arXiv subjects

William D. Langer

Publications and source records attributed to William D. Langer.

17 recordsLinked to original sources

arXiv2025

An 18-25 GHz spectroscopic survey of dense cores in the Chamaeleon I molecular cloud

We extend the survey for organics in the southern hemisphere by observing two cores in the Chamaeleon complex using NASA's Deep Space Network 70-m antenna in Canberra, Australia, over the frequency range of 18 to 25 GHz. We surveyed the class 0 protostar Cha-MMS1 and the prestellar core Cha-C2, which represent two stages in the evolution of dense cores. We detect several molecules including HC$_3$N, HC$_5$N, C$_4$H, CCS, C$_3$S, NH$_3$, and c-C$_3$H$_2$. A longer cyanopolyyne, HC$_7$N, is detected with high confidence via spectral stacking analysis. While molecular column densities in the two Chamaeleon cores are typically an order of magnitude lower compared to the cynaopolyyne peak in TMC-1, the molecular abundance ratios are in general agreement with the TMC-1 values. The two exceptions are c-C$_3$H$_2$, which is enhanced by a factor of \about 25 with respect to cyanopolyynes in the Chamaeleon cores, and ammonia, which is enhanced by a factor of ~ 125. The deuterated species c-C$_3$HD is detected in both cores, with a high D/H ratio of ~0.23 in c-C$_3$H$_2$. A rare isotopologue of ammonia, $^{15}$NH$_3$, is also detected in Cha-MMS1 suggesting a high $^{14}$N/$^{15}$N ratio of ~ 690 in ammonia. However, this ratio may be artificially enhanced due to the high optical depth of the $^{14}$NH$_3$ (1,1) line, which increases the effective source size. We use the detections of ammonia, cyanopolyynes, and far-infrared dust continuum to characterize the density and temperature in the Chamaeleon cores and calculate the molecular column densities and their relative ratios. The ring molecule benzonitrile is not detected in either Chamaeleon core. The $3 σ$ upper limits for its column density are a factor of 2 higher than the value derived for TMC-1 and the upper limits for its relative abundance with respect to HC$_5$N are a factor of 3 higher than the TMC-1 value.

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arXiv2024

Nitrogen Abundance Distribution in the inner Milky Way

We combine a new Galactic plane survey of Hydrogen Radio Recombination Lines (RRLs) with far-infrared (FIR) surveys of ionized Nitrogen, N+, to determine Nitrogen abundance across Galactic radius. RRLs were observed with NASA DSS-43 70m antenna and the Green Bank Telescope in 108 lines-of-sight spanning -135 degrees < l < 60 degrees, at b=0 degrees. These positions were also observed in [N II] 122 um and 205 um lines with the Herschel Space Observatory. Combining RRL and [N II] 122 um and 205 um observations in 41 of 108 samples with high signal-to-noise ratio, we studied ionized Nitrogen abundance distribution across Galactocentric distances of 0-8 kpc. Combined with existing Solar neighborhood and Outer galaxy N/H abundance determinations, we studied this quantity's distribution within the Milky Way's inner 17 kpc for the first time. We found a Nitrogen abundance gradient extending from Galactocentric radii of 4-17 kpc in the Galactic plane, while within 0-4 kpc, the N/H distribution remained flat. The gradient observed at large Galactocentric distances supports inside-out galaxy growth with the additional steepening resulting from variable star formation efficiency and/or radial flows in the Galactic disk, while the inner 4 kpc flattening, coinciding with the Galactic bar's onset, may be linked to radial flows induced by the bar potential. Using SOFIA/FIFI-LS and Herschel/PACS, we observed the [N III] 57 um line to trace doubly ionized gas contribution in a sub-sample of sightlines. We found negligible N++ contributions along these sightlines, suggesting mostly singly ionized Nitrogen originating from low ionization H II region outskirts.

