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Wider and Deeper at Green Bank: The New Argus-144 Instrument

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September 22 – 24, 2020

10 am – 1 pm EDT

Recordings of workshop sessions are shared on YouTube, see the complete playlist here [1], or click directly on presentation titles below to go to individual videos.

Meeting Outline

The Green Bank Observatory is holding a virtual three-day workshop to discuss the scientific and technical aspects of its planned 144 element 3mm band spectroscopic radio camera. With a 6′ × 6′ field of view, Argus-144 will provide high spatial dynamic range maps of interstellar molecules that are critical in understanding the physical processes and astrochemistry associated with star formation, from the sub-parsec scale of interstellar filaments and dense cores to 10 kpc galactic disks. 

With a beam size of 6.5” to 10” over the intended frequency range of 74-116 GHz, the GBT with Argus-144 will be unequaled worldwide for wide-area 3mm spectroscopic mapping, and will be a critical complement to ALMA, which has high angular resolution but a small field of view.

Learn more about the Argus-144 receiver here [2].

This meeting will last for three days, each of which will have two hours of presentations, each 20 minutes in length. Each of these sessions will be followed by one hour of discussion in groups to encourage collaboration and discussion. The first session of presentations will cover the technical requirements and aspects of the receiver, while the second and third will focus more on the potential scientific benefits that the instrument will deliver. Abstracts were submitted under the loose topic headings of:

Topics for the discussion groups are expected to include:

Posters and Poster Abstracts

Poster Flash Talks Recording [3]

Workshop Schedule

Tuesday, September 22 – Technical Talks

TimeSpeakerTopic
10:00 EDTKaren O’Neil
GBO Site Director		Welcome & Opening Remarks [9]
10:10 Andy HarrisKeynote: [10]The Technical Requirements and Development Areas for the Argus 144 Receiver [11]

Abstract: Pending
10:30GBO: Dave FrayerThe GBT Performance at 3mm [12]

I will summarize the performance and calibration of the GBT within the 3mm band using the current Argus instrument and highlight the possibility in carrying-out very large wide-area Galactic surveys with Argus-144.    Measurements of the GBT aperture efficiency, small-source main beam efficiency, large-source main beam efficiency, and beam size as a function of frequency will be presented.   The surface performance, Gravity-Zernike model, elevation gain curve, pointing model performance, and limitations due to offset pointing will be discussed.   With 144 beams and the sensitivity of the GBT, Argus-144 could map 100’s of sq-deg in 13CO to interesting levels with about 1 hr per sq-deg of observing time for studies of the Galactic ISM.
10:50GBO: Steve WhiteThe Technical Requirements for the Argus 144 Receiver and its Proposed Digital Backend [13]

As the Argus+ proposal is an expansion to 144 elements of the original 16 element design, the working assumption is a duplication of the existing RF downconversion module and feed package. An increase in the number of elements requires a larger Dewar and associated vacuum window. The increased radiation load and resistive load from the amplifiers requires more lift and thus more cryogenic refrigeration capacity. Also the number of IF channels far exceeds the capability of the current GBT IF system. All these present challenges in managing size, weight, and heat load for the receiver room. This presentation will address our current work on vacuum windows, cryogenic capacity, and thoughts on the IF system design and the work needed to address IF transmission. Also the justification for the proposed 600 MHz bandwidth digital backend specifically designed for the instrument will be discussed.
11:10GBO: Pedro SalasThe laser surface scanning instrument: LASSI [14]

Observations with the Green Bank telescope (GBT) at 3 mm are possible thanks to its ability to correct for deformations of its primary reflector. Terrestrial laser scanners (TLS) have the potential to measure these deformations more efficiently than currently used methods (Out-of-focus holography, OOF). The laser surface scanning instrument (LASSI) is exploring the use of a TLS for this purpose. To enable more efficient 3 mm observations LASSI should match the performance of OOF, while being faster and able to operate over a broader range of conditions. During the last year we quantified the ability of LASSI to measure deformations under different observing conditions, and this summer the instrument was installed on the telescope. In this presentation we will introduce LASSI, what it is and how it works, present preliminary results and the next steps in its development.
11:30Mikko VaronenCryogenic SiGe MMIC Receiver Development

