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At least 883 records · Page 49

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DOE OSTI2016

HIGHLY VARIABLE YOUNG MASSIVE STARS IN ATLASGAL CLUMPS

High-amplitude variability in young stellar objects (YSOs) is usually associated with episodic accretion events. It has not been observed so far in massive YSOs. Here, the high-amplitude variable star sample of Contreras Peña et al. has been used to search for highly variable (Δ K ≥ 1 mag) sources coinciding with dense clumps mapped using the 850 μ m continuum emission by the ATLASGAL survey. A total of 18 variable sources are centered on the submillimeter clump peaks and coincide (<1″) with a 24 μ m point or compact (<10″) source. Of these 18 sources, 13 can be fit by YSO models. The 13 variable YSOs (VYSOs) have luminosities of ∼10{sup 3} L {sub ⊙}, an average mass of 8 M {sub ⊙}, and a range of ages up to 10{sup 6} yr. A total of 11 of these 13 VYSOs are located in the midst of infrared dark clouds. Nine of the 13 sources have Δ K > 2 mag, significantly higher compared to the mean variability of the entire VVV sample. The light curves of these objects sampled between 2010 and 2015 display rising, declining, or quasi-periodic behavior but no clear periodicity. Light-curve analysis using the Plavchan method shows that the most prominent phased signals have periods of a few hundred days. The nature and timescale of variations found in 6.7 Ghz methanol maser emission in massive stars are similar to that of the VYSO light curves. We argue that the origin of the observed variability is episodic accretion. We suggest that the timescale of a few hundred days may represent the frequency at which a spiraling disk feeds dense gas to the young massive star.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

The H{sub 2} + CO ↔ H{sub 2}CO reaction: Rate constants and relevance to hot and dense astrophysical media

A theoretical thermochemical and kinetic investigation of the thermal H{sub 2} + CO ↔ H{sub 2}CO reaction was performed for a temperature range from 200 to 4000 K. Geometries and vibrational frequencies of reactants, product, and transition state (TS) were obtained at CCSD/cc-pVxZ (x = T and Q) levels and scaling factors were employed to consider anharmonicity effects on vibrational frequencies, zero-point energies, and thermal corrections provided by these methodologies. Enthalpies Gibbs energies, and rate constants for this reaction were determined by including a complete basis set extrapolation correction for the electronic properties calculated at CCSD(T)/cc-pVyZ (y = Q and 5) levels. Our study indicates that enthalpy changes for this reaction are highly dependent on temperature. Moreover, forward and reverse (high-pressure limit) rate constants were obtained from variational TS theory with quantum tunneling corrections. Thus, modified Arrhenius’ equations were fitted by means of the best forward and reverse rate constant values, which provide very reliable estimates for these quantities within the temperature range between 700 and 4000 K. To our knowledge, this is the first kinetic study done for the forward H{sub 2} + CO → H{sub 2}CO process in a wide temperature range. Finally, these results can be used to explain the formaldehyde abundance in hot and dense interstellar media, possibly providing data about the physical conditions associated with H{sub 2}CO masers close to massive star-forming regions.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2021

The Mass-loss History of the Red Hypergiant VY CMa

Imaging and spectroscopy of the knots, clumps, and extended arcs in the complex ejecta of VY CMa confirm a record of high mass-loss events over the past few hundred years. Hubble Space Telescope/Space Telescope Imaging Spectrograph spectroscopy of numerous small knots close to the star allow us to measure their radial velocities from the strong K i emission and determine their separate motions, spatial orientations, and time since ejecta. Their ages concentrate around 70, 120, 200, and 250 yr ago. A K i emission knot only 50 mas from the star ejected as recently as 1985–1995 may coincide with an H{sub 2}O maser. Comparison with VY CMa’s historic light curve from 1800 to the present shows several knots with ejection times that correspond with extended periods of variability and deep minima. The similarity of this correspondence in VY CMa with the remarkable recent dimming of Betelgeuse and an outflow of gas is apparent. The evidence for similar outflows from the surface of a more typical red supergiant suggests that discrete ejections are more common and surface or convective activity is a major source of mass loss for red supergiants.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2016

High-resolution observations of molecular lines in ARP 220: kinematics, morphology, and limits on the applicability of the ammonia thermometer

