arXiv subjects

Neal J. Evans II

Publications and source records attributed to Neal J. Evans II.

At least 91 records · Page 5Linked to original sources

arXiv2016

The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds

We present the results of the Gould's Belt Distances Survey (GOBELINS) of young star forming regions towards the Orion Molecular Cloud Complex. We detected 36 YSOs with the Very Large Baseline Array (VLBA), 27 of which have been observed in at least 3 epochs over the course of 2 years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions towards these stars to study the structure and kinematics of the Complex. We measured a distance of 388$\pm$5 pc towards the Orion Nebula Cluster, 428$\pm$10 pc towards the southern portion L1641, 388$\pm$10 pc towards NGC 2068, and roughly $\sim$420 pc towards NGC 2024. Finally, we observed a strong degree of plasma radio scattering towards $λ$ Ori.

astro-ph.SR↗

arXiv2016

The Mid-Infrared Evolution of the FU Orionis Disk

We present new SOFIA-FORCAST observations obtained in Feburary 2016 of the archetypal outbursting low mass young stellar object FU Orionis, and compare the continuum, solid state, and gas properties with mid-IR data obtained at the same wavelengths in 2004 with Spitzer-IRS. In this study, we conduct the first mid-IR spectroscopic comparison of an FUor over a long time period. Over a 12 year period, UBVR monitoring indicates that FU Orionis has continued its steady decrease in overall brightness by ~ 14%. We find that this decrease in luminosity occurs only at wavelengths < 20 microns. In particular, the continuum short ward of the silicate emission complex at 10 microns exhibits a ~ 12% (~ 3 sigma) drop in flux density, but no apparent change in slope; both the Spitzer and SOFIA spectra are consistent with a 7200 K blackbody. Additionally, the detection of water absorption is consistent with the Spitzer spectrum. The silicate emission feature at 10 microns continues to be consistent with unprocessed grains, unchanged over 12 years. We conclude that either the accretion rate in FU Orionis has decreased by ~ 12-14% over this time baseline, or that the inner disk has cooled, but the accretion disk remains in a superheated state outside of the innermost region.

astro-ph.SR↗

arXiv2016

Star Formation Relations in the Milky Way

The relations between star formation and properties of molecular clouds are studied based on a sample of star forming regions in the Galactic Plane. Sources were selected by having radio recombination lines to provide identification of associated molecular clouds and dense clumps. Radio continuum and mid-infrared emission were used to determine star formation rates, while 13CO and submillimeter dust continuum emission were used to obtain masses of molecular and dense gas, respectively. We test whether total molecular gas or dense gas provides the best predictor of star formation rate. We also test two specific theoretical models, one relying on the molecular mass divided by the free-fall time, the other using the free-fall time divided by the crossing time. Neither is supported by the data. The data are also compared to those from nearby star forming regions and extragalactic data. The star formation "efficiency," defined as star formation rate divided by mass, spreads over a large range when the mass refers to molecular gas; the standard deviation of the log of the efficiency decreases by a factor of three when the mass of relatively dense molecular gas is used rather than the mass of all the molecular gas.

astro-ph.SR↗

arXiv2016

The abundance and thermal history of water ice in the disk surrounding HD142527 from the DIGIT Herschel Key Program

The presence or absence of ice in protoplanetary disks is of great importance for the formation of planets. By enhancing the solid surface density and increasing the sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the time scale for giant planet core accretion. Aims: In this paper we analyse the composition of the outer disk around the Herbig star HD~142527. We focus on the composition of the water ice, but also analyse the abundances of previously proposed minerals. Methods: We present new Herschel far infrared spectra and a re-reduction of archival data from the Infrared Space Observatory (ISO). We model the disk using full 3D radiative transfer to obtain the disk structure. Also, we use an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component. Results: The water ice in the disk around HD~142527 contains a large reservoir of crystalline water ice. We determine the local abundance of water ice in the outer disk (i.e. beyond 130\,AU). The re-reduced ISO spectrum differs significantly from that previously published, but matches the new Herschel spectrum at their common wavelength range. In particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims. Conclusions: The amount of water ice detected in the outer disk requires $\sim80\,$\% of the oxygen atoms. This is comparable to the water ice abundance in the outer solar system, in comets and in dense interstellar clouds. The water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant time scales. We discuss the implications of this finding.