In this presentation, we focus on the recent advances in W-band cryogenic SiGe low-noise amplifier and receiver development. Although amplifiers based on InP techonologies have better noise figures, the motivation for using silicon based technologies is the possibility to integrate more functions on the same chip. A hybrid combination of the two technologies can be used to obtain a high level of integration with a low-noise figure. When cryogenically cooled to 20 K and measured on-wafer the SiGe amplifier shows 95-116-K noise temperature from 77 to 116 GHz. This means 6 to 7 times improvement in noise temperature compared to room temperature noise. The measured gain is around 20 dB for frequency range of 71 to 116 GHz with unprecedented low power consumption of 2.8 mW. The noise performance of our SiGe amplifier is low enough for cascading it with a preceding InP amplifier. Because of the outstanding low power consumption of the SiGe amp more functions such as mixers with LO multiplier chains could be integrated on the same silicon chip and assembled inside the cryogenic Dewar without excessive increase in overall power consumption. This would relax system complexity inside the cryogenic Dewar for a future generation of large receiver arrays with hundreds of elements.
11:50Flash Poster Talks (Pre-Recorded) – 1 min each
Videos will be shown in session, and are available on YouTube [3]
12:00Breakout Discussions

Wednesday, September 23 – Science/Survey Astronomy

TimeSpeakerTopic
10:00Che-Yu ChenKeynote: Gas Kinematics in Molecular Clouds [15]

In molecular clouds, multi-scale supersonic turbulent flows compress low density material to initiate creation of overdense regions, which may grow to become prestellar cores and then collapse gravitationally to form protostellar systems. Turbulence is therefore considered one of the key agents affecting the dynamics of the star forming process, and there have been several projects, both observational and theoretical, aimed at understanding the velocity structure within molecular clouds at all physical scales and throughout different evolutionary stages. In particular, the kinematics of star-forming cores is one of the critical properties needed to understand the star-forming process, because it provides the initial conditions for all protostellar evolution, including disk formation and stellar multiplicity. However, gas kinematics at sub-pc scales have not been well characterized because of a lack of the required sensitivity and high spatial and spectral resolution until now. With our ongoing survey with GBT-Argus, we aim to establish a broad statistical sample of the internal velocity structure within dense cores, to address a key open question in star formation: the origin and distribution of angular momenta of star-forming cores. 
10:20Peter BarnesPerspectives on Survey Astronomy at 3mm: Potential Niches for Argus-144 [16]

I review some selected experiences from 3mm survey science of molecular clouds from the past 10-15 years. Almost the entire Galactic Plane has now been surveyed at ~arcmin or better resolution, not only in the 3 mm iso-CO lines, but also several 3 mm “dense gas” tracers as well as higher-J CO and other lines at 1.2 and 0.8 mm, with FCRAO, Mopra, JCMT, APEX, Nobeyama and other ground-based facilities. Despite this, we are still only scratching the surface of deeper, Big Data science questions that can be answered with such surveys. Even if only available in 4 years, an Argus-144 receiver presents an opportunity to start the 2nd generation of such surveys, with an angular resolution about 5x finer than the above and focusing perhaps not so much on areal coverage and fundamental cartography in the Milky Way, but on specific science themes that nevertheless will still need their own dedicated Big Data survey projects. I highlight some sample science objectives that may be optimal for Argus-144’s particular strengths, and consider how such goals impact survey design through their observing time (eg, calibration, adaptive real-time mapping strategy, data-taking efficiency) and data reduction pipeline (automation of standard modes, non-standard options) requirements.
10:40Alberto BolattoGas Galaxy Surveys at z=0 to Study Galaxy Evolution [17]

The evolution of galaxies is driven by the rate and efficiency of star formation, controlled by accretion and feedback. Trends in gas fraction and star formation activity are some of the clearest patterns along the Hubble sequence, yet gas sample sizes are pitifully small compared to optical data. The last decade has seen progress in fairly unbiased but unresolved studies, and in resolved but biased or otherwise limited studies of local samples. The natural next step is a resolved gas-focused survey of a large, representative sample of galaxies spanning a large local volume and a wide range of conditions, to study the physics of molecular gas and star formation in the full galactic evolution context.
11:00Amanda KepleyDense Extragalactic GBT+Argus Survey: Current Status [18]

