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Ronald J. Allen

Publications and source records attributed to Ronald J. Allen.

At least 19 recordsLinked to original sources
arXiv2021

Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy

We present the serendipitous discovery of an extremely broad ($ΔV_{LSR} \sim 150$ km/s), faint ($T_{mb} < 10 \textrm{mK}$), and ubiquitous 1667 and 1665 MHz ground-state thermal OH emission towards the 2nd quadrant of the outer Galaxy ($R_{gal}$ > 8 kpc) with the Green Bank Telescope. Originally discovered in 2015, we describe the redundant experimental, observational, and data quality tests of this result over the last five years. The longitude-velocity distribution of the emission unambiguously suggests large-scale Galactic structure. We observe a smooth distribution of OH in radial velocity that is morphologically similar to the HI radial velocity distribution in the outer Galaxy, showing that molecular gas is significantly more extended in the outer Galaxy than previously expected. Our results imply the existence of a thick ($-200< z < 200$ pc) disk of diffuse ($n_{H_{2}}$ $\sim$ 5 $\times$ 10$^{-3}$ cm$^{-3}$) molecular gas in the Outer Galaxy previously undetected in all-sky CO surveys.

astro-ph.GA↗
arXiv2019

The Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of "CO-Dark" Gas in the Perseus Arm

We report the results from a new, highly sensitive ($ΔT_{mb} \sim 3 $mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9-pixel grid covering a $1°$ x $1°$ patch of sky in the direction of $l = 105°, b = +2.50°$ towards the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope (GBT) 1667 MHz OH results with archival CO J=1-0 observations from the Five College Radio Astronomy Observatory (FCRAO) Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically-independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an A$_{\rm v}$ of 0.2, but the CO emission does not appear until the volume density exceeds 100-200 cm$^{-3}$. These results demonstrate that a combination of low column density A$_{\rm v}$ and low volume density $n_{H}$ can explain the lack of CO emission along sight lines exhibiting OH emission. The 18-cm OH main lines, with their low critical density of $n^{*}$ $ \sim 1 $ cm$^{-3}$, are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically-thin radio tracer for diffuse H$_2$.

astro-ph.GA↗
arXiv2019

OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5

We report column densities of molecular gas in the W5 star-forming region as traced with OH 18-cm emission in a grid survey using the Green Bank Telescope. OH appears to trace a greater column density than does CO in 8 out of 15 cases containing OH emission detections; the two molecules trace the same column densities for the other 7 cases. OH and CO trace a similar morphology of molecular gas with a nearly one-to-one correspondence. The mass of molecular gas traced by OH in the portion of the survey containing OH emission is $1.7$ (+ 0.6 or - 0.2) $\times 10^4 M_{\odot}$, whereas the corresponding CO detections trace $9.9 \times 10^3 M_{\odot} (\pm 0.7) \times 10^3$. We find that for lines observed in absorption, calculations assuming uniform gas and continuum distributions underestimate column density values by 1 to 2 orders of magnitude, making them unreliable for our purposes. Modeling of this behavior in terms of OH cloud structure on a scale smaller than telescopic resolution leads us to estimate that the filling factor of OH gas is a few to 10 percent. Consideration of filling factor effects also results in a method of constraining the excitation temperature values. The total molecular gas content of W5 may be approximately two to three times what we report from direct measurement, because we excluded absorption line detections from the mass estimate.

astro-ph.GA↗
arXiv2018

OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of the OH 18-cm Main Lines in W5

We present excitation temperatures $T_{ex}$ for the OH 18-cm main lines at 1665 and 1667 MHz measured directly in front of the W5 star-forming region, using observations from the Green Bank Telescope and the Very Large Array. We find unequivocally that $T_{ex}$ at 1665 MHz is greater than $T_{ex}$ at 1667 MHz. Our method exploits variations in the continuum emission from W5, and the fact that the continuum brightness temperatures $T_C$ in this nebula are close to the excitation temperatures of the OH lines in the foreground gas. The result is that an OH line can appear in emission in one location and in absorption in a neighboring location, and the value of $T_C$ where the profiles switch from emission to absorption indicates $T_{ex}$. Absolute measurements of $T_{ex}$ for the main lines were subject to greater uncertainty because of unknown effects of geometry of the OH features. We also employed the traditional "expected profile" method for comparison with our "continuum background" method, and found that the continuum background method provided more precise results, and was the one to definitively show the $T_{ex}$ difference. Our best estimate values are: $T^{65}_{ex} = 6.0 \pm 0.5$ K, $T^{67}_{ex} = 5.1 \pm 0.2$ K, and $T^{65}_{ex} - T^{67}_{ex} = 0.9 \pm 0.5$ K. The $T_{ex}$ values we have measured for the ISM in front of W5 are similar to those found in the quiescent ISM, indicating that proximity to massive star-forming regions does not generally result in widespread anomalous excitation of OH emission.

