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2020 Science Highlights

by Davis Murphy last modified Dec 02, 2020

The Formation of Exo-comets

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. To date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here, the authors present high-resolution interferometric observations of Comet 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15–16 December 2019. These observations reveal emission from hydrogen cyanide (HCN) and carbon monoxide (CO) coincident with the expected position of 2I/Borisov’s nucleus, with production rates Q(HCN) = (7.0 ± 1.1) × 1023 per second and Q(CO) = (4.4 ± 0.7) × 1026 per second. While the HCN abundance relative to water (0.06–0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35–105%) is among the highest observed in any comet within 2 Astronomical Units (AU) of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment—probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small icy bodies analogous to our Solar System’s own proto-Kuiper belt.

Image: ALMA HCN [Left] and CO [Center] observations of Comet 2I/Borisov. [ Right ] Hubble Space Telescope image of 2I/Borisov.

Publication: M.A. Cordiner (Catholic University) et al., Unusually High CO Abundance of the First Active Interstellar Comet, Nature Astronomy, 4, 861 (20 April 2020).

NRAO Press Release: ALMA Reveals Unusual Composition of Interstellar Comet 2I/Borisov

New Insight into Brown Dwarf Atmospheres

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

Zonal (latitudinal) winds dominate the bulk flow of planetary atmospheres. For gas giant planets such as Jupiter, the motion of clouds can be compared with radio emissions from the magnetosphere, which is connected to the planet’s interior, to determine the wind speed. In principle, this technique can be applied to brown dwarfs, the most common stellar type, and/or directly imaged exoplanets, if periods can be determined for the infrared and radio emissions. In this paper, the authors apply this method to measure the wind speeds on the 40MJ brown dwarf 2MASS J10475385+2124234. The difference between the radio period of 1.751 to 1.765 hours measured with the Jansky Very Large Array, and infrared period of 1.741 ± 0.007 hours measured with the Spitzer Space Telescope, implies a strong wind proceeding eastward of 650 ± 310 meters per second, 6X the winds on Jupiter. This could be due to atmospheric jet streams and/or low frictional drag at the bottom of the atmosphere.

Image: [Left] Brown dwarf and [Right] Jupiter. Artist's conception of brown dwarf illustrates magnetic field and atmosphere's top, which were observed at different wavelengths to determine wind speeds. Credit: Bill Saxton, NRAO/AUI/NSF.

Publication: Katelyn Allers (Bucknell University) et al., A Measurement of the Wind Speed on a Brown Dwarf, Science, 368, Issue 6487, 169 (10 April 2020).

NRAO Press Release: Astronomers Measure Wind Speed on a Brown Dwarf

The Lukewarm Chromosphere of Antares

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

The authors present spatially resolved ALMA and Jansky VLA continuum observations of the early-M red supergiant Antares to search for the presence of a chromosphere at radio wavelengths. The free-free emission of the Antares atmosphere is resolved at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA S band). The projected angular diameter is found to continually increase with increasing wavelength, from a low of 50.7 milli-arcseconds (mas) at 0.7 mm, up to a diameter of 431 mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular diameter, respectively.

All four ALMA measurements show that the shape of the atmosphere is elongated, with a flattening of 15% at a similar position angle. The disk-averaged gas temperature of the atmosphere initially rises from a value of 2700 K at 1.35 R (i.e., 0.35 R above the photosphere) to a peak value of 3800 K at ∼2.5 R, after which it then more gradually decreases to 1650 K at 11.6 R. The rise in gas temperature between 1.35 R and ∼2.5 R is evidence for a chromospheric temperature rise above the photosphere of a red supergiant. The authors detect a clear change in the spectral index across the sampled wavelength range, with the flux density Sν∝ ν1.42 between 0.7 mm and 1.4 cm, which they associate with chromosphere-dominated emission, while the flux density Sν ∝ ν0.8 between 4.3 cm and 10 cm, which they associate with wind-dominated emission. The authors show that Antares’s outer atmosphere is transparent at their observed wavelengths, and the lukewarm chromosphere that was detected is thus real and not just an average of the cool molecular shell and hot ultraviolet emitting gas. We then perform nonlocal thermal equilibrium modeling of the far-ultraviolet radiation field of another early-M red supergiant, Betelgeuse, and find that an additional hot (i.e., > 7000 K) chromospheric photoionization component with a much smaller filling factor must also exist throughout the chromospheres of these stars.

