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Benbow, Wystan: Very High Energy Gamma-Ray Astronomy with VERITAS

Project Title: Very High Energy Gamma-Ray Astronomy with VERITAS 

Project Advisor: Dr. Wystan Benbow, 617-496-7597, wbenbow@cfa.harvard.edu

Background: VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a stereoscopic array of four atmospheric-Cherenkov telescopes that are sensitive to very high energy (VHE; E> 100 GeV) gamma rays. Located at the F.L.Whipple Observatory in southern Arizona, USA, the array began operation in 2007, and is currently the most sensitive VHE observatory in the world. The VERITAS Collaboration, which consists of ~80 scientists from institutions in the U.S.A., Canada, Germany, and Ireland, is carrying out observations that cover a broad range of science topics. VERITAS seeks to both identify new sources of VHE gamma rays, and to perform in-depth studies (e.g. spectral, temporal and morphological measurements) of the known VHE sources to better understand their underlying fundamental processes. VERITAS continues to lead the emergent field of VHE gamma-ray astrophysics, where in the past twenty years the VHE source catalog has grown from ~10 to ~250 objects. VERITAS is a also keystone facility for the high-growth field of multi-messenger astrophysics.

Scientific Questions: What are the sources of high-energy neutrinos and gravitational waves? What is the population of extragalactic very high energy gamma-ray emitters? What are the underlying non-thermal mechanisms behind these powerful particle accelerators? How do supermassive black holes accrete matter and produce powerful jets? How do AGN jets accelerate particles and are they sources of ultra-high energy cosmic rays? What is the origin of, and the timescales of, the extreme variability observed in VHE gamma-ray emitting blazars? 

Scientific Methodology: The SAO VERITAS group focuses on VHE observations of extragalactic objects including: active galactic nuclei (primarily blazars), radio galaxies, starburst galaxies, gamma-ray bursts and dark-matter dominated structures (e.g. galaxy clusters and dwarf galaxies). Since VHE gamma-ray sources emit radiation over ~20 orders of magnitude in energy, these studies often involve collaboration with experiments at lower energies (e.g., the Fermi Gamma-ray Space Telescope, several X-ray satellites (Chandra, Swift, NuSTAR, IXPE), and numerous optical and radio facilities). The multi-wavelength data are used to search for temporal flux correlations and variability time scales, and to generate spectral energy distributions enabling the non-thermal processes behind the observed emission to be modeled. Nearly every VERITAS observation also has multi-messenger astrophysics implications, and these science efforts also often involve correlation analyses of high-energy signals across all known astronomical messengers: photons, neutrinos, cosmic rays, and gravitational waves. A major goal of the SAO group is to publish the VERITAS AGN catalog, the first long-term, intensive multi-wavelength study of the entire (Northern) VHE AGN catalog, and an interpretation of these data. Members of the SAO group are expected to spend time at the VERITAS site observing and assisting with upgrades to various subsystems of the array, as well as in developing the next-generation of VHE gamma-ray instrumentation.

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Doeleman, Sheperd: Imaging Supermassive Black Holes

Project Title: Imaging Supermassive Black Holes

Project Advisor: Dr. Sheperd Doeleman, 617-496-7762, Observatory M215, sdoeleman@cfa.harvard.edu

Background: The Event Horizon Telescope (EHT) is a Very Long Baseline Interferometry (VLBI) array operating at the shortest possible wavelengths, which can resolve the event horizons of the nearest supermassive black holes. Initial observations with the EHT have revealed Schwarzschild radius scale structure in SgrA*, the 4 million solar mass black hole at the Galactic Center, and in the much more luminous and massive black hole at the center of the giant elliptical galaxy M87. Over the next 2 years, this international project will add new sites and increase observing bandwidth to focus on astrophysics at the black hole boundary. The EHT will have an unprecedented combination of sensitivity and resolution with excellent prospects for imaging strong GR signatures near the horizon, detecting magnetic field structures through full polarization observations, time-resolving black hole orbits, testing GR, and modeling black hole accretion, outflow and jet production. In April 2017, the EHT team completed its first observing campaign with the potential for horizon imaging. 

