Dissertation: The Role of Nonideal Magnetohydrodynamic Effects, Gravitational Instability, and Episodic Accretion in Star-Formation

(Cassiopeia – Autumn 2022)

by Dr. Indrani Das
Thesis defended on July 25, 2022
Department of Physics and Astronomy, Western University
Thesis advisor: Prof. Shantanu Basu

Link to the Electronic thesis: https://ir.lib.uwo.ca/etd/8728/

My dissertation focuses on the effect of magnetic fields on disk and core evolution during star-formation. We investigate the fragmentation scales of gravitational instability of a rotationally-supported self-gravitating protostellar disk using linear perturbation analysis in the presence of two nonideal magnetohydrodynamic (MHD) effects: Ohmic dissipation and ambipolar diffusion. Our results show that molecular clouds exhibit a preferred lengthscale for collapse that depends on mass-to-flux ratio, magnetic diffusivities, and the Toomre-Q parameter. In addition, the influence of the magnetic field on the preferred mass for collapse leads to a modified threshold for the fragmentation mass, as opposed to a Jeans mass, that might lead to giant planet formation in the early embedded phase. Furthermore, we apply the nonideal MHD threshold for fragmentation scales to fit the data of prestellar core lifetimes and as well as the number of enclosed cores formed in a clump, as found with the observations of Herschel and Submillimeter Array (SMA), respectively. Our results show that the trend found in the observed lifetime and fragmentation mass cannot be explained in a purely hydrodynamic scenario. Our best-fit model exhibits B ~ n0.43, which signifies a means to indirectly infer the effect of the ambipolar diffusion on mildly supercritical dense regions of molecular clouds. We also develop a semi-analytic formalism of episodic mass accretion (therefore episodic luminosity) from a disk to star, which provides a good match to the observed luminosity distribution of protostars. In contrast, neither a constant nor a time-dependent but smoothly varying mass accretion rate is able to do so. Our analytic work provides insight into global MHD simulations of protoplanetary disks that we carry out using the FEOSAD numerical code. Our numerical results demonstrate the long-term evolution of disks, including the formation and evolution of clumps, and especially the episodic nature of accretion, which might explain the origin of observed knots in the molecular jet outflows.

Keywords: ISM: clouds – magnetic fields – magnetohydrodynamics (MHD) – stars: formation, gravitational collapse, disk evolution: episodic accretion, young stellar objects (YSOs)

Square Kilometre Array (SKA) Update

By Kristine Spekkens (Canadian SKA Science Director) and the AACS
(Cassiopeia – Autumn 2022)

Artist’s impression of the SKA, combining elements from South Africa and Australia from left to right in the image. Photos of real hardware have been blended with realizations of the future SKA antennas. Image credit: SKA Observatory.

The SKA Observatory (SKAO) is now 15 months into the construction phase of SKA Phase 1 (=SKA1), with the staged construction plan anticipating the first correlated SKA1-Low stations and SKA1-Mid dishes in 2024, the first data from scientifically competitive arrays in 2026, and science readiness reviews of completed arrays underway by 2028.  Forty-one contracts for a total commitment of over 150 million euros have now been awarded, with a focus on the components and subsystems needed for the initial integration phases for SKA1-Mid and SKA1-Low as well as on the development of software infrastructure for telescope management and delivery. Full production contracts for the major infrastructure components are expected to be secured by the end of 2022, with mitigation plans being implemented to navigate exceptional global challenges including inflation, shipping costs/times, labour shortages and component availability.