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arXiv2023

Structure of the W3A Low Density Foreground Region

We present analysis of OI 63 micron and CO $J$ = 5-4 and 8-7 multi-position data in the W3A region and use it to develop a model for the extended low-density foreground gas that produces absorption features in the OI and $J$ = 5-4 CO lines. We employ the extinction to the exciting stars of the background HII region to constrain the total column density of the foreground gas. We have used the Meudon PDR code to model the physical conditions and chemistry in the region employing a two-component model with high density layer near the HII region responsible for the fine structure line emission, and an extended low density foreground layer. The best-fitting total proton density, constrained largely by the CO lines, is $n$(H) = 250 cm$^{-3}$ in the foreground gas, and 5$\times$10$^5$ cm$^{-3}$ in the material near the HII region. The absorption is distributed over the region mapped in W3A, and is not restricted to the foreground of either the embedded exciting stars of the HII region or the protostar W3 IRS5. The low-density material associated with regions of massive star formation, based on an earlier study by Goldsmith et al. (2021), is quite common, and we now see that it is extended over a significant portion of W3A. It thus should be included in modeling of fine structure line emission, including interpreting low-velocity resolution observations made with incoherent spectrometer systems, in order to use these lines as accurate tracers of massive star formation.

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arXiv2019

Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [NII] Fine-structure and Hydrogen Recombination lines

We present a method for deriving the electron density of ionized gas using the ratio of the intensity of the [NII] 205um line to that of Hydrogen radio recombination lines (RRL). We use this method to derive electron densities of 21 velocity components in 11 lines of sight through the Galaxy, including the Galactic center. We observed, at high--spectral resolution, the [NII] 205um line with the Herschel/HIFI and SOFIA/GREAT instruments and the radio recombination lines with the Green Bank Telescope and the NASA Deep Space Network Deep Space Station 43 (DSS--43) telescope. We find typical electron densities between 6 to 170 cm^-3, which are consistent with those derived at low spectral resolution using the [NII] 205um/122um line ratio with Herschel/PACS on a larger sample of sight lines in the Galactic plane. By matching the electron densities derived from the [NII] 205um/RRL intensity ratio and the [NII] 122um/205um intensity ratio, we derive the nitrogen fractional abundance for most of the velocity components. We investigate the dependence of the N/H ratio with Galactocentric distance in the inner Galaxy (R_gal<6 kpc, which is inaccessible in optical studies due to dust extinction. We find that the distribution of nitrogen abundances in the inner galaxy derived from our data has a slope that is consistent to that found in the outer Galaxy in optical studies. This result is inconsistent with some suggestions of a flatter distribution of the nitrogen abundance in the inner galaxy.

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arXiv2019

Probing ISM Structure in Trumpler 14 & Carina I Using The Stratospheric Terahertz Observatory 2

We present observations of the Trumpler 14/Carina I region carried out using the Stratospheric Terahertz Observatory 2 (STO2). The Trumpler 14/Carina I region is in the west part of the Carina Nebula Complex, which is one of the most extreme star-forming regions in the Milky Way. We observed Trumpler 14/Carina I in the 158 $μ$m transition of [C\,{\sc ii}] with a spatial resolution of 48$''$ and a velocity resolution of 0.17 km s$^{-1}$. The observations cover a 0.25$^\circ$ by 0.28$^\circ$ area with central position {\it l} = 297.34$^\circ$, {\it b} = -0.60$^\circ$. The kinematics show that bright [C\,{\sc ii}] structures are spatially and spectrally correlated with the surfaces of CO clouds, tracing the photodissociation region and ionization front of each molecular cloud. Along 7 lines of sight that traverse Tr 14 into the dark ridge to the southwest, we find that the [C\,{\sc ii}] luminosity from the HII region is 3.7 times that from the PDR. In same los we find in the PDRs an average ratio of 1:4.1:5.6 for the mass in atomic gas:dark-CO gas: molecular gas traced by CO. Comparing multiple gas tracers including HI 21cm, [C\,{\sc ii}], CO, and radio recombination lines, we find that the HII regions of the Carina Nebula Complex are well-described as HII regions with one-side freely expanding towards us, consistent with the champagne model of ionized gas evolution. The dispersal of the GMC in this region is dominated by EUV photoevaporation; the dispersal timescale is 20-30 Myr.

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arXiv2018

The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics

Using visible, radio, microwave, and sub-mm data, we study several lines of sight toward stars generally closer than 1 kpc on a component-by-component basis. We derive the component structure seen in absorption at visible wavelengths from Ca II, Ca I, K I, CH, CH$^{+}\!,$ and CN and compare it to emission from H I, CO and its isotopologues, and C$^{+}$ from the GOT C+ survey. The correspondence between components in emission and absorption help create a more unified picture of diffuse atomic and molecular gas in the interstellar medium. We also discuss how these tracers are related to the CO-dark H$_{2}$ gas probed by C$^{+}$ emission and discuss the kinematic connections among the species observed.

