplyd 133-353, which is near the massive star θ {sup 1} Ori C at the center of the Orion Nebula Cluster (ONC). The object was known to have extended emission pointing away from θ {sup 1} Ori C, indicating ongoing external photoevaporation. Our near-infrared spectroscopic data and the location on the H–R diagram suggest that the central source of Proplyd 133-353 is substellar (∼M9.5) and has a mass probably less than 13 Jupiter mass and an age younger than 0.5 Myr. Proplyd 133-353 shows a similar ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC with a similar stellar luminosity and has a similar proper motion to the mean one of confirmed ONC members. We propose that Proplyd 133-353 formed in a very low-mass dusty cloud or an evaporating gas globule near θ {sup 1} Ori C as a second generation of star formation, which can explain both its young age and the presence of its disk.

79 ASTRONOMY AND ASTROPHYSICS↗