astro-ph.GA↗
arXiv2015

The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l \approx 105^°, b \approx +1^°$

We report the first results from a survey for 1665, 1667, and 1720 MHz OH emission over a small region of the Outer Galaxy centered at $l \approx 105.0°, b \approx +1.0°$ . This sparse, high-sensitivity survey ($ΔTa \approx ΔTmb \approx 3.0 - 3.5$ mK rms in 0.55 km/s channels), was carried out as a pilot project with the Green Bank Telescope (GBT, FWHM $\approx 7.6'$) on a 3 X 9 grid at $0.5°$ spacing. The pointings chosen correspond with those of the existing $^{12}$CO(1-0) CfA survey of the Galaxy (FWHM $\approx 8.4'$). With 2-hr integrations, 1667 MHz OH emission was detected with the GBT at $\gtrsim 21$ of the 27 survey positions ($\geq 78\%$ ), confirming the ubiquity of molecular gas in the ISM as traced by this spectral line. With few exceptions, the main OH lines at 1665 and 1667 MHz appear in the ratio of 5:9 characteristic of LTE at our sensitivity levels. No OH absorption features are recorded in the area of the present survey, in agreement with the low levels of continuum background emission in this direction. At each pointing the OH emission appears in several components extending over a range of radial velocity and coinciding with well-known features of Galactic structure such as the Local Arm and the Perseus Arm. In contrast, little CO emission is seen in the survey area; less than half of the $\gtrsim 50$ identified OH components show detectable CO at the CfA sensitivity levels, and these are generally faint. There are no CO profiles without OH emission. With few exceptions, peaks in the OH profiles coincide with peaks in the GBT HI spectra (obtained concurrently, FWHM $8.9'$), although the converse is not true. We conclude that main-line OH emission is a promising tracer for the "dark molecular gas" in the Galaxy discovered earlier in Far-IR and gamma-ray emission. Further work is needed to establish the quantitative details of this connection.

astro-ph.GA↗
arXiv2012

Faint Extended OH Emission from the Local Interstellar Medium in the Direction l \approx 108\circ, b \approx 5\circ

We have mapped faint 1667 OH line emission (TA \approx 20 - 40 mK in our \approx 30' beam) along many lines of sight in the Galaxy covering an area of \approx 4\circ \times 4\circ in the general direction of l \approx 108\circ, b \approx 5\circ. The OH emission is widespread, similar in extent to the local HI (r </= 2 kpc) both in space and in velocity. The OH profile amplitudes show a good general correlation with those of HI in spectral channels of \approx 1 km/s; this relation is described by TA(OH) \approx 1.50 \times 10^{-4} TB(HI) for values of TB(HI) </\approx 60 - 70 K. Beyond this the HI line appears to "saturate", and few values are recorded above \approx 90 K. However, the OH brightness continues to rise, by a further factor \approx 3. The OH velocity profiles show multiple features with widths typically 2 - 3 km/s, but less than 10% of these features are associated with CO(1-0) emission in existing surveys of the area smoothed to comparable resolution.

astro-ph.CO↗
arXiv2012

Radio & Optical Interferometry: Basic Observing Techniques and Data Analysis

Astronomers usually need the highest angular resolution possible, but the blurring effect of diffraction imposes a fundamental limit on the image quality from any single telescope. Interferometry allows light collected at widely-separated telescopes to be combined in order to synthesize an aperture much larger than an individual telescope thereby improving angular resolution by orders of magnitude. Radio and millimeter wave astronomers depend on interferometry to achieve image quality on par with conventional visible and infrared telescopes. Interferometers at visible and infrared wavelengths extend angular resolution below the milli-arcsecond level to open up unique research areas in imaging stellar surfaces and circumstellar environments. In this chapter the basic principles of interferometry are reviewed with an emphasis on the common features for radio and optical observing. While many techniques are common to interferometers of all wavelengths, crucial differences are identified that will help new practitioners avoid unnecessary confusion and common pitfalls. Concepts essential for writing observing proposals and for planning observations are described, depending on the science wavelength, angular resolution, and field of view required. Atmospheric and ionospheric turbulence degrades the longest-baseline observations by significantly reducing the stability of interference fringes. Such instabilities represent a persistent challenge, and the basic techniques of phase-referencing and phase closure have been developed to deal with them. Synthesis imaging with large observing datasets has become a routine and straightforward process at radio observatories, but remains challenging for optical facilities. In this context the commonly-used image reconstruction algorithms CLEAN and MEM are presented. Lastly, a concise overview of current facilities is included as an appendix.