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arXiv2017

Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO

We present and analyze deep Herschel/HIFI observations of the [CII] 158um, [CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. These observations are used to determine the physical conditions of the line--emitting gas, which we use to study the transition from atomic to molecular gas and from C^+ to C^0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1 0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C^0 and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [CII] intensity in our sample represents about 1% of the total far-infrared intensity from the LOSs observed in both Magellanic Clouds.

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arXiv2016

L1599B: Cloud Envelope and C+ Emission in a Region of Moderately Enhanced Radiation Field

We study the effects of an asymmetric radiation field on the properties of a molecular cloud envelope. We employ observations of carbon monoxide (12CO and 13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the chemical and physical properties of the core and envelope of L1599B, a molecular cloud forming a portion of the ring at approximately 27 pc from the star Lambda Ori. The O III star provides an asymmetric radiation field that produces a moderate enhancement of the external radiation field. Observations of the [CII] fine structure line with the GREAT instrument on SOFIA indicate a significant enhanced emission on the side of the cloud facing the star, while the [Ci], 12CO and 13CO J = 1-0 and 2-1, and 12CO J = 3-2 data from the PMO and APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic, and molecular line centroid velocities track each other very closely, and indicate that the cloud may be undergoing differential radial motion. The HI data from the Arecibo GALFA survey and the SOFIA/GREAT [CII] data do not suggest any systematic motion of the halo gas, relative to the dense central portion of the cloud traced by 12CO and 13CO.

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arXiv2015

Herschel Galactic plane survey of [NII] fine structure emission

We present the first large scale high angular resolution survey of ionized nitrogen in the Galactic Plane through emission of its two fine structure transitions ([NII]) at 122 $μ$m and 205 $μ$m. The observations were largely obtained with the PACS instrument onboard the Herschel Space Observatory. The lines-of-sight were in the Galactic plane, following those of the Herschel OTKP project GOT C+. Both lines are reliably detected at the 10$^{-8}$ - 10$^{-7}$ $W$m$^{-2}$sr$^{-1}$ level over the range -60$^{o}$ $\leq$ $l$ $\leq$ 60$^{o}$. The $rms$ of the intensity among the 25 PACS spaxels of a given pointing is typically less than one third of the mean intensity, showing that the emission is extended. [NII] is produced in gas in which hydrogen is ionized, and collisional excitation is by electrons. The ratio of the two fine structure transitions provides a direct measurement of the electron density, yielding $n(e)$ largely in the range 10 to 50 cm$^{-3}$ with an average value of 29 cm$^{-3}$ and N$^+$ column densities 10$^{16}$ to 10$^{17}$ cm$^{-2}$. [NII] emission is highly correlated with that of [CII], and we calculate that between 1/3 and 1/2 of the [CII] emission is associated with the ionized gas. The relatively high electron densities indicate that the source of the [NII] emission is not the Warm Ionized Medium (WIM), which has electron densities more than 100 times smaller. Possible origins of the observed [NII] include the ionized surfaces of dense atomic and molecular clouds, the extended low density envelopes of HII regions, and low-filling factor high-density fluctuations of the WIM.

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arXiv2015

[C II] emission from galactic nuclei in the presence of X-rays

The luminosity of [C II] is used to probe the star formation rate in galaxies, but the correlation breaks down in some active galactic nuclei (AGNs). Models of the [C II] emission from galactic nuclei do not include the influence of X-rays on the carbon ionization balance, which may be a factor in reducing the [C II] luminosity. We calculate the [C II] luminosity in galactic nuclei under the influence of bright sources of X-rays. We solve the balance equation of the ionization states of carbon as a function of X-ray flux, electron, atomic hydrogen, and molecular hydrogen density. These are input to models of [CII] emission from the interstellar medium (ISM) in galactic nuclei. We also solve the distribution of the ionization states of oxygen and nitrogen in highly ionized regions. We find that the dense warm ionized medium (WIM) and dense photon dominated regions (PDRs) dominate the [C II] emission when no X-rays are present. The X-rays in galactic nuclei can affect strongly the C$^+$ abundance in the WIM converting some fraction to C$^{2+}$ and higher ionization states and thus reducing its [C II] luminosity. For an X-ray luminosity > 10$^{43}$ erg/s the [C II] luminosity can be suppressed by a factor of a few, and for very strong sources, >10$^{44}$ erg/s, such as found for many AGNs by an order of magnitude. Comparison of the model with extragalactic sources shows that the [C II] to far-infrared ratio declines for an X-ray luminosity >10$^{43}$ erg/s, in reasonable agreement with our model.