Scientific Questions: Our group is focusing on some of the most fundamental questions in astronomy that can only be answered with observations that resolve the event horizon of a black hole. How do black holes accrete matter? Simulations show that an interplay between magnetic fields and hot gas surrounding a black hole results in instabillities that drive matter to the event horizon, and the EHT will look for signatures of these physical processes. How do black holes launch jets that pierce entire galaxies? Some supermassive black holes power directed outflows that redistribute matter and energy on galactic scales, but the process is not well understood. By imaging the magnetic fields thought to accelerate charged particles at the black hole boundary, the EHT will test models for how jets are launched. Does General Relativity hold at the event horizon - was Einstein right? Close to the black hole, the strong gravity distorts light emitted by the infalling gas into a 'silhouette' or 'shadow'. The EHT is aiming to image this shadow whose shape and size is predicted by Einstein's Field Equations. Detection of this silhouette feature would confirm that millions of solar masses can be contained within a few Schwarzschild radii - all but cementing the existence of black holes. How does matter orbit black holes? Separate confirmation and testing of GR can be accomplished by time-resolving the orbits of material close to the black hole. The EHT can use non-imaging techniques to search for orbital signatures near the Innermost Stable Circular Orbit. 

Scientific Methodology: Our group uses numerical simulations to refine imaging algorithms and tests of strong field GR near a black hole. We also develop cutting edge instrumentation that we bring to remote mountain tops and install at mm and submm wavelength observatories. Each site has an atomic clock that enables us to synchronize and compare recordings made at sites around the Globe, each observing the same black hole at the same time. This technique, known as VLBI, synthesizes a virtual telescope as big as the Earth with unparalleled magnifying power. Students interested in instrumentation, signal processing algorithms and black hole astrophysics will find a lot to do in this project.

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Drake, Jeremy: High Energy Stellar Physics

Project Title: High Energy Stellar Physics

Project Advisor: Dr. Jeremy J. Drake, 617-496-7850, Observatory B-441, jdrake@cfa.harvard.edu

Background: Stars exhibit a range of energetic phenomena: hot coronae found on young protostars and stars like the Sun accretion thermal radiation from hot white dwarfs, novae and neutron stars. These phenomena are all characterised by plasmas that radiate copiously in the X-ray range and can be studied with satellite observatories above the Earths atmosphere.

Scientific Questions: What heats the coronae of stars? How do stellar outer atmospheric phenomena affect stellar and planetary evolution - star formation itself, protoplanetary disks, angular momentum loss through stellar winds and mass ejections, and the evolution of binary systems? What is the nature of the outer layers of neutron stars? What is happening in violent novae explosions?

Scientific Methodology: Our studies have recently concentrated on X-ray observations of stars using the Chandra and XMM-Newton observatories, and multi-dimensional photoionisation and radiative transfer models of protoplanetarty disks. High resolution X-ray diffraction grating spectra provide detailed information on individual objects, whereas CCD imaging spectroscopy provides lower resolution information on larger samples of objects, such as young pre-main sequence star clusters. Other observations compliment these studies for example, optical high resolution spectroscopy has been used to obtain information on elemental abundances that are of interest for probing outer atmospheric abundance anomalies in stars. Protoplanetary disk models are employed to investigate disk structure and ionisation under the influence of energetic phenomena.

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Elvis, Martin: Astronomical Prospecting: Steps to Asteroid Mining

Project Title: Astronomical Prospecting: Steps to Asteroid Mining

Project Advisor: Dr. Martin Elvis, 617-495-7442, Observatory B-424, melvis@cfa.harvard.edu

Background: Asteroids number in the millions and the total mass of industrially useful raw materials they contain is far vaster than the accessible materials in the Earth's crust. There are many potentially ore-bearing asteroids, but as a fraction of the total they are quite rare. As a result asteroid mining is likely to proceed in a multi-step process, like terrestrial mining, from initial surveys to final extraction. Astronomical techniques must be the first step in prospecting the asteroids.