astro-ph.IM↗
arXiv2011

Measuring the Sizes, Shapes, Surface Features and Rotations of Solar System Objects with Interferometry

We consider the application of interferometry to measuring the sizes and shapes of small bodies in the solar system that cannot be spatially resolved by today's single-dish telescopes. Assuming ellipsoidal shapes, our results indicate that interferometers can measure the size of an object to better than 15% uncertainty if the limb-darkening is unknown. Assuming a Minnaert scattering model, one can theoretically derive the limb-darkening parameters from simultaneous measurements of visibilities at several different projected baseline lengths to improve the size and shape determination to an accuracy of a few percent. With a 3-D shape model for the dwarf planet Haumea, we demonstrate that when photometric light curve, visibility light curve, and visibility phase center displacement are combined, the rotational period and sense of rotation can all be derived, and the rotational pole can be estimated. Because of its elongated shape and the dark red spot, the rotation of Haumea causes its optical photocenter to move in a loop on the sky, extending of ~80 \muas without the dark red spot, and ~200 \muas with it. Such movements are easily detectable by space-based astrometric interferometer designed e.g. for planet detection. As an example, we consider the possible contributions to the study of small bodies in the solar system by the Space Interferometry Mission. We show that such a mission could make substantial contributions in characterizing the fundamental physical properties of the brightest Kuiper Belt Objects and Centaurs as well as a large number of main belt asteroids. We compile a list of Kuiper Belt Objects and Centaurs that are potentially scientifically interesting and observable by such missions.

astro-ph.EP↗
arXiv2010

The Metal-Enriched Outer Disk of NGC 2915

We present optical emission-line spectra for outlying HII regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-foming core is similarly under-abundant for its gas fraction. Star formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that the low-level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider 3 plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.

astro-ph.CO↗
arXiv2009

Mass Transport Processes and their Roles in the Formation, Structure, and Evolution of Stars and Stellar Systems

We summarize some of the compelling new scientific opportunities for understanding stars and stellar systems that can be enabled by sub-mas angular resolution, UV/Optical spectral imaging observations, which can reveal the details of the many dynamic processes (e.g., variable magnetic fields, accretion, convection, shocks, pulsations, winds, and jets) that affect their formation, structure, and evolution. These observations can only be provided by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible - and since they require observations at wavelengths (UV) not accessible from the ground. Two mission concepts which could provide these invaluable observations are NASA's Stellar Imager (SI; http://hires.gsfc.nasa.gov/si/) interferometer and ESA's Luciola sparse aperture hypertelescope, which each could resolve hundreds of stars and stellar systems. These observatories will also open an immense new discovery space for astrophysical research in general and, in particular, for Active Galactic Nuclei (Kraemer et al. Decadal Survey Science Whitepaper). The technology developments needed for these missions are challenging, but eminently feasible (Carpenter et al. Decadal Survey Technology Whitepaper) with a reasonable investment over the next decade to enable flight in the 2025+ timeframe. That investment would enable tremendous gains in our understanding of the individual stars and stellar systems that are the building blocks of our Universe and which serve as the hosts for life throughout the Cosmos.

astro-ph.SR↗
arXiv2008

The volume densities of giant molecular clouds in M83

Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm HI column densities, along with estimates of the local dust abundances, we measure the volume densities of a sample of actively star-forming giant molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution scale of 170 pc. Our approach is based on an equilibrium model for the cycle of molecular hydrogen formation on dust grains and photodissociation under the influence of the FUV radiation on the cloud surfaces of GMCs. We find a range of total volume densities on the surface of GMCs in M83, namely 0.1 - 400 cm-3 inside R25, 0.5 - 50 cm-3 outside R25 . Our data include a number of GMCs in the HI ring surrounding this galaxy. Finally, we discuss the effects of observational selection, which may bias our results.

astro-ph↗
arXiv2008

Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved Instrument Model

In a previous paper we described a method of estimating the single-measurement bias to be expected in astrometric observations of targets in crowded fields with the future Space Interferometry Mission (SIM). That study was based on a simplified model of the instrument and the measurement process involving a single-pixel focal plane detector, an idealized spectrometer, and continuous sampling of the fringes during the delay scanning. In this paper we elaborate on this ``instrument model'' to include the following additional complications: spectral dispersion of the light with a thin prism, which turns the instrument camera into an objective prism spectrograph; a multiple-pixel detector in the camera focal plane; and, binning of the fringe signal during scanning of the delay. The results obtained with this improved model differ in small but systematic ways from those obtained with the earlier simplified model. We conclude that it is the pixellation of the dispersed fringes on the focal plane detector which is responsible for the differences. The improved instrument model described here suggests additional ways of reducing certain kinds of confusion, and provides a better basis for the evaluation of instrumental effects in the future.