From both a theoretical and an observation perspective, dense gas plays an important role in star formation. Almost all theories of star formation have gas density as a key variable, while observations in the Milky Way and nearby galaxies suggest a close link between dense gas and star formation. Early unresolved observations of nearby galaxies suggested a simple relationship between the amount of dense gas and the amount of star formation in a galaxy. Recent resolved surveys of dense molecular gas in nearby galaxies have shown that the dense gas fraction and the dense gas star formation efficiency ‘ratio of the star formation rate to amount of dense gas’ vary within individual galaxies and among different galaxies. However, the samples for these studies have been limited to 5-10 sources since the most common dense molecular gas tracers are faint. The goal of DEGAS “Dense Extragalactic GBT+Argus Survey” is map the dense molecular gas in central 2arcmin of 36 nearby galaxies at moderate (10arcsec) resolution. When complete, it will be the largest survey of dense molecular gas to date. In this presentation, we will provide an update on the current status of the project and initial results from our upcoming first data release. We conclude with thoughts on how a large mm camera on the GBT like Argus-144 could further push this important science.
11:20Rachel FriesenBeyond Ammonia: 3mm line surveys of molecular clouds with Argus-144 [19]

Recent surveys of dust continuum emission of Galactic star-forming regions have revealed the ubiquity of high column density filamentary structures within molecular clouds, within which most star-forming cores are embedded. With the Green Bank Telescope, surveys such as the Green Bank Ammonia Survey (GAS) and KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE) have mapped large areas of low- and high-mass star forming regions within our galaxy in emission from ammonia and other excited lines at 1cm. These surveys complement continuum surveys by revealing the kinematic and temperature structure of dense filaments and cores, allowing us to probe the stability of these structures as a function of scale, track the dissipation of turbulence and the evolution of angular momentum, and quantitatively test predictions of core and filament formation via mass flows and accretion. I will present results from GAS and KEYSTONE, and discuss how Argus-144 will enable surveys of the emission from higher density tracers and deuterated species to further probe the physics and efficiency with which dense gas forms stars in Galactic molecular clouds.
11:40Brian SvobodaDense gas on the scale of dense cores: Galactic Plane surveys with Argus-144

Argus144’s unique combination of fast mapping speed, high angular resolution, and lack of spatial filtering will enable the first 3mm Galactic Plane surveys of traditional “dense gas” molecular tracers: C18O, HCO+, HNC, N2H+, and HCN, among others, at the scale of dense cores (~0.1 pc). I shall discuss the rich scientific potential for dense gas Galactic Plane surveys with Argus144 and highlight an accepted Argus pilot program targeting N2H+ in a 0.5×0.5deg field. This program complements the IRAM 30m Large Program GASTON that is blindly observing the Galactic Plane at 1.2mm at an unprecedented angular resolution and mass completeness. The Argus observations will provide precise gas kinematics from N2H+ at beam-matched resolution uniformly over all molecular cloud clump evolutionary stages: a key strength of a blind survey design. On-going and future mm/FIR dust continuum surveys require such kinematic followup to measure the dynamic, large-scale flows feeding the sites of embedded star and cluster formation. Estimates show that Argus144 would be able to map the entire 75deg2 footprint of the BU-FCRAO Galactic Ring Survey at 8″ resolution within the limits of a GBT Large Program (500 hr). Such a survey would yield invaluable scientific legacy value and provide high-fidelity short-spacing measurements for the ngVLA.
12:00Breakout Discussions

Thursday, September 24 – Science/Software Considerations

TimeSpeakerTopic
10:00Erik RosolowskyKeynote: Data reduction and Software Requirements for a 144 Element 3mm Receiver [20]

I will discuss the design considerations for Argus-144 software to allow for rapid calibration, reduction, and inspection of data. To provide context, I will describe the software developed in python for the GAS, KEYSTONE, and DEGAS surveys to produce high quality data using spectroscopic data pipelines. For DEGAS, this includes the approaches developed by the survey team for low overhead calibration and aggressive correction for atmospheric and receiver stability issues. For GAS and KEYSTONE, I will present our approaches for multiplexed spectroscopy and extraction of physical parameters. Finally, I will point out the bottlenecks that we would encounter in scaling current solutions to Argus-144 and where new development would be required.
10:20Dane SizemoreBringing a New Device to the Observer-Supported Level [21]