Scientific Questions: How can we identify potential ore-bodes among the many asteroids given that most are just "dumb rock"? We are investigating two approaches: (1) Remote prospecting via large astronomical telescopes are preferred as they are cheap and can prospect large numbers of asteroids rapidly. However the information returned is limited. (2) Proximity prospecting, using instruments on spacecraft within a kilometer or so of the asteroid, provides far more detailed information, if the right instruments are used. But this approach is expensive to apply to many asteroids.

Scientific Methodology: (1) for telescopic prospecting we are beginning a campaign with the PISCO instrument on a 6.5m Magellan telescope in Chile; PISCO takes 4-color images simultaneously, and gets high signal-to-noise in 2 minutes, allowing both spectral types and accurate orbits to be obtained from the same data. (2) CfA scientists have developed miniature X-ray optics and radiation hard X-ray sensors that will make great proximity prospecting tools as well as enabling X-ray navigation for deep space missions; we are developing these into a system and will propose it at every opportunity.

Papers related to this project:

(1) Elvis, M., 2016, "Astronomical Prospecting: A Necessary Precursor to Asteroid Mining", 66th International Astronautical Congress, IAC-15-D4.3.10.

(2) Galache J.L., Beeson, C.L., McLeod, K.K., and Elvis, M., 2015, "The need for speed in Near-Earth Asteroid characterization", Planetary and Space Science, Volume 111, p. 155-166.

Golub, Leon: Dynamics of the Solar Corona 

Project Title: Dynamics of the Solar Corona 

Project Advisor: Dr. L. Golub, 617-495-7177, Observatory P-132, lgolub@cfa.harvard.edu, 

Background: Hot, X-ray emitting plasmas are ubiquitous throughout astrophysics, and the mechanism(s) responsible for their heating is poorly understood. 

Scientific Questions: What causes the heating and dynamics of the hot, magnetized solar outer atmosphere? What combinations of observations and modeling can be carried out to determine the mechanisms involved? 

Scientific Methodology: A combination of theory, modelling and experiment: calculate the plasma properties resulting from proposed instability mechanisms, model the observable effects, and compare to observations of the solar corona. 

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Golub, Leon: Heating of Hot Magnetized Plasmas

Project Title: Heating of Hot Magnetized Plasmas 

Project Advisor: Dr. L. Golub 

Background: Hot, X-ray emitting plasmas are ubiquitous throughout astrophysics, and the mechanism(s) responsible for their heating is poorly understood. 

Scientific Questions: What are the observable consequences of the different mechanisms proposed for heating of the solar coronal plasma? Can we distinguish among them via direct observation? 

Scientific Methodology: A combination of theory, modelling and experiment: calculate the plasma properties resulting from proposed dissipation mechanisms, model the observable effects, and compare to observations of the solar corona. 

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Hora, Joseph: Sgr A as Particle Accelerator: What Drives the Black Holes Variable IR and X-ray Emission?

Project Title: Sgr A as Particle Accelerator: What Drives the Black Holes Variable IR and X-ray Emission?

Project Advisor: Dr. Joseph. Hora, jhora@cfa.harvard.edu

Background: Sgr A, the central supermassive black hole (SMBH) of our Galaxy, is more than 100 times closer than any other massive black hole and therefore can be studied in far greater detail. Sgr As accretion flow is detected in the radio, submm, near-IR, and X-ray regimes and it is variable at all wavelengths. The relatively rapid (minutes to hours) fluctuations and temporal correlations between the NIR, submm, and X-ray indicate that the variable emission originates in the innermost regions of the accretion flow, not far outside the event horizon. Observations with GRAVITY/VLT have detected positional and polarization changes consistent with emission from a hot spot just outside the innermost stable circular orbit of the SMBH. Recent observations from ALMA, collected in concert with the Event Horizon Telescope, also show polarization variations, including loops that seem to trace a second instance of an orbiting hot spot. The emission and accretion mechanisms in this critical regime can be understood via targeted study of the variability.