astro-ph.EP↗

arXiv2016

The inflow signature toward different evolutionary phases of massive star formation

We analyzed both HCN J=1-0 and HNC J=1-0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar object (HMPOs), and 54 ultra-compact HII regions (UCHIIs). The inflow asymmetry in HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in blue profile. In the case of HNC spectra, the prevalence of blue sources does not appear, excepting for IRDCs. We suggest that this line is not appropriate to trace inflow motion in evolved stages of massive star formation because the abundance of HNC decreases at high temperatures. This result spotlights the importance of considering chemistry in the dynamics study of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also found that the absorption dips of the HNC spectra in 6 out of 7 blue sources are red-shifted relative to their systemic velocities. These red-shifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.

astro-ph.GA↗

arXiv2016

A Catalog of Low-Mass Star-Forming Cores Observed with SHARC-II at 350 microns

We present a catalog of low-mass dense cores observed with the SHARC-II instrument at 350 microns. Our observations have an effective angular resolution of 10", approximately 2.5 times higher than observations at the same wavelength obtained with the Herschel Space Observatory, albeit with lower sensitivity, especially to extended emission. The catalog includes 81 maps covering a total of 164 detected sources. For each detected source, we tabulate basic source properties including position, peak intensity, flux density in fixed apertures, and radius. We examine the uncertainties in the pointing model applied to all SHARC-II data and conservatively find that the model corrections are good to within ~3", approximately 1/3 of the SHARC-II beam. We examine the differences between two array scan modes and find that the instrument calibration, beam size, and beam shape are similar between the two modes. We also show that the same flux densities are measured when sources are observed in the two different modes, indicating that there are no systematic effects introduced into our catalog by utilizing two different scan patterns during the course of taking observations. We find a detection rate of 95% for protostellar cores but only 45% for starless cores, and demonstrate the existence of a SHARC-II detection bias against all but the most massive and compact starless cores. Finally, we discuss the improvements in protostellar classification enabled by these 350 micron observations.

astro-ph.GA↗

arXiv2016

Probing Planet Forming Zones with Rare CO Isotopologues

The gas near the midplanes of planet-forming protostellar disks remains largely unprobed by observations due to the high optical depth of commonly observed molecules such as CO and H$_2$O. However, rotational emission lines from rare molecules may have optical depths near unity in the vertical direction, so that the lines are strong enough to be detected, yet remain transparent enough to trace the disk midplane. Here we present a chemical model of an evolving T-Tauri disk and predict the optical depths of rotational transitions of $^{12}$C$^{16}$O, $^{13}$C$^{16}$O, $^{12}$C$^{17}$O and $^{12}$C$^{18}$O. The MRI-active disk is primarily heated by the central star due to the formation of the dead zone. CO does not freeze out in our modeled region within $70$AU around a sunlike star. However, the abundance of CO decreases because of the formation of complex organic molecules (COM), producing an effect that can be misinterpreted as the "snow line". These results are robust to variations in our assumptions about the evolution of the gas to dust ratio. The optical depths of low-order rotational lines of C$^{17}$O are around unity, making it possible to see into the disk midplane using C$^{17}$O. Combining observations with modeled C$^{17}$O$/$H$_2$ ratios, like those we provide, can yield estimates of protoplanetary disks' gas masses.

astro-ph.EP↗

arXiv2016

The CDF Archive: Herschel PACS and SPIRE Spectroscopic Data Pipeline and Products for Protostars and Young Stellar Objects