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arXiv2014

A Herschel [CII] Galactic plane survey III: [CII] as a tracer of star formation

We study the relationship between the [CII] emission and the star formation rate (SFR) in the Galactic plane and separate the relationship of different ISM phases to the SFR. We compare these relationships to those in external galaxies and local clouds, allowing examinations of these relationships over a wide range of physical scales. We compare the distribution of the [CII] emission, with its different contributing ISM phases, as a function of Galactocentric distance with the SFR derived from radio continuum observations. We also compare the SFR with the surface density distribution of atomic and molecular gas, including the CO-dark H2 component. The [CII] and SFR are well correlated at Galactic scales with a relationship that is in general agreement with that found for external galaxies. By combining [CII] and SFR data points in the Galactic plane with those in external galaxies and nearby star forming regions, we find that a single scaling relationship between the [CII] luminosity and SFR applies over six orders of magnitude. The [CII] emission from different ISM phases are each correlated with the SFR, but only the combined emission shows a slope that is consistent with extragalactic observations. These ISM components have roughly comparable contributions to the Galactic [CII] luminosity: dense PDRs (30%), cold HI (25%), CO-dark H2 (25%), and ionized gas (20%). The SFR-gas surface density relationship shows a steeper slope compared to that observed in galaxies, but one that it is consistent with those seen in nearby clouds. The different slope is a result of the use of a constant CO-to-H2 conversion factor in the extragalactic studies, which in turn is related to the assumption of constant metallicity in galaxies. We find a linear correlation between the SFR surface density and that of the dense molecular gas.

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arXiv2013

A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components

[Abridged] The [CII] 158um line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium, including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [CII] line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [CII] emission for Galactic latitudes b=0deg, +-0.5deg, and +-1.0deg. [CII] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10 kpc. We estimate that most of the observed [CII] emission is produced by dense PDRs (47%), with smaller contributions from CO-dark H2 gas (28%), cold atomic gas (21%), and ionized gas (4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium (WNM) gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is 43%. We find that the warm and diffuse CO-dark H2 is distributed over a larger range of Galactocentric distances (4-11 kpc) than the cold and dense H2 gas traced by 12CO and 13CO (4-8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from 20% at 4 kpc to 80% at 10 kpc. On average, CO-dark H2 accounts for 30% of the molecular mass of the Milky Way. When the CO-dark H2 component is included, the radial distribution of the CO-to-H2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [CII] emission emerging from dense PDRs is associated with modest far-ultraviolet fields in the range chi0~1-30.

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arXiv2012

Collisional Excitation of the [CII] Fine Structure Transition in Interstellar Clouds

We analyze the collisional excitation of the 158 micron (1900.5 GHz) fine structure transition of ionized carbon (C+) in terms of line intensities produced by simple cloud models. The single C+ fine structure transition is a very important coolant of the atomic interstellar medium and of photon dominated regions in which carbon is partially or completely in ionized form. The [CII] line is widely used as a tracer of star formation in the Milky Way and other galaxies. Excitation of the [CII] fine structure transition can be via collisions with hydrogen molecules, atoms, and electrons. Velocity-resolved observations of [CII] have become possible with the HIFI instrument on Herschel and the GREAT instrument on SOFIA. Analysis of these observations is complicated by the fact that it is difficult to determine the optical depth of the [CII] line due to the relative weakness and blending of the components of the analogous transition of 13C$+. We discuss the excitation and radiative transition of the [CII] line, deriving analytic results for several limiting cases and carry out numerical solutions using a large velocity gradient model for a more inclusive analysis. We show that for antenna temperatures up to 1/3 of the brightness temperature of the gas kinetic temperature, the antenna temperature is linearly proportional to the column density of C+ irrespective of the optical depth of the transition, which can be referred to as the effectively optically thin (EOT) approximation. We review the critical densities for excitation of the [CII] line by various collision partners. We briefly analyze C+ absorption and conclude with a discussion of C+ cooling and how the considerations for line intensities affect the behavior of this important coolant of the ISM.