Scientific Questions: Recent modeling has suggested two possible scenarios to explain the emission: magnetic reconnection near the event horizon, or secondary magnetic reconnection in the flux tube/hot spot ejected by the event horizon reconnection layer. The questions we will answer are: what is the origin and energy distribution of the non-thermal electrons? What are the variable SED properties from the submm to the NIR and X-ray? Are there delays between the emission from different wavelengths, and can they be explained by expansion or the reconnection mechanisms? Do bright flares have a different physical nature than the low level variability? And on the side of emission models: Are single-zone models adequate? Can we differentiate between the two reconnection scenarios?

Scientific Methodology: We have approved programs with JWST/MIRI, Chandra, and the SMA to obtain simultaneous mid-IR, X-ray, and submm monitoring observations of Sgr A, expected to be obtained in March 2024. The project will include reducing and analyzing these datasets to determine the emission mechanisms at work.

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Jarmak, Stephanie: Modeling Grainflow on Mars to Explore Enigmatic Streaks

Project Title: Modeling Grainflow on Mars to Explore Enigmatic Streaks

Project Advisor: Stephanie Jarmak, stephanie.jarmak@cfa.harvard.edu

Background: Goal: The overarching scientific goal of this project is to justify or refute a dry grainflow mechanism that would form enigmatic slope features on Mars, namely Recurring Slope Lineae (RSL) and Slope Streaks (SS). We address the following science questions in order to achieve our project goal:

  1. How does aeolian sand transport influence RSL sites?
  2. If RSL and SS are generated by dry grainflow, why do they start and stop on different slope angles and differ in scale?

Methodology: In the proposed research we investigate grainflow and aeolian conditions that would produce and modulate these slope features with a three-pronged approach including 1) contextual geomorphology with remote sensing, 2) experimental testing with a wind tunnel, and 3) simulated granular flow with an established numerical model.

The predoctoral researcher would be responsible for assisting the PI on the granular flow numerical model portion of the project.

Task 1 of the project (carried out by Co-I David Stillman at SwRI) constrains the minimum upslope sand transport observed at RSL sites and provides slope profiles and slope feature properties to fit via numerical modeling in Task 3 (to be carried out by the PI and predoctoral student at SAO). Task 2 (to be carried out by Co-I Mackenzie Day and their graduate student at UCLA) calibrates the numerical models in Task 3 and experimentally tests the aeolian-grainflow model of RSL incremental lengthening. Task 3 models flow volumes and grain properties that would match observed RSL and SS properties for comparison to sand transport estimates from Tasks 1 and 2.

Significance of the proposed work: The proposed research would test a compelling formation mechanism for RSL and SS along with a modulation mechanism for RSL that has yet to be robustly explored by the community. By numerically modeling the flow volumes that would produce RSL and SS we would provide the first numerical constraints on the physics of dry granular flows that produce features on Mars that have been under investigation for over a decade. RSL are abundant on Mars, and are potential targets of future missions, so better constraining/understanding how they work is a high-impact goal. The proposed research would provide the scientific community with detailed, self-consistent aeolian feature maps in the immediate vicinity of a significant number of known RSL and SS sites, along with interpretations and insight aided by grainflow modeling and experiments. Even if clear support for a purely dry grainflow mechanism that explains RSL and SS behavior does not result from this proposed work, our findings provide additional constraints that are needed by the community to formulate hypothetical mechanisms that do sufficiently explain the comprehensive observed behavior of these slope features.

Funding: Yes

Fund Length: Three years

McCarthy, Michael: Laboratory Spectroscopy of Highly-Reactive Molecules of Astrophysical Interest

Project Title: Laboratory Spectroscopy of Highly-Reactive Molecules of Astrophysical Interest

Project Advisor: Michael C. McCarthy, 617-495-7262 or 617-495-9848, P-256, mmccarthy@cfa.harvard.edu

Background: Understanding the chemical composition in the interstellar medium can provide much insight into a variety of astrophysical processes, allowing one to derive important physical properties such as mass loss, temperature, density, fractional ionization, etc. Many of the key chemical intermediates found in space are highly reactive or unstable species, generally unknown or unfamiliar on Earth, such as radicals, carbenes, and positively and negatively-charged ions. Unambiguous astronomical detection of these reactive intermediates frequently requires highly accurate measurements of their rotational spectra throughout the radio band. Using highly sensitive laboratory instrumentation and production techniques developed at SAO, such measurements are undertaken, yielding precisely rest frequencies to guide dedicated radio astronomical searches for new molecules.