We present the COPS-DIGIT-FOOSH (CDF) Herschel spectroscopy data product archive, and related ancillary data products, along with data fidelity assessments, and a user-created archive in collaboration with the Herschel-PACS and SPIRE ICC groups. Our products include datacubes, contour maps, automated line fitting results, and best 1-D spectra products for all protostellar and disk sources observed with PACS in RangeScan mode for two observing programs: the DIGIT Open Time Key Program (KPOT_nevans_1 and SDP_nevans_1; PI: N. Evans), and the FOOSH Open Time Program (OT1_jgreen02_2; PI: J. Green). In addition, we provide our best SPIRE-FTS spectroscopic products for the COPS Open Time Program (OT2_jgreen02_6; PI: J. Green) and FOOSH sources. We include details of data processing, descriptions of output products, and tests of their reliability for user applications. We identify the parts of the dataset to be used with caution. The resulting absolute flux calibration has improved in almost all cases. Compared to previous reductions, the resulting rotational temperatures and numbers of CO molecules have changed substantially in some sources. On average, however, the rotational temperatures have not changed substantially (< 2%), but the number of warm (Trot ~ 300 K) CO molecules has increased by about 18%.

astro-ph.IM↗

arXiv2015

Infrared emission from tidal disruption events --- probing the pc-scale dust content around galactic nuclei

Recent UV-optical surveys have been successful in finding tidal disruption events (TDEs), in which a star is tidally disrupted by a supermassive black hole (BH). These TDEs release a huge amount of radiation energy ~ 10^51-52 erg into the circum-nuclear medium. If the medium is dusty, most of the radiation energy will be absorbed by dust grains within ~ 1 pc from the BH and re-radiated in the infrared. We calculate the dust emission lightcurve from a 1-D radiative transfer model, taking into account the time-dependent heating, cooling and sublimation of dust grains. We show that the dust emission peaks at 3-10 microns and has typical luminosities ~ 10^42-43 erg/s (with sky covering factor of dusty clouds ranging from 0.1-1). This is detectable by current generation of telescopes. In the near future, James Webb Space Telescope will be able to perform photometric and spectroscopic measurements, in which silicate or polycyclic aromatic hydrocarbon (PAH) features may be found. Observations at rest-frame wavelength > 2 microns have only been reported from two TDE candidates, SDSS J0952+2143 and Swift J1644+57. Although consistent with the dust emission from TDEs, the mid-infrared fluxes of the two events may be from other sources. Long-term monitoring is needed to draw a firm conclusion. We also point out two nearby TDE candidates (ASSASN-14ae and -14li) where the dust emission may be currently detectable. The dust infrared emission can give a snapshot of the pc-scale dust content around weakly- or non-active galactic nuclei, which is hard to probe otherwise.

astro-ph.HE↗

arXiv2015

Detection of Infall in the Protostar B335 with ALMA

Observations of the isolated globule B335 with ALMA have yielded absorption features against the continuum that are redshifted from the systemic velocity in both HCN and HCO$^+$ lines. These features provide unambiguous evidence for infall toward a central luminosity source. Previously developed models of inside-out collapse can match the observed line profiles of HCN and HCO$^+$ averaged over the central 50 AU. At the new distance of 100 pc, the inferred infall radius is 0.012 pc, the mass infall rate is $3 \times 10^{-6}$ solar masses per year, the age is 50,000 years, and the accumulated mass in the central zone is 0.15 solar masses, most of which must be in the star or in parts of a disk that are opaque at 0.8 mm. The continuum detection indicates an optically thin mass (gas and dust) of only $7.5\times 10^{-4}$ solar masses in the central region, consistent with only a very small disk mass.