Image: Comparison of the temperature structure of the extended atmospheres of the early-M supergiants Antares and Betelgeuse. The filled red circles are the measurements summarized in this work for Antares, and the filled blue diamonds are the measurements for Betelgeuse from previous work. The gas temperature error bars include the uncertainty in absolute flux density scale. The overlapping filled black circles at R = 1 represent the photospheric effective temperatures of Antares and Betelgeuse. The solid black line is the temperature profile of the semiempirical model for the extended atmosphere of Betelgeuse discussed in this paper.

Publication: E. O’Gorman (Dublin Institute for Advanced Studies) et al., ALMA and VLA Reveal the Lukewarm Chromospheres of the Nearby Red Supergiant Antares and Betelgeuse, Astronomy & Astrophysics, 638, A65 (2019).

NRAO Press Release: Supergiant Atmosphere of Antares Revealed by Radio Telescopes

A Census of the Orion Protostars

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

Tobin et al. conducted a survey of 328 protostars in the Orion molecular clouds with ALMA at 0.87 mm at a resolution of ~0.14 milli-arcseconds (40 AU), including observations with the Jansky VLA at 9 mm toward 148 protostars at a resolution of ~0.08 milli-arcseconds (32 AU). This is the largest multiwavelength survey of protostars at this resolution by an order of magnitude. The authors use the dust continuum emission at 0.87 and 9 mm to measure the dust disk radii and masses toward the Class 0, Class I, and flat-spectrum protostars, characterizing the evolution of these disk properties in the protostellar phase. The mean dust disk radii for the Class 0, Class I, and flat-spectrum protostars are 44+5.83.4, 37+4.93.0 and 28.5+3.72.3  AU, respectively, and the mean protostellar dust disk masses 25.9+7.74.0 , 14.9+3.82.2, and 11.6+3.51.9 M, respectively. The decrease in dust disk masses is expected from disk evolution and accretion, but the decrease in disk radii may point to the initial conditions of star formation not leading to the systematic growth of disk radii or that radial drift is keeping the dust disk sizes small. At least 146 of these protostellar disks – 35% of 379 detected 0.87 mm continuum sources plus 42 nondetections – have disk radii greater than 50 AU. These properties are not found to vary significantly between different regions within Orion. The protostellar dust disk mass distributions are systematically larger than those of Class II disks by a factor of more than four, providing evidence that the cores of giant planets may need to at least begin their formation during the protostellar phase.

Image: Herschel infrared image of the Orion Molecular Cloud [Insets] VLA 32 GHz and ALMA 250 GHz, sample images of protostars in the VANDAM sample.

Publication: John J. Tobin (National Radio Astronomy Observatory)  et al., The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. II. A Statistical Characterization of Class 0 and Class I Protostellar Disks, Astrophysical Journal, 890, 130  (20 February 2020).

 NRAO Press Release: How Newborn Stars Prepare for the Birth of Planets

Megamaser Cosmology: A Better Hubble Constant

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

The authors present a measurement of the Hubble constant made using geometric distance measurements to megamaser-hosting galaxies. They have applied an improved approach for fitting maser data and obtained better distance estimates for four galaxies previously published by the Megamaser Cosmology Project: UGC 3789, NGC 6264, NGC 6323, and NGC 5765b. Combining these updated distance measurements with those for the maser galaxies CGCG 074-064 and NGC 4258, and assuming a fixed velocity uncertainty of 250 km s1 associated with peculiar motions, this work constrains the Hubble constant to be H0 = 73.9 ± 3.0 km s1 Mpc1 independent of distance ladders and the cosmic microwave background. This best value relies solely on maser-based distance and velocity measurements, and it does not use any peculiar velocity corrections. Different approaches for correcting peculiar velocities do not modify H0 by more than ±1σ, with the full range of best-fit Hubble constant values spanning 71.8­ – 76.9 km s1 Mpc1. The authors corroborate prior indications that the local value of H0 exceeds the early-Universe value, with a confidence level varying from 95% to 99% for different treatments of the peculiar velocities.

Image: [Left] Distance to the megamaser galaxies. [Right] Probability distribution for H0 for the six galaxies.

Publication: D.W. Pesce (Harvard-Smithsonian Center for Astrophysics) et al., The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints, Astrophysical Journal Letters, 891, L1 (1 March 2020).