Scientific Questions: What are the key chemical intermediates in astronomical sources? What methods and techniques can be used to detect this species in the laboratory? How can these intermediates be used to provide new insight into astrophysical process?

Scientific Methodology: Chemically unstable molecules of astronomical interest are produced and detected in the radio band using custom instrumentation. Laboratory searches are often undertaken in collaboration with leading theoretical groups here and abroad because computational predictions serve as a useful guide to experiment. Target reactive species are synthesized by applying an electrical discharge to a mixture of precursor gases, as the gas mixture rapidly expands to form an ultra-cold molecular beam. Fourier transform microwave spectroscopy is used in the centimeter-wave band to conduct spectral surveys at frequencies predicted by theory. Follow-up investigations to confirm the carrier of the rotational lines or to extend the frequency range of the laboratory measurements are often undertaken as part of this effort.

 

 

McCollough, Michael: A Multi-Wavelength Study of the Relativistic Jet Source Cygnus X-3

Project Title: A Multi-Wavelength Study of the Relativistic Jet Source Cygnus X-3

Project Advisor: Dr. Michael L. McCollough, 617-496-2119, Observatory B-240, mmccollough@cfa.harvard.edu

Background: Cygnus X-3 is one of the most enigmatic X-ray binaries to have been studied. Its X-ray flux shows a 4.79 hr modulation associated with its orbital period. While the period is typical of a low mass system IR observations have shown that the mass donating companion is a massive Wolf-Rayet star. Cygnus X-3 has two major X-ray states (low/hard and high/soft), shows correlative activity between the radio and hard X-ray, and relativistic jets have been observed in the system (~0.9c).

Scientific Questions: Among the issues we are seeking to address in this study are:

Hard X-Ray/Gamma-Ray Continuum: We seek to understand the nature of the hard X-ray/gamma-Ray continuum associated with major radio flares. Is it due to synchrotron or inverse Compton? Are the processes producing this emission nonthermal or thermal in nature?

Annihilation Lines: The major radio flares in Cygnus X-3 have been linked to relativistic jets. The composition of these jets is a major point of interest. Are they a pair plasma (electrons and positrons) or do baryons play major role in their makeup? The detection of annihilation lines make help answer this question.

Timing Properties: Do the major flares have a distinctive timing signature? The RXTE observations probe times very close to the creation of these major flares.

Properties of Cygnus X-3's Wind: The Chandra observations (supported by the RXTE) observations will allow a detailed (phased resolved) measurement of the parameters and nature of the wind associated with Cygnus X-3.

Multi-Wavelength Correlations: We will look for and study the correlations between the different wavelenghts (radio, X-ray, Gamma-Ray, IR). These will be examined relative to previously discovered correlations.

Scientific Methodology: Since early 2002 Cygnus X-3 had been in an unusually long quiescent state (~ 1300 days). At the start of 2006 Cygnus X-3 transitioned from a radio quiescent (low/hard) state to a flaring (high/soft) state. Among the activities that have been observed are an extended quenched state (high X-ray, very low radio, and very low hard X-ray emission), rapid (< 1 day) bright flares (~ 3 Jy), and three major radio flares (~ 14 Jy). 

During this active state, a major international multi-wavelength observing campaign has been undertaken. This campaign includes observations in the radio (Ryle, RATAN-600), IR (PAIRITEL), UV/Optical (Swift), X-ray (Chandra, RXTE, INTEGRAL, Swift), hard X-ray (RXTE, INTEGRAL, Swift), and Gamma-ray (INTEGRAL, Whipple). 

This project involves the analysis of these various data sets with particular emphasis on the spacecraft data (Chandra, INTEGRAL, RXTE, and Swift). We will be using XSPEC, FTOOLS, CIAO, and OSA (INTEGRAL Data Analysis) software to analyze the various data sets.