We observe Arp 220, the nearest ultra-luminous infrared galaxy, over 4 GHz in the K and Ka bands, providing constraints for the kinematics and morphology, and identifying molecular species on scales resolving both nuclei (0.″6 or 230 pc). We detect multiple molecular species, including hydroxyl (OH {sup 2}Π{sub 3/2}J=9/2F=4−4;5−5) in both cores, and tentatively detect H{sub 2}O(6{sub 15}-5{sub 23}) at ∼21.84 GHz in both nuclei, indicating the likely presence of maser emission. The observed frequency range also contains metastable ammonia transitions from (J, K) = (1, 1)–(5, 5), as well as the (9, 9) inversion line; together, they are a well-known thermometer of dense molecular gas. Furthermore, the non-metastable (4, 2) and (10, 9), and possibly the (3, 1), lines are also detected. We apply a standard temperature analysis to Arp 220; however, the analysis is complicated in that standard local thermal equilibrium (LTE) assumptions do not hold. There are indications that a substantial fraction of ammonia could be in the non-metastable transitions, as opposed to only the metastable ones. Thus, the non-metastable transitions could be essential to constraining the temperature. We compare all of these data to ALMA observations of this source, confirming the outflow previously observed by other tracers in both nuclei.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

The Relation Between Globular Cluster Systems and Supermassive Black Holes in Spiral Galaxies: The Case Study of NGC 4258

We aim to explore the relationship between globular cluster total number, N{sub GC}, and central black hole mass, M {sub •}, in spiral galaxies, and compare it with that recently reported for ellipticals. We present results for the Sbc galaxy NGC 4258, from Canada–France–Hawaii Telescope data. Thanks to water masers with Keplerian rotation in a circumnuclear disk, NGC 4258 has the most precisely measured extragalactic distance and supermassive black hole mass to date. The globular cluster (GC) candidate selection is based on the (u{sup ∗}−i{sup ′}) versus (i{sup ′}−K{sub s}) diagram, which is a superb tool to distinguish GCs from foreground stars, background galaxies, and young stellar clusters, and hence can provide the best number counts of GCs from photometry alone, virtually free of contamination, even if the galaxy is not completely edge-on. The mean optical and optical-near-infrared colors of the clusters are consistent with those of the Milky Way and M 31, after extinction is taken into account. We directly identify 39 GC candidates; after completeness correction, GC luminosity function extrapolation, and correction for spatial coverage, we calculate a total N{sub GC}=144±31{sub −36}{sup +38} (random and systematic uncertainties, respectively). We have thus increased to six the sample of spiral galaxies with measurements of both M {sub •} and N{sub GC}. NGC 4258 has a specific frequency S{sub N}=0.4±0.1 (random uncertainty), and is consistent within 2σ with the N{sub GC} versus M {sub •} correlation followed by elliptical galaxies. The Milky Way continues to be the only spiral that deviates significantly from the relation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

The Lifetimes of Phases in High-mass Star-forming Regions

High-mass stars form within star clusters from dense, molecular regions (DMRs), but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of DMRs above a column density associated with high-mass star formation, N(H{sub 2}) > 0.4–2.5 × 10{sup 22} cm{sup −2}, in the “starless” (no signature of stars ≳10 M{sub ⊙} forming) and star-forming phases in a 2° × 2° region of the Galactic Plane centered at ℓ = 30°. Of regions capable of forming high-mass stars on ∼1 pc scales, the starless (or embedded beyond detection) phase occupies about 60%–70% of the DMR lifetime, and the star-forming phase occupies about 30%–40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetime of the starless phase to be 0.2–1.7 Myr (about 0.6–4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1–0.7 Myr (about 0.4–2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high column density gas pixel by pixel, and our results demonstrate that the majority of high column density gas is in a starless or embedded phase.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Occultations of Astrophysical Radio Sources as Probes of Planetary Environments: A Case Study of Jupiter and Possible Applications to Exoplanets

Properties of planetary atmospheres, ionospheres, and magnetospheres are difficult to measure from Earth. Radio occultations are a common method for measuring these properties, but they traditionally rely on radio transmissions from a spacecraft near the planet. Here, we explore whether occultations of radio emissions from a distant astrophysical radio source can be used to measure magnetic field strength, plasma density, and neutral density around planets. In a theoretical case study of Jupiter, we find that significant changes in polarization angle due to Faraday rotation occur for radio signals that pass within 10 Jupiter radii of the planet and that significant changes in frequency and power occur from radio signals that pass through the neutral atmosphere. There are sufficient candidate radio sources, such as pulsars, active galactic nuclei, and masers, that occultations are likely to occur at least once per year. For pulsars, time delays in the arrival of their emitted pulses can be used to measure plasma density. Exoplanets, whose physical properties are very challenging to observe, may also occult distant astrophysical radio sources, such as their parent stars.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Inferring the Composition of Super-Jupiter Mass Companions of Pulsars with Radio Line Spectroscopy