The SKAO’s mission is “to build and operate cutting-edge radio telescopes to transform our understanding of the Universe, and deliver benefits to society through global collaboration and innovation”. The broader impacts of the SKA are structured around the UN Sustainable Development Goals, and include building partnerships with Indigenous and local communities at the remote sites where the dishes and antennas will be located. In Australia, an Indigenous Land Use Agreement (ILUA) with the Wajarri Yamaji, on whose traditional lands SKA1-Low will be located, provides ongoing consent for the Murchison Radio Observatory (MRO) where ASKAP and the MWA currently operate. The completion of a new SKA1-Low-specific ILUA – the outcome of years of negotiations – is expected by the end of this year. In South Africa, a Memorandum of Understanding (MOU) with representatives of the San people, whose early ancestors walked the land on which SKA1-Mid will be located, was signed to protect and promote San culture and heritage. An MOU is also in place with Agri-SA, many of whose members own farms which border or will host SKA1-Mid antennas. NRC, SKAO, and LCRIC are organising a webinar this fall to provide CASCA members with additional information regarding the ongoing process of respectful engagement with Indigenous peoples and local communities on which the SKAO will continue to build across the lifetime of the project.

Canada is currently an Observer of the SKAO Council that governs the project. A cooperation agreement between NRC and the SKAO allows Canada’s scientific and engineering communities to continue participating in the SKA through March 2023, while longer-term SKAO membership is given full consideration by the federal government. Work under the cooperation agreement is fully funded and proceeding on schedule, with the Canadian correlator team from NRC and industry partner MDA on track to provide the backends to support the initial four-dish Array Assembly (= AA0.5) and the subsequent 8-dish Array Assembly (= AA1) for SKA1-Mid. A prototype system integration facility has been set up at MDA, with a compute cluster installed and digitizer and correlator hardware to be delivered in mid-September.  A Critical Design Review for the SKA1-Mid Single Pixel Feed and Receiver was held July 5-7, resulting in a conditional pass to proceed with construction of the Band 1 & 2 design pending the implementation of the recommendations generated by the review panel.

In order to maintain our leading role in SKA1-Mid correlator work, a commitment to construction and operations beyond the cooperation agreement will soon be required from our government. Bilateral meetings between NRC and SKAO held over the summer discussed a broad range of topics related to fulfilling the LPR2020 recommendation regarding Canadian participation in SKA1. These meetings have provided the ministries and agencies involved in the file with the most detailed, up-to-date information available to aid our government in its decision on long-term participation in the SKA.

New Eyes on the Universe, an international conference that will highlight the complementarity and synergies between the SKA and the ngVLA, will be held in Vancouver the week of April 30, 2023. The meeting will explore the science opportunities enabled by the unprecedented combined frequency coverage, sensitivity and resolution of the ngVLA and the SKA.  Plenary talks will feature the areas of greatest synergy between the two observatories, while contributed talks will focus on topics that highlight each facility’s strengths. Additional details regarding the conference as well as opportunities to participate will be circulated to CASCA members as they become available.

For more information and updates on Canada and the SKA:

ngVLA Update

By Erik Rosolowsky (U Alberta), Joan Wrobel (NRAO)
(Cassiopeia – Autumn 2022)

More details on all items may be found here.

ngVLA Key Science Goal 1: Unveiling the Formation of Solar System Analogues on Terrestrial Scales. Credit: NRAO/AUI/NSF

New Eyes on the Universe: SKA & ngVLA Conference

The SKA and the ngVLA are pleased to announce a landmark radio astronomy science conference designed to highlight the complementarity and synergies between these premier radio observatories of the 21st century. To be held in Vancouver, Canada the week of 30 April 2023, this conference will review, discuss, and extend the cutting-edge science opportunities enabled by the unprecedented SKA-ngVLA coverage across three decades of radio frequency (50 MHz to 116 GHz).

ngVLA Special Session: Chemical Probes of Astrophysical Systems

The NRAO and the ngVLA will convene a Special Session titled “Chemical Probes of Astrophysical Systems” at the January 2023 American Astronomical Society meeting in Seattle, WA. Invited presentations by Eliza Kempton (UMD), Brett McGuire (MIT), David Meier (NMT), Stefanie Milam (GSFC), Dominique Segura-Cox (MPE), and Kamber Schwarz (MPIA) will be featured. Contributed iPoster presentations on any ngVLA theme are welcomed.