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Stark, Antony: The South Pole Telescope (SPT)

Project Title: The South Pole Telescope (SPT)

Project Advisor: Antony Stark, 617-495-7256, Observatory M-205, astark@cfa.harvard.edu

Background: The SPT is a 10m diameter millimeter wave telescope located at Amundsen-Scott South Pole Station in Antarctica. It has been in continual operation since 2007, resulting in over 40 major publications on a variety of topics that are fundamental to cosmology and high-redshift astrophysics. SPT is operated by an informal consortium of 70 scientists from 20 institutions including the Harvard-Smithsonian Center for Astrophysics. CfA astronomer Antony A. Stark, as one of the founders of the project, can provide unrestricted access to all SPT consortium data. The project is manpower limited, with a great many interesting projects available to pre-doctoral students. Consortium policy is to encourage independent work by students and to reward those efforts with first-authorships.

Scientific Questions: SPT science falls into three broad categories: cosmology by direct observation of features in the Cosmic Microwave Background including E- and B-mode polarization and lensing; cosmology and astrophysics of galaxy clusters discovered via the Sunyev-Zeldovich effect; and the astrophysics of highly-redshifted galaxies that happen to be unusually bright because they are behind a strong gravitational lens. The SPT is among the few instruments in the world that is currently constraining cosmological models and the properties of neutrinos. Future observations will determine the tensor-scalar ratio, running, kinetic S-Z effect, the structure of matter between z = 0 and z = 1000, the timescale of reionization, the number and masses of neutrino species, and the history of Dark Energy. Galaxy cluster projects will study the ensemble of clusters in the context of cosmology as well as the physics of intergalactic gas, star formation and populations of stars in cluster galaxies. Our sample of highly-magnified high-z galaxies allow study of star and galaxy formation in the very early Universe. That data can be used, for example, to study the possible existence of a large-scale gradient in the fine structure constant.

Scientific Methodology: The SPT is engaged in several long-term survey projects to produce deep (~ 3 microK rms) images of 10% of the sky near the south galactic pole at 90, 150, and 230 GHz. Six years of data, comprising the first Sunyaev-Zeldovich effect survey and the first two deep polarization surveys are complete. Survey work is expected to continue for at least the next 5 years. The sensitivity of the SPT has recently been greatly improved with the successful commissioning of the SPT3G detector system. SPT is by far the most sensitive CMB instrument, currently operating at brightness levels 30X deeper than Planck at 3X higher resolution. Detections in the survey are followed up with a wide variety of observations in the radio, infrared, visual, UV and X-ray. Harvard-Smithsonian participants in this project routinely observe with the Hubble, Spitzer, and Chandra Space Telescopes, the Magellan, Gemini, and VLT telescopes, and the ALMA and ATCA radio telescopes.

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Tong, Edward: Next Generation Instrumentation for the Submillimeter Array

Project Title: Next Generation Instrumentation for the Submillimeter Array

Project Advisor: Edward Tong, 617-496-7641, etong@cfa.harvard.edu

Background: The Submillimeter Array (SMA) is an 8-element radio interferometer on Mauna Kea, Hawaii, operating at submillimeter wavelengths. The array is a pioneer in wideband radio interferometry. For our second generation instrumentation, wSMA, that we are currently developing, the superconducting receiver, based on the Superconductor-Insulator-Superconductor (SIS) junction will deliver an intermediate frequency (IF) from near DC to close to 20 GHz for processing. In the future, further enhancements of performance are possible by moving to a dual-sideband (2SB) configuration, even wider IF bandwidth and simultaneous multi-band operations.

Scientific Questions: The present SIS mixers used in SMA are fabricated on crystalline quartz. With the advent of mature silicon chip processing, one can envision that the SIS mixers can be made on silicon wafer and other functionality can be integrated on the same chip, for example, coupler for local oscillator injection, quadrature hybrid for 2SB mixers, and planar orthomode transducer. While the individual technology exists, the goal of integrating them remains aloof. One further idea we are keen to pursue is to design silicon chip multiplexers that channelize the signal collected by the telescope into multiple SIS receivers that can be operated simultaneously.