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arXiv2010

A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields observed by Herschel

The [CII] fine--structure line at 158um is an excellent tracer of the warm diffuse gas in the ISM and the interfaces between molecular clouds and their surrounding atomic and ionized envelopes. Here we present the initial results from Galactic Observations of Terahertz C+ (GOTC+), a Herschel Key Project devoted to study the [CII] fine structure emission in the galactic plane using the HIFI instrument. We use the [CII] emission together with observations of CO as a probe to understand the effects of newly--formed stars on their interstellar environment and characterize the physical and chemical state of the star-forming gas. We collected data along 16 lines--of--sight passing near star forming regions in the inner Galaxy near longitudes 330 degrees and 20 degrees. We identify fifty-eight [CII] components that are associated with high--column density molecular clouds as traced by 13CO emission. We combine [CII], 12CO, and 13CO observations to derive the physical conditions of the [CII]--emitting regions in our sample of high--column density clouds based on comparison with results from a grid of Photon Dominated Region (PDR) models. From this unbiased sample, our results suggest that most of [CII] emission originates from clouds with H2 volume densities between 10e3.5 and 10e5.5 cm^-3 and weak FUV strength (CHI_0=1-10). We find two regions where our analysis suggests high densities >10e5 cm^-3 and strong FUV fields (CHI=10e4-10e6), likely associated with massive star formation. We suggest that [CII] emission in conjunction with CO isotopes is a good tool to differentiate between regions of massive star formation (high densities/strong FUV fields) and regions that are distant from massive stars (lower densities/weaker FUV fields) along the line--of--sight

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arXiv2008

Dichotomy in the Dynamical Status of Massive Cores in Orion

To study the evolution of high mass cores, we have searched for evidence of collapse motions in a large sample of starless cores in the Orion molecular cloud. We used the Caltech Submillimeter Observatory telescope to obtain spectra of the optically thin (\H13CO+) and optically thick (\HCO+) high density tracer molecules in 27 cores with masses $>$ 1 \Ms. The red- and blue-asymmetries seen in the line profiles of the optically thick line with respect to the optically thin line indicate that 2/3 of these cores are not static. We detect evidence for infall (inward motions) in 9 cores and outward motions for 10 cores, suggesting a dichotomy in the kinematic state of the non-static cores in this sample. Our results provide an important observational constraint on the fraction of collapsing (inward motions) versus non-collapsing (re-expanding) cores for comparison with model simulations.

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arXiv2007

ExoPTF Science Uniquely Enabled by Far-IR Interferometry: Probing the Formation of Planetary Systems, and Finding and Characterizing Exoplanets

By providing sensitive sub-arcsecond images and integral field spectroscopy in the 25 - 400 micron wavelength range, a far-IR interferometer will revolutionize our understanding of planetary system formation, reveal otherwise-undetectable planets through the disk perturbations they induce, and spectroscopically probe the atmospheres of extrasolar giant planets in orbits typical of most of the planets in our solar system. The technical challenges associated with interferometry in the far-IR are greatly relaxed relative to those encountered at shorter wavelengths or when starlight nulling is required. A structurally connected far-IR interferometer with a maximum baseline length of 36 m can resolve the interesting spatial structures in nascent and developed exoplanetary systems and measure exozodiacal emission at a sensitivity level critical to TPF-I mission planning. The Space Infrared Interferometric Telescope was recommended in the Community Plan for Far-IR/Submillimeter Space Astronomy, studied as a Probe-class mission, and estimated to cost 800M dollars. The scientific communities in Europe, Japan, and Canada have also demonstrated a keen interest in far-IR interferometry through mission planning workshops and technology research, suggesting the possibility of an international collaborative effort.

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arXiv2006

Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

Maps of estimated dust column density in molecular clouds are usually assumed to reliably trace the total gas column density structure. In this work we present results showing a clear discrepancy between the dust and the gas distribution in the Taurus molecular cloud complex. We compute the power spectrum of a 2MASS extinction map of the Taurus region and find it is much shallower than the power spectrum of a 13CO map of the same region previously analyzed. This discrepancy may be explained as the effect of grain growth on the grain extinction efficiency. However, this would require a wide range of maximum grain sizes, which is ruled out based on constraints from the extinction curve and the available grain models. We show that major effects due to CO formation and depletion are also ruled out. Our result may therefore suggest the existence of intrinsic spatial fluctuations of the dust to gas ratio, with amplitude increasing toward smaller scales. Preliminary results of numerical simulations of trajectories of inertial particles in turbulent flows illustrate how the process of clustering of dust grains by the cloud turbulence may lead to observable effects. However, these results cannot be directly applied to large scale supersonic and magnetized turbulence at present.

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