ngVLA Site Selection Begins in Mexico

In September 2022, the NRAO began supporting a Mexican astronomer’s work to select and develop antenna sites in northern Mexico for the ngVLA.

ngVLA Completes Its Technical Conceptual Design Review

The ngVLA successfully completed its Technical Conceptual Design Review in July 2022. This verifies that the technical design is likely to meet all the performance-driving requirements. This successful milestone sets the stage for the Programmatic Conceptual Design Review, to be run in March 2023 by the US National Science Foundation.

Computational Astrophysics in the ngVLA Era: Synergistic Simulations, Theory, and Observations

Held in June 2022 at the Simons Foundation’s Flatiron Institute in Manhattan, New York, USA, this conference brought together 80+ theoreticians, modellers, and observers. Their 50+ presentations dealt with the computational astrophysics and observational challenges for the next generation of observatories, with a focus on the ngVLA.

VLA/VLBA to ngVLA Transition Advisory Group (TAG)

This group will determine how to transition operations between the currently operating VLA/VLBA to the ngVLA during construction process. TAG membership, including Stefi Baum (Manitoba), was announced in May 2022. The TAG will develop, quantitatively assess, and evaluate a set of possible transition options that can be prioritized on their scientific promise, cost, and technical/personnel impacts. It is anticipated that the TAG’s final report will be delivered by mid-2023 to the NRAO and the US National Science Foundation.

CATAC Update on the Thirty Meter Telescope

By / par Michael Balogh (CATAC Chair)
(Cassiopeia – Autumn / l’automne 2022)

Site Update

On July 7, with Act 255 the Governor of the State of Hawai’i signed bill HB2024 into law, establishing a new Authority responsible for managing Mauna Kea lands. The Authority will include 11 voting members, including: a lineal descendent of a practitioner of Native Hawaiian traditional and customary practices associated with Mauna Kea; an individual who is a recognized practitioner of Native Hawaiian traditional and customary practices; and a representative who shall be appointed by the governor from a list of three names submitted by Maunakea Observatories. The process to appoint these members is underway. There will be a transition period of up to five years, during which time no new leases or subleases can be renewed or issued.

Partnership Update

On July 16, the National Science Foundation (NSF) issued a notice of intent to proceed with the Environmental Impact Statement (EIS) and Section 106 processes. Four scoping meetings were held on the Big Island, to receive feedback on the process. Both written and oral testimony was provided to the visiting panel. More information about this process can be found here. You are invited and welcome to provide feedback on both the environmental review and the Draft Community Engagement Plan, via links on that site. An Environmental Assessment will also be made for the alternative site, at La Palma in the Canary Islands.

The Project is preparing for the NSF’s Preliminary Design Review, anticipated for the end of this year. This will be a comprehensive review of the whole US-ELTP project (TMT, GMT and NOIRLab), including governance and operations. A final site decision will not be required by the time of this review. If successful, the PDR will be followed by a Final Design Review (FDR) about a year later.

CATAC Membership

Michael Balogh (University of Waterloo), Chair, mbalogh@uwaterloo.ca
Bob Abraham (University of Toronto; TIO SAC)
Stefi Baum (University of Manitoba)
Laura Ferrarese (NRC)
Harvey Richer (UBC)
Kim Venn (University of Victoria)
Luc Simard (Director General of NRC-HAA, non-voting, ex-officio)
Don Brooks (Executive Director of ACURA, non-voting, ex-officio)
Christine Wilson (CASCA President, non-voting, ex-officio)
Stan Metchev (TIO SAC, non-voting, ex-officio)
Tim Davidge (TIO SAC Canadian co-chair; NRC, observer)
Greg Fahlman (NRC, observer)

Update on CASTOR

By John Hutchings, Patrick Côté (NRC Herzberg Astronomy & Astrophysics Research Centre)
(Cassiopeia – Autumn 2022)

CASTOR continues to make progress on several fronts. The following list summarizes activities since the last e-CASS report in June.