We propose using radio line spectroscopy to detect molecular absorption lines (such as OH at 1.6–1.7 GHz) before and after the total eclipse of black widow and other short orbital period binary pulsars with low-mass companions. The companion in such a binary may be ablated away by energetic particles and high-energy radiation produced by the pulsar wind. The observations will probe the eclipsing wind being ablated by the pulsar and constrain the nature of the companion and its surroundings. Maser emission from the interstellar medium stimulated by a pulsar beam might also be detected from the intrabinary medium. The short temporal resolution allowed by the millisecond pulsars can probe this medium with the high angular resolution of the pulsar beam.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

CTIO, ROSAT HRI, and Chandra ACIS Observations of the Archetypical Mixed-morphology Supernova Remnant W28 (G6.4–0.1)

We present a joint analysis of optical emission-line and X-ray observations of the archetypical Galactic mixed-morphology supernova remnant (MMSNR) W28 (G6.4–0.1). MMSNRs comprise a class of sources whose shell-like radio morphology contrasts with a filled center in X-rays; the origin of these contrasting morphologies remains uncertain. Our CTIO images reveal enhanced [S ii] emission relative to H α along the northern and eastern rims of W28. Hydroxyl (OH) masers are detected along these same rims, supporting prior studies suggesting that W28 is interacting with molecular clouds at these locations, as observed for several other MMSNRs. Our ROSAT HRI mosaic of W28 provides almost complete coverage of the supernova remnant (SNR). The X-ray and radio emission is generally anti-correlated, except for the luminous northeastern rim, which is prominent in both bands. Our Chandra observation sampled the X-ray-luminous central diffuse emission. Spectra extracted from the bright central peak and from nearby annular regions are best fit with two overionized recombining plasma models. We also find that while the X-ray emission from the central peak is dominated by swept-up material, that from the surrounding regions shows evidence for oxygen-rich ejecta, suggesting that W28 was produced by a massive progenitor. We also analyze the X-ray properties of two X-ray sources (CXOU J175857.55−233400.3 and 3XMM J180058.5–232735) projected into the interior of W28 and conclude that neither is a neutron star associated with the SNR. The former is likely to be a foreground cataclysmic variable or a quiescent low-mass X-ray-binary, while the latter is likely to be a coronally active main-sequence star.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Young Stellar Objects in the Massive Star-forming Regions W51 and W43

We present the results of our investigation of the star-forming complexes W51 and W43, two of the brightest in the first Galactic quadrant. In order to determine the young stellar object (YSO) populations in W51 and W43 we used color–magnitude relations based on Spitzer mid-infrared and 2MASS/UKIDSS near-infrared data. We identified 302 Class I YSOs and 1178 Class II/transition disk candidates in W51, and 917 Class I YSOs and 5187 Class II/transition disk candidates in W43. We also identified tens of groups of YSOs in both regions using the Minimal Spanning Tree (MST) method. We found similar cluster densities in both regions, even though Spitzer was not able to probe the densest part of W43. By using the Class II/I ratios, we traced the relative ages within the regions and, based on the morphology of the clusters, we argue that several sites of star formation are independent of one another in terms of their ages and physical conditions. We used spectral energy distribution-fitting to identify the massive YSO (MYSO) candidates since they play a vital role in the star formation process, and then examined them to see if they are related to any massive star formation tracers such as UCH ii regions, masers, or dense fragments. We identified 17 MYSO candidates in W51, and 14 in W43, respectively, and found that groups of YSOs hosting MYSO candidates are positionally associated with H ii regions in W51, though we do not see any MYSO candidates associated with previously identified massive dense fragments in W43.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Persistence of Precursor Waves in Two-dimensional Relativistic Shocks

We investigated the efficiency of coherent upstream large-amplitude electromagnetic wave emission via synchrotron maser instability in relativistic magnetized shocks using two-dimensional particle-in-cell simulations. We considered a purely perpendicular shock in an electron–positron plasma. The coherent wave emission efficiency was measured as a function of the magnetization parameter σ , which is defined as the ratio of the Poynting flux to the kinetic energy flux. The wave amplitude was systematically smaller than that observed in one-dimensional simulations. However, it continued to persist, even at a considerably low magnetization rate, where the Weibel instability dominated the shock transition. The emitted electromagnetic waves were sufficiently strong to disturb the upstream medium, and transverse filamentary density structures of substantial amplitude were produced. Based on this result, we discuss the possibility of the wakefield acceleration model to produce nonthermal electrons in a relativistic magnetized ion–electron shock.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