astro-ph↗
arXiv2007

Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest

Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument that delivers parallaxes at about 4 microarcsec on targets as faint as V = 20, differential accuracy of 0.6 microarcsec on bright targets, and with flexible scheduling. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed all of the enabling technologies needed for the flight instrument in 2005. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. SIM will search for planets with masses as small as an Earth orbiting in the `habitable zone' around the nearest stars using differential astrometry, and could discover many dozen if Earth-like planets are common. It will be the most capable instrument for detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. SIM will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the galactic mass distribution and the formation and evolution of the Galactic halo. (abridged)

astro-ph↗
arXiv2007

Crowded-Field Astrometry with the Space Interferometry Mission - I. Estimating the Single-Measurement Astrometric Bias Arising from Confusion

The accuracy of position measurements on stellar targets with the future Space Interferometry Mission (SIM) will be limited not only by photon noise and by the properties of the instrument (design, stability, etc.) and the overall measurement program (observing strategy, reduction methods, etc.), but also by the presence of other "confusing" stars in the field of view (FOV). We use a simple "phasor" model as an aid to understanding the main effects of this "confusion bias" in single observations with SIM. This analytic model has been implemented numerically in a computer code and applied to a selection of typical SIM target fields drawn from some of the Key Projects already accepted for the Mission. We expect that less than 1% of all SIM targets will be vulnerable to confusion bias; we show that for the present SIM design, confusion may be a concern if the surface density of field stars exceeds 0.4 star/arcsec^2. We have developed a software tool as an aid to ascertaining the possible presence of confusion bias in single observations of any arbitrary field. Some a priori knowledge of the locations and spectral energy distributions of the few brightest stars in the FOV is helpful in establishing the possible presence of confusion bias, but the information is in general not likely to be available with sufficient accuracy to permit its removal. We discuss several ways of reducing the likelihood of confusion bias in crowded fields. Finally, several limitations of the present semi-analytic approach are reviewed, and their effects on the present results are estimated. The simple model presented here provides a good physical understanding of how confusion arises in a single SIM observation, and has sufficient precision to establish the likelihood of a bias in most cases.

astro-ph↗
arXiv2007

The volume densities of GMCs in M81

HI features near young star clusters in M81 are identified as the photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young stars have recently formed. The HI column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10^21 cm^-2 near R ~ 7.5 kpc. This trend is similar to that of the radially-averaged HI distribution in this galaxy, and implies a constant area covering factor of ~ 0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the HI column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm^-3 are derived, with a geometric mean of 17 cm^-3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.

astro-ph↗
arXiv2007

The Opacity of Spiral Galaxy Disks VIII: Structure of the Cold ISM

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations and dust emissivity are all free parameters in galaxy Spectral Energy Distribution (SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in HST images through the foreground disk. We measure the IR flux in images from the {\it Spitzer} Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree. The ratios between the measured average and apparent optical depths of the disk sections imply optically thin clouds in these disks. Optically thick disks, are likely to have more than a single cloud along the line-of-sight.

astro-ph↗
arXiv2007

Station-Keeping Requirements for Constellations of Free-Flying Collectors Used for Astronomical Imaging in Space

The accuracy requirements on station-keeping for constellations of free-flying collectors coupled as (future) imaging arrays in space for astrophysics applications are examined. The basic imaging element of these arrays is the two-element interferometer. Accurate knowledge of two quantities is required: the \textit{projected baseline length}, which is the distance between the two interferometer elements projected on the plane tranverse to the line of sight to the target; and the \textit{optical path difference}, which is the difference in the distances from that transverse plane to the beam combiner. ``Rules-of-thumb'' are determined for the typical accuracy required on these parameters. The requirement on the projected baseline length is a \textit{knowledge} requirement and depends on the angular size of the targets of interest; it is generally at a level of half a meter for typical stellar targets, decreasing to perhaps a few centimeters only for the widest attainable fields of view. The requirement on the optical path difference is a \textit{control} requirement and is much tighter, depending on the bandwidth of the signal; it is at a level of half a wavelength for narrow (few %) signal bands, decreasing to $\approx 0.2 λ$ for the broadest bandwidths expected to be useful. Translation of these requirements into engineering requirements on station-keeping accuracy depends on the specific details of the collector constellation geometry. Several examples are provided to guide future application of the criteria presented here. Some implications for the design of such collector constellations and for the methods used to transform the information acquired into images are discussed.

astro-ph↗