  1. The ongoing work towards a collaboration with India featured a meeting between CSA and ISRO in July. Enthusiasm for the collaboration was expressed on both sides, and schedules exchanged. The next formal step awaits a meeting of the ISRO advisory committee APEX this month to formalize the joint mission. In the meantime, the Canadian and Indian teams continue to work on a common design and science plan.
  2. The UK Space Agency has issued an Announcement of Opportunity to fund bilateral missions of interest that specifically include CSA, and a proposal to fund CASTOR scientific and technical work is in preparation now. This is a timely step towards the UK participation that has been in discussion for some years.
  3. CSA will attend at International Astronautical Congress meeting in Paris later this month, where bilateral discussions are planned with these and other potential partners (NASA/JPL, ESA) for CASTOR. In addition, CASTOR will be on the agenda for regular CSA meetings with NASA.
  4. The ongoing CSA technical and phase 0 contracts continue to make progress. The work on acquiring, doping and coating prototype detectors involves Canadian industry, Teledyne-E2V, JPL, and HAA. Science working groups are detailing specific surveys and investigations as a design reference mission, aided by an extensive set of simulation and planning tools (FORECASTOR) being developed at HAA. A detailed costing exercise for the entire mission is also under way as part of the Phase 0 study.
  5. Overall, progress is on track to have a well-defined mission with international partners by about a year from now, as a powerful, Canadian-led mission of unique capability that will serve the community and the world over a wide range of research interests. These range from deep UV imaging surveys and spectra for cosmology, galactic evolution and AGN, to detailed studies of nearby galaxies, exoplanets, the outer solar system, and also enable unique time-domain and multi-messenger astronomy in the UV-blue.

For more information on the mission, see the CASTOR mission website.

BRITE-Constellation Mission Update

By Gregg Wade (on behalf of the Canadian BRITE team)
(Cassiopeia – Autumn / 2022)

BRITE-Constellation is an international space astronomy mission consisting of a fleet of 20x20x20 cm nanosatellites dedicated to precision optical photometry of bright stars in two photometric colours. The mission continues in full science operations, with 64 datasets available in the public domain from the BRITE public archive. As of April of 2020, all data is made public as soon as decorrelation is complete, with no proprietary period.

The BRITE mission is a collaboration between Canadian, Austrian and Polish astronomers and space scientists. The Canadian partners represent University of Toronto, Université de Montréal, Mount Allison University, and Royal Military College of Canada. The mission was built, and the Canadian satellites operated, by the University of Toronto Institute for Aerospace Studies Space Flight Lab (UTIAS-SFL). The Canadian Space Agency funded the construction of the Canadian satellites and continues to support their day-to-day operations.


There are five BRITE satellites in the Constellation, which work together to obtain well-sampled, long term continuous (~6 months) light curves in both red and blue band passes across a variety of sky fields.

As this issue of Cassiopeia went to press, the assignments of the BRITE nanosats were:

  • BRITE Toronto (Canada): This satellite observes with a red filter. It is currently observing the Cyg V with good pointing performance. (As indicated by the roman numeral, Cygnus is a BRITE legacy field being observed for the 5th time.)
  • BRITE Lem (Poland): Lem observes with a blue filter, but is currently idle due to unresolved stability issues.
  • BRITE Heweliusz (Poland): Heweliusz observes with a red filter. It is currently being set up on the Ori IX field.
  • BRITE Austria (Austria): BRITE Austria observes with a blue filter. It is currently observing the Sgr VIII field.
  • UniBRITE (Austria): Currently out of order.

The BRITE Constellation observing program is currently set through mid-2022. Details of the observing plan will be available on the BRITE photometry Wiki page.