On the Origin of Fast Radio Bursts (FRBs)

We derive stringent constraints on the persistent source associated with FRB 121102: size 0.3<R{sub 17.5} =(R/10{sup 17.5} cm)<3, age <10{sup 2.5} year, energy E≈10{sup 49}(ε{sub e}/0.2GeV){sup 3} erg, characteristic electron energy 0.1⩽ε{sub e}/1 GeV⩽0.5; the radiating plasma is confined by a cold plasma of mass M{sub c}<10{sup −1.5}R{sub 17.5}{sup 4} M{sub ⊙}; these properties are inconsistent with typical “magnetar wind nebulae” model predictions. The fact that ε{sub e}∼m{sub p}c{sup 2} suggests that the hot plasma was created by the ejection of a mildly relativistic, M≈E/c{sup 2}≈10{sup −5} M{sub ⊙} shell, which propagated into an extended ambient medium or collided with a pre-ejected shell. Independent of the persistent source model, we suggest a physical mechanism for the generation of fast radio bursts (FRBs): the ejection from an underlying compact object, R{sub s}=10{sup 6}R{sub s,6} cm, of highly relativistic shells with energy E{sub s}=10{sup 41}E{sub 41} erg and Lorentz factor γ{sub s}=10{sup 3}E{sub 41}{sup 1/8}R{sub s,6}{sup −3/8}, into a surrounding e − p plasma with density n∼10{sup −1} cm{sup −3} (consistent with that inferred for the persistent source). For E {sub s} similar to observed FRB energies, plasma conditions appropriate for strong synchrotron maser emission at ν{sub coh.}≈0.5E{sub 41}{sup 1/4}R{sub s,6}{sup −3/4} GHz are formed. A significant fraction of the deposited energy is converted to an FRB with duration R{sub s}/c, accompanied by ∼10 MeV gamma-rays carrying less energy than the FRB. The inferred energy and mass associated with the source suggest some type of a “weak stellar explosion,” where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out M{sub c}∼1 M{sub ⊙}, or the ejection of a larger mass well before the ejection of the confining shell.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Monitor Variability of Millimeter Lines in IRC+10216

A single dish monitoring of millimeter maser lines SiS J = 14-13 and HCN ν{sub 2}=1{sup f} J = 3-2 and several other rotational lines is reported for the archetypal carbon star IRC+10216. Relative line strength variations of 5% ∼ 30% are found for eight molecular line features with respect to selected reference lines. Definite line-shape variations are found in limited velocity intervals of the SiS and HCN line profiles. The asymmetrical line profiles of the two lines are mainly due to the varying components. The dominant varying components of the line profiles have similar periods and phases to the IR light variation, though both quantities show some degree of velocity dependence; there is also variability asymmetry between the blue and red line wings of both lines. Combining the velocities and amplitudes with a wind velocity model, we suggest that the line profile variations are due to SiS and HCN masing lines emanating from the wind acceleration zone. The possible link of the variabilities to thermal, dynamical, and/or chemical processes within or under this region is also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

A Flaring Magnetar in FRB 121102?

The persistent radio counterpart of fast radio burst (FRB) 121102 is estimated to have N∼10{sup 52} particles, energy E{sub N}∼10{sup 48} erg, and size R∼10{sup 17} cm. The source can be nebula inflated and heated by an intermittent outflow from a magnetar—a neutron star powered by its magnetic (rather than rotational) energy. The object is young and frequently liberating energy in magnetic flares driven by accelerated ambipolar diffusion in the neutron star core, feeding the nebula and producing bright millisecond bursts. The particle number in the nebula is consistent with ion ejecta from giant flares. The nebula may also contain the freeze-out of electron–positron pairs N{sub ±}∼10{sup 51} created months after the neutron star birth; the same mechanism offers an explanation for N{sub ±} in the Crab Nebula. The persistent source around FRB 121102 is likely heated by magnetic dissipation and internal waves excited by the magnetar ejecta. The volumetric heating by waves explains the nebula’s enormous efficiency in producing radio emission. The repeating radio bursts are suggested to occur much closer to the magnetar, as a result of ultrarelativistic internal shocks in the magnetar wind, which are launched by the magnetospheric flares. The shocks are mediated by Larmor rotation, which forms a GHz maser with the observed ms duration. Furthermore, the flare ejecta can become charge-starved and then convert to electromagnetic waves.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1963