astro-ph.GA↗

arXiv2015

Young Stellar Objects in the Gould Belt

We present the full catalog of Young Stellar Objects (YSOs) identified in the 18 molecular clouds surveyed by the Spitzer Space Telescope "cores to disks" (c2d) and "Gould Belt" (GB) Legacy surveys. Using standard techniques developed by the c2d project, we identify 3239 candidate YSOs in the 18 clouds, 2966 of which survive visual inspection and form our final catalog of YSOs in the Gould Belt. We compile extinction corrected SEDs for all 2966 YSOs and calculate and tabulate the infrared spectral index, bolometric luminosity, and bolometric temperature for each object. We find that 326 (11%), 210 (7%), 1248 (42%), and 1182 (40%) are classified as Class 0+I, Flat-spectrum, Class II, and Class III, respectively, and show that the Class III sample suffers from an overall contamination rate by background AGB stars between 25% and 90%. Adopting standard assumptions, we derive durations of 0.40-0.78 Myr for Class 0+I YSOs and 0.26-0.50 Myr for Flat-spectrum YSOs, where the ranges encompass uncertainties in the adopted assumptions. Including information from (sub)millimeter wavelengths, one-third of the Class 0+I sample is classified as Class 0, leading to durations of 0.13-0.26 Myr (Class 0) and 0.27-0.52 Myr (Class I). We revisit infrared color-color diagrams used in the literature to classify YSOs and propose minor revisions to classification boundaries in these diagrams. Finally, we show that the bolometric temperature is a poor discriminator between Class II and Class III YSOs.

astro-ph.GA↗

arXiv2015

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XII. The Perseus YSO Population as Observed with IRAC and MIPS

The Spitzer Space Telescope mapped the Perseus molecular cloud complex with IRAC and MIPS as part of the c2d Spitzer Legacy project. This paper combines the observations from both instruments giving an overview of low-mass star formation across Perseus from 3.6 to 70 micron. We provide an updated list of young stellar objects with new classifications and source fluxes from previous works, identifying 369 YSOs in Perseus with the Spitzer dataset. By synthesizing the IRAC and MIPS maps of Perseus and building on the work of previous papers in this series (Jorgensen et al. 2006, Rebull et al. 2007), we present a current census of star formation across the cloud and within smaller regions. 67% of the YSOs are associated with the young clusters NGC 1333 and IC 348. The majority of the star formation activity in Perseus occurs in the regions around the clusters, to the eastern and western ends of the cloud complex. The middle of the cloud is nearly empty of YSOs despite containing regions of high visual extinction. The western half of Perseus contains three-quarters of the total number of embedded YSOs (Class 0+I and Flat SED sources) in the cloud and nearly as many embedded YSOs as Class II and III sources. Class II and III greatly outnumber Class 0+I objects in eastern Perseus and IC 348. These results are consistent with previous age estimates for the clusters. Across the cloud, 56% of YSOs and 91% of the Class 0+I and Flat sources are in areas where Av > 5 mag, indicating a possible extinction threshold for star formation.

astro-ph.SR↗

arXiv2015

Evidence for Decay of Turbulence by MHD Shocks in Molecular Clouds via CO Emission

We utilize observations of sub-millimeter rotational transitions of CO from a Herschel Cycle 2 open time program ("COPS", PI: J. Green) to identify previously predicted turbulent dissipation by magnetohydrodynamic (MHD) shocks in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar. Two models fit about equally well: model 1 has a density of 10$^{3}$ cm$^{-3}$, a shock velocity of 3 km s$^{-1}$, and a magnetic field strength of 4 $μ$G; model 2 has a density of 10$^{3.5}$ cm$^{-3}$, a shock velocity of $2$ km s$^{-1}$, and a magnetic field strength of 8 $μ$G. Timescales for decay of turbulence in this region are comparable to crossing times. Transitions of CO up to $J$ of 8, observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially-distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars. We find significant agreement with predictions of models of turbulent dissipation in slightly denser (10$^{3.5}$ cm$^{-3}$) material with a stronger magnetic field (24 $μ$G) than in the general molecular cloud.

astro-ph.GA↗

arXiv2015

The Bolocam Galactic Plane Survey. XIII. Physical Properties and Mass Functions of Dense Molecular Cloud Structures