Recent Science Results

“Towards a consistent model of the hot quadruple system HD 93206 = QZ Carinae – I. Observations and their initial analyses” (Harmanec et al. 2022, A&A, in press)

Fit to BRITE and TESS light curves of HD 93206 with the synthetic light curve computed with PHOEBE 1.

The hot nine-component system HD 93206, which contains a gravitationally bounded eclipsing Ac1+Ac2 binary (P=5.9987d) and a spectroscopic Aa1+Aa2 (P=20.734d) binary can provide important insights into the origin and evolution of massive stars. Using archival and new spectra, and a rich collection of ground-based and space photometric observations, we carried out a detailed study of this object. We provide a much improved description of both short orbits and a good estimate of the mutual period of both binaries of about 14500d (i.e. 40 years). For the first time, we detected weak lines of the fainter component of the 6.0d eclipsing binary in the optical region of the spectrum, measured their radial velocities, and derived a mass ratio of MAc2/MAc1=1.29, which is the opposite of what was estimated from the International Ultraviolet explorer (IUE) spectra. We confirm that the eclipsing subsystem Ac is semi-detached and is therefore in a phase of large-scale mass transfer between its components. The Roche-lobe filling and spectroscopically brighter component Ac1 is the less massive of the two and is eclipsed in the secondary minimum. We show that the bulk of the Hα emission, so far believed to be associated with the eclipsing system, moves with the primary O9.7I component Aa1 of the 20.73d spectroscopic binary. However, the weak emission in the higher Balmer lines seems to be associated with the accretion disc around component Ac2. We demonstrate that accurate masses and other basic physical properties including the distance of this unique system can be obtained but require a more sophisticated modelling.

Conferences, Resources, and Social Media


No conferences are planned for the immediate future.


The BRITE Public Data Archive, based in Warsaw, Poland, at the Nikolaus Copernicus Astronomical Centre, can be accessed here.

The mission Wiki (including information on past, current and future fields) can be accessed here.

BRITE Constellation is on Facebook, at @briteconstellation.

The BRITE International Advisory Science Team

The BRITE International Advisory Science Team (BIAST), which consists of BRITE scientific PIs, technical authorities, amateur astronomers, and mission fans, advises the mission executive on scientific and outreach aspects of the mission. If you’re interested in joining BIAST, contact Catherine Lovekin, the chair of BEST.

President’s Message

By Chris Wilson (CASCA President)
(Cassiopeia – Autumn 2022)

The summer is generally a quieter time for society business, as many of us take the opportunity for some vacation and to attend conferences and (dare I say it) do some research. Many of our committees do not meet unless something urgent comes up. The Board skipped its July meeting but had a productive meeting on August 17.

As a regular feature of these reports, I plan to provide a short summary on the steps that the Board is taking to improve and clarify CASCA procedures and governance, for itself, for its committees, and for the membership as a whole. Over the past 3 months, the Board has re-constituted its Action Items list and is starting to work its way through items systematically. There are a lot of items, many of them simple housekeeping, but everything takes some time and attention. The Vice-President and I are each circulating a written report to the Board ahead of the monthly Board meeting, so that the meetings themselves use less time for simple reporting of events and have more time for discussion. CASCA committee membership is now mostly complete, with just a few members remaining to be finalized for LCRIC and CATAC.

The Coalition for Canadian Astronomy has been fairly quiet over the summer. We took the opportunity of the extensive media coverage around the JWST early images to send LRP2020 and a cover letter to about 40 ministers, staff, and department officials in July. As we do every year, we are working on a 2023 pre-budget submission, which is due October 8. When Parliament resumes Sept 19, I expect Coalition activities will ramp up; we had no meetings with government over the summer.