Deploying a new instrument on the GBT involves creating a manager application for monitoring and control of the instrument, as well as modifications to integrate the new receiver into the existing system. GBO’s Software Division has been using Attribute Driven Design (ADD) methodology to capture requirements for large projects. We will present a brief overview of this process, major software considerations for the proposed Argus-144 instrument, and the envisioned iterative process for meeting the requirements from a software perspective.
10:40Gaspar GalazThe relevance to detect molecular gas in giant low surface brightness galaxies: From IRAM/30m to GBT/100m [22]

In this brief talk, I summarize the importance and the efforts to detect molecular gas in giant low surface brightness galaxies. I show how new wide-field instruments like Argus-144 can help with this task.
11:00Samantha ScibelliDetecting Complex Organic Molecules in Starless and Prestellar Cores in the Taurus Molecular Cloud [23]

Before stars like our Sun are born, they are conceived inside dense clumps of gas and dust known as starless and gravitationally bound prestellar cores. The detection of complex organic molecules (COMs) toward these cores has sparked interest in the fields of astrochemistry and astrobiology, yet detection rates and degrees of complexity within a larger sample of cores (i.e., more than a few) have not been fully explored. With the 3mm receiver on the Arizona Radio Observatory’s 12m telescope, we looked for COMs in 31 starless and prestellar cores, spanning a wide range of dynamical and chemical evolutionary stages, all within the localized L1495-B218 Taurus Star Forming Region. We found a prevalence of COMs, detecting methanol (CH3OH) in 100% of the cores targeted and acetaldehyde (CH3CHO) in 70%. Additionally, a deep survey in the nearby young prestellar core L1521E exposed additional complexity, with detections of even larger molecules including dimethyl ether (CH3OCH3), methyl formate (HCOOCH3) and vinyl cyanide (CH2CHCN). Results from both studies indicate that future high-resolution maps of COMs in prestellar cores are critical for 1) studying the chemical connection between COMs, by comparing against modeled radial profiles and spatial morphologies at the core level, and 2) correcting for source size (i.e., filling fraction) errors when calculating column densities, which we find can lead to factors of a few in uncertainty if not taken into account.
11:20Jaime PinedaIons-Neutrals connection in Dense Cores [24]

Dense cores within molecular clouds are the places where stars are formed and the final place where turbulence is dissipated. Several (large) programs have focused on the dense gas kinematics to constrain the angular momentum on dense cores, however, little has been done to understand the effect of magnetic fields on ions (affected by magnetic fields) when compared to neutrals. It is widely assumed that the non-thermal velocity dispersion (a.k.a. turbulence) should be narrower both for molecular ions (compared to neutrals) when the magnetic field inside the core is static. A few of the previous observations of starless cores and IRDCs have shown suggestive evidence for broader linewidths in ions, however, it remained unexplained. I will discuss the new opportunities opened up with ARGUS and ARGUS-144 to study the gas kinematics of ions and neutrals, and their implications for star-formation. I will show the early results of the comparing N2H+ (1-0) taken with GBT ARGUS and the NH3 (1,1) and (2,2) taken with GBT+VLA in the dense core Barnard 5. Surprisingly, the non-thermal velocity dispersion of the ion is subsonic and systematically higher than that of the neutral by ~20%. We explore a new and surprising possibility, that the magnetic field inside dense cores is not static, but oscillating. And, the ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader linewidth.
11:40Meeting Wrap Up (Larry Morgan/SOC) [25]
12:00Breakout Discussions, with Q&A Session for Poster Presenters

An online poster session is planned for attendees to peruse at their leisure, with a short, interactive session to allow the presenters to discuss their work with interested parties. Keynote speakers include:

Scientific Organizing Committee:

Abstract Submission and Registration

Registration for the workshop is now closed. Please contact Will Armentrout at warmentr [at] nrao [.] edu for connection details.

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