We use the distance probability density function (DPDF) formalism of Ellsworth-Bowers et al. (2013, 2015) to derive physical properties for the collection of 1,710 Bolocam Galactic Plane Survey (BGPS) version 2 sources with well-constrained distance estimates. To account for Malmquist bias, we estimate that the present sample of BGPS sources is 90% complete above 400 $M_\odot$ and 50% complete above 70 $M_\odot$. The mass distributions for the entire sample and astrophysically motivated subsets are generally fitted well by a lognormal function, with approximately power-law distributions at high mass. Power-law behavior emerges more clearly when the sample population is narrowed in heliocentric distance (power-law index $α= 2.0\pm0.1$ for sources nearer than 6.5 kpc and $α= 1.9\pm0.1$ for objects between 2 kpc and 10 kpc). The high-mass power-law indices are generally $1.85 \leq α\leq 2.05$ for various subsamples of sources, intermediate between that of giant molecular clouds and the stellar initial mass function. The fit to the entire sample yields a high-mass power-law $\hatα = 1.94_{-0.10}^{+0.34}$. Physical properties of BGPS sources are consistent with large molecular cloud clumps or small molecular clouds, but the fractal nature of the dense interstellar medium makes difficult the mapping of observational categories to the dominant physical processes driving the observed structure. The face-on map of the Galactic disk's mass surface density based on BGPS dense molecular cloud structures reveals the high-mass star-forming regions W43, W49, and W51 as prominent mass concentrations in the first quadrant. Furthermore, we present a 0.25-kpc resolution map of the dense gas mass fraction across the Galactic disk that peaks around 5%.

astro-ph.GA↗

arXiv2015

The Bolocam Galactic Plane Survey. XI. Temperatures and Substructure of Galactic Clumps Based on 350 micron Observations

We present 107 maps of continuum emission at 350 microns from Galactic molecular clumps. Observed sources were mainly selected from the Bolocam Galactic Plane Survey (BGPS) catalog, with 3 additional maps covering star forming regions in the outer Galaxy. The higher resolution of the SHARC-II images (8.5" beam) compared with the 1.1 mm images from BGPS (33" beam) allowed us to identify a large population of smaller substructures within the clumps. A catalog is presented for the 1386 sources extracted from the 350 micron maps. The color temperature distribution of clumps based on the two wavelengths has a median of 13.3 K and mean of 16.3 +- 0.4 K, assuming an opacity law index of 1.7. For the structures with good determination of color temperatures, the mean ratio of gas temperature, determined from NH3 observations, to dust color temperature is 0.88 and the median ratio is 0.76. About half the clumps have more than two substructures and 22 clumps have more than 10. The fraction of the mass in dense substructures seen at 350 microns compared to the mass of their parental clump is ~0.19, and the surface densities of these substructures are, on average, 2.2 times those seen in the clumps identified at 1.1 mm. For a well-characterized sample, 88 structures (31%) exceed a surface density of 0.2 g cm^(-2), and 18 (6%) exceed 1.0 g cm^(-2), thresholds for massive star formation suggested by theorists.