NRAO Press Release: New Distance Measurements Bolster Challenge to Basic Model of Universe

A New Type of Cosmic Explosion

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

The authors discuss ZTF18abvkwla (the "Koala"), a fast blue optical transient (FBOT) discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey. ZTF18abvkwla has features in common with the groundbreaking transient AT 2018cow: blue colors at peak light, a short rise time from half-max of under two days, a decay time to half-max of only three days, a high optical luminosity, a hot featureless spectrum at peak light, and a luminous radio counterpart. At late times (Dt > 80 days), the radio luminosity of ZTF18abvkwla at 10 GHz is most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy (M ~ 5 x 108 M and SFR ~ 7 M per year) that is moderately metal-enriched (log[O/H] ~ 8.5), similar to the hosts of Gamma Ray Bursts (GRBs) and superluminous supernovae.

As in AT2018cow, the radio and optical emission in ZTF18abvkwla likely arises from two separate components: the radio from fast-moving ejecta and the optical from shock-interaction with confined dense material. These FBOTs likely begin the same way as certain supernovae and gamma-ray bursts. The differences are seen in the aftermath of the initial explosion. FBOTS are much brighter than supernovae, but not as bright as GRBs, suggesting a mildly relativistic jet engine in a dense environment. The authors find that transients in the FBOT rise-luminosity phase space are at least two to three orders of magnitude less common than core-collapse supernovae. They discuss strategies for identifying such events with future facilities such as the Vera C. Rubin Observatory, as well as prospects for detecting accompanying X-ray and radio emission.

Image: [Top] Radio light curves for several cosmic explosion types. The Koala is marked with an arrow. The FBOTS (black stars) are all much brighter than supernovae, but not as bright as GRBs, suggesting a mildly relativistic jet engine in a dense environment. [Bottom] Artist’s impression of an FBOT.

Publication: Anna Ho (California Institute of Technology) et al., The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z=0.27, Astrophysical Journal, 895, 49 (26 May 2020).

NRAO Press Release: Astronomers Discover New Class of Cosmic Explosions

Finding the Youngest Radio Jets

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

The authors present new sub-arcsecond Jansky Very Large Array (VLA) imaging at 10 GHz of 155 ultra-luminous (Lbol∼1011.714.2L) and heavily obscured quasars with redshifts z ∼ 0.4−3. The sample was selected to have extremely red mid-infrared to optical color ratios based on data from the Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) Survey. High-resolution VLA observations have revealed that the majority of the sources in this sample (93 of 155) are compact on angular scales < 0.2 arcseconds (≤ 1.7 kpc at z ∼ 2). The radio luminosities, linear extents, and lobe pressures of these sources are similar to young radio active galactic nuclei (AGN) but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests relatively young dynamical ages (∼1047 years), relatively high ambient interstellar medium densities (∼1−104 cm3), and modest lobe expansion speeds (∼30−10,000 km s1). The authors find their sources to be consistent with a population of newly-triggered, young jets caught in a unique evolutionary stage in which they still reside within the dense gas reservoirs of their hosts. Based on their radio luminosity function and dynamical ages, the authors estimate only ∼20% of classical large scale FRI/II radio galaxies could have evolved directly from these objects. They speculate that the WISE-NVSS sources might first become Gigahertz Peaked Spectrum or Compact Steep Spectrum sources, of which some might ultimately evolve into larger radio galaxies.

Image: [Left] VLA Sky Survey and FIRST images of the emerging radio loud quasar population. [Right] VLA spectrum of one source showing the flat radio spectrum.

Publication: Pallavi Patil (University of Virginia) et al., High-resolution VLA Imaging of Obscured Quasars: Young Radio Jets Caught in a Dense ISM, Astrophysical Journal, 896, 18 (9 June 2020).

A Massive Rotating Disk in the Early Universe

by Mark Adams last modified Nov 13, 2020 by Davis Murphy

Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers. Observationally, it has been difficult to identify disk galaxies in emission at high redshift  to discern between competing models of galaxy formation. In this contribution, the authors report imaging, with a resolution of ~1.3 kiloparsecs, the 158-micrometre emission line from singly ionized carbon, the far-infrared dust continuum, and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. These observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of ~ 272 kilometers per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of ~ 72 billion M. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favors formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations.

Image: [Far left & left center]  VLA CO contours, and ALMA contours of the [CII] and thermal dust emission from the z=4.3 Wolfe galaxy.  [Right center & far right]: [CII] velocity field, and the rotation curve.

Publication: Marcel Neeleman (Max Planck Institute for Astronomy) et al., A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years after the Big Bang, Nature, 581, 269 (20 May 2020).

NRAO Press Release: ALMA Discovers Massive Rotating Disk in Early Universe