In TMT news, the National Science Foundation announced the beginning of the scoping process for the TMT project. To quote from an AAS email August 22, “The scoping process is a public comment period for identifying issues to be analyzed during the creation of the Environmental Impact Statement (EIS) and for consultation under Section 106 of the National Historic Preservation Act to evaluate anticipated effects on historic properties on the summit of Maunakea, Hawaiʻi Island, Hawaiʻi. While the scoping process is a standard procedure for all NSF large facilities construction, NSF is also working to implement the community astronomy model outlined in the most recent decadal survey. To that end, NSF also invites the public to comment on NSF’s plans to engage the public in its EIS and Section 106 compliance processes through review of and comment on NSF’s Draft Community Engagement Plan.”

Other reports on progress with our major new initiatives (TMT, SKA, CASTOR, etc.) can be found elsewhere in this issue (or the 2022 Summer Solstice issue).

I want to close this article with a few thoughts on the subject of land and consent, a topic that is the focus of Recommendation #1 from LRP2020 and was the focus of a policy document that the LCRIC shared with the membership in advance of the May 2022 CASCA AGM. It is important to acknowledge my own settler background, as that background affects my perspective on this topic.

LRP Recommendation #1 states (in part): “We recommend that the Canadian astronomical community (e.g., ACURA, CASCA, and NRC-HAA) work together with Indigenous representatives and other relevant communities to develop and adopt a comprehensive set of guiding principles for the locations of astronomy facilities and associated infrastructure in which Canada participates. These principles should be centred on consent from the Indigenous Peoples and traditional title holders who would be affected by any astronomy project. “

In thinking about the topic of land and consent in the context of LRP Recommendation #1, probably the most important question that arises is who gives consent for a new facility to be built on Indigenous lands.

UNDRIP, the United Nations Declaration on the Rights of Indigenous Peoples, gives the answer: Indigenous peoples and communities. This is admittedly a very general answer and does not attempt to answer questions such as: which Indigenous peoples and communities; who speaks for those Indigenous peoples and communities; what to do if there are disagreements between the groups who are asked to consent to a project; etc. But it is appropriate that UNDRIP does not address these details: it is up to the Indigenous peoples and communities to decide if and how they wish to give consent.

A secondary question that follows from the question “who gives consent” is who decides/judges/acknowledges that “free, prior, and informed consent” has been received from Indigenous peoples and communities for a new facility to be built on Indigenous lands.

In Canada, the act that established the National Research Council (NRC) mandated that NRC should “operate and administer any astronomical observatories established or maintained by the Government of Canada”. So, for projects in which Canada is involved at a national level, the decision on whether consent has been given belongs with the Government of Canada and its agencies such as the NRC.

So you may ask, what is our role, as Canadian astronomers, in the process of land and consent? Personally, I feel that it is important that Canadian astronomers be reasonably confident that consent has been given for a new facility. This confidence helps to ensure that Canadian astronomers continue to support the facility that is being constructed. Confidence can be gained by educating ourselves: by reading reports and emails shared by the facility, by CASCA and its committees or by NRC; by attending informational webinars; by participating and asking questions; and so on. As a community (through CASCA and its committees, for example), we also have a role to play in working with Indigenous peoples and communities, as well as other organizations such as NRC, ACURA, etc., to help develop the guiding principles called for in LRP Recommendation #1. The policy document circulated by LCRIC in spring 2022 is an example of the type of work that is required from our community.

But ultimately it is not up to us as individual astronomers to make the call that consent has been given. We don’t have the necessary expertise (social, political, etc.) or resources. We don’t have the connections with the local Indigenous people and communities, certainly not for new telescope facilities that will be based in distant countries.

I think the important point is that we, the Canadian astronomical community, have said through LRP2020 Recommendation #1 that it is essential to obtain consent from Indigenous peoples and communities for new facilities that Canada is involved in. The Government of Canada, through NRC and other avenues, is taking this statement from our community very seriously in evaluating potential new national telescope facilities. Our role as individual Canadian astronomers is to stay informed about what is going on, to participate and ask questions, and to continue to educate ourselves.

Wishing you all a safe fall semester,