astro-ph.GA↗

arXiv2015

The Gould Belt 'MISFITS' Survey: The Real Solar Neighborhood Protostars

We present an HCO$^{+}$ J=3-2 survey of Class 0+I and Flat SED young stellar objects (YSOs) found in the Gould Belt clouds by surveys with Spitzer. Our goal is to provide a uniform Stage 0+I source indicator for these embedded protostar candidates. We made single point HCO$^{+}$ J = 3-2 measurements toward the source positions at the CSO and APEX of 546 YSOs (89% of the Class 0+I + Flat SED sample). Using the criteria from van Kempen et al. (2009), we classify sources as Stage 0+I or bona fide protostars and find that 84% of detected sources meet the criteria. We recommend a timescale for the evolution of Stage 0+I (embedded protostars) of 0.54 Myr. We find significant correlations of HCO$^{+}$ integrated intensity with $α$ and $T_{bol}$ but not with $L_{bol}$. The detection fraction increases smoothly as a function of $α$ and $L_{bol}$, while decreasing smoothly with $T_{bol}$. Using the Stage 0+I sources tightens the relation between protostars and high extinction regions of the cloud; 89% of Stage I sources lie in regions with $A_{V}$ >8 mag. Class 0+I and Flat SED YSOs that are not detected in HCO$^{+}$ have, on average, a factor of ~2 higher $T_{bol}$ and a factor of ~5 lower $L_{bol}$ than YSOs with HCO$^{+}$ detections. We find less YSO contamination, defined as the number of undetected YSOs divided by the total number surveyed, for sources with $T_{bol}\lesssim$ 600 K and $L_{bol} \gtrsim$ 1 $L_{\odot}$. The contamination percentage is >90% at $A_{V}$< 4 mag and decreases as $A_{V}$ increases.

astro-ph.GA↗

arXiv2015

Infrared and Radio observations of a small group of protostellar objects in the molecular core, L1251-C

We present a multi-wavelength observational study of a low-mass star-forming region, L1251-C, with observational results at wavelengths from the near-infrared to the millimeter. Spitzer Space Telescope observations confirmed that IRAS 22343+7501 is a small group of protostellar objects. The extended emission to east-west direction with its intensity peak at the center of L1251A has been detected at 350 and 850 micron with the CSO and JCMT telescopes, tracing dense envelope materials around L1251A. The single-dish data from the KVN and TRAO telescopes show inconsistencies between the intensity peaks of several molecular line emission and that of the continuum emission, suggesting complex distributions of molecular abundances around L1251A. The SMA interferometer data, however, show intensity peaks of CO 2-1 and 13CO 2-1 located at the position of IRS 1, which is both the brightest source in IRAC image and the weakest source in the 1.3 mm dust continuum map. IRS 1 is the strongest candidate for the driving source of the newly detected compact CO 2-1 outflow. Over the whole region (14' by 14') of L125l-C, 3 Class I and 16 Class II sources have been detected, including three YSOs in L1251A. A comparison with the average projected distance among 19 YSOs in L1251-C and that among 3 YSOs in L1251A suggests L1251-C is an example of low-mass cluster formation, where protostellar objects are forming in a small group.

astro-ph.SR↗

arXiv2015

"Dust, Ice, and Gas In Time" (DIGIT) Herschel Observations of GSS30-IRS1 in Ophiuchus

As a part of the "Dust, Ice, and Gas In Time" (DIGIT) key program on Herschel, we observed GSS30-IRS1, a Class I protostar located in Ophiuchus (d = 120 pc), with Herschel/Photodetector Array Camera and Spectrometer (PACS). More than 70 lines were detected within a wavelength range from 50 micron to 200 micron, including CO, H2O, OH, and two atomic [O I] lines at 63 and 145 micron. The [C II] line, known as a tracer of externally heated gas by the interstellar radiation field, is also detected at 158 micron. All lines, except [O I] and [C II], are detected only at the central spaxel of 9.4" X 9.4". The [O I] emissions are extended along a NE-SW orientation, and the [C II] line is detected over all spaxels, indicative of external PDR. The total [C II] intensity around GSS30 reveals that the far-ultraviolet radiation field is in the range of 3 to 20 G0, where G0 is in units of the Habing Field, 1.6 X 10^{-3} erg cm^{-2} s^{-1}. This enhanced external radiation field heats the envelope of GSS30-IRS1, causing the continuum emission to be extended, unlike the molecular emission. The best-fit continuum model of GSS30-IRS1 with the physical structure including flared disk, envelope, and outflow shows that the internal luminosity is 10 Lsun, and the region is externally heated by a radiation field enhanced by a factor of 130 compared to the standard local interstellar radiation field.

astro-ph.SR↗