Scientific Methodology: The research starts with a basic understanding of the operation of SIS mixers and various submillimeter components used in submillimeter receivers. It is followed by investigation of various receiver components in silicon, and experimenting with different configurations that would lead to a compact and practical receiver with multi-band capability.

Tran, Vy: Illuminating the Dark Universe with Gravitational Lensing

Project Title: Illuminating the Dark Universe with Gravitational Lensing

Project Advisor: Vy Tran, 617-384-7689, kim-vy.tran@cfa.harvard.edu

Background: Gravitational lensing has matured into a powerful cosmic tool for exploring a wide range of astrophysical phenomena such as multiply imaging a single supernova, identifying the highest redshift galaxies, and mapping dark matter distributions. With upcoming all-sky surveys, we are at the brink of a revolution where deep high resolution imaging of vast cosmological volumes is becoming widely available.

I currently lead the ASTRO 3D Galaxy Evolution with Lenses (AGEL) Survey (Tran et al. 2022) to deliver a benchmark sample of new gravitational lenses that can be observed by both northern and southern hemispheres to obtain high quality follow-up with upcoming Adaptive Optics and space telescopes. The AGEL observations include an ongoing Hubble Space Telescope program to image 500 strong lenses, extensive ground-based spectroscopy of the deflectors and sources, and complementary multi-wavelength observing campaigns to capture the interplay between gas and stars.

Potential thesis research projects include measuring the dark matter profiles of the foreground deflector halos mapping the Circum-Galactic Medium associated with the foreground deflector quantifying the changing conditions of the Inter-Stellar Medium of the background lensed sources and measuring the Hubble constant for a subset of compound lenses that have two sources at different redshifts.

About me: I have graduated 5 PhD students, served on 15 PhD committees, and supervised 30+ undergrads in research including Honors theses. I develop and support professional development opportunities for early career researchers as part of my current role as the Associate Director for Internal Relations at the CfA.

Funding: Yes

Fund Length: Two years

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Zhang, Qizhou: How Do Most Massive Stars Form?

Project Title: How Do Most Massive Stars Form?

Project Advisor: Qizhou Zhang, qzhang@cfa.harvard.edu

Background: Massive protostars must accrete more than 30 Msun to become early O-type stars, but they begin core hydrogen burning well before reaching their final masses. This poses a serious challenge to the current paradigm that massive stars form through accretion of molecular gas. The detection of resolved molecular disks around protostars more massive than 20 Msun shows that this scenario is valid for the formation of stars up to about a few tens of solar masses however, it likely breaks down for the formation of more massive stars. After the protostars have reached several tens of solar masses, the amount of EUV photons is large enough to ionize the otherwise molecular accretion flow(s) and the cloud in which they reside. This raises several outstanding questions: How does star formation proceed in such an environment? Is it through an ionized accretion disk? Does this disk surround a single or a multiple stellar system? When does active (proto)stellar accretion stop due to the increasing photoionization feedback?

This project will utilize sub-arcsecond observations of molecular and ionized gas in luminous star forming regions in the Milky Way obtained from the SMA, ALMA and VLA. Through analyses of kinematics of both molecular and ionized gas surrounding massive protostars, the study holds promise to unravel accretion flows feuling the formation of O stars. The findings will expand our knowledge of Galactic star formation and have implications to the formation of most massive stars in the early universe.

References

Zhang, Qizhou: Extreme Star Formation in the Galactic Center

Project Title: Extreme Star Formation in the Galactic Center

Project Advisor: Qizhou Zhang, qzhang@cfa.harvard.edu

Background: The central molecular zone (CMZ), the inner 200 pc of the Milky Way, harbors the most extreme physical conditions for star formation in the Galaxy. The gas properties, radiation field, and cosmic ray ionization rate are more similar to those in the center of other galaxies, starbursts or galaxies of redshifts around 2 than in the Solar neighborhood clouds. It is the only extreme environment where the small-scale physics of individual star formation can be resolved and be linked with the galactic-scale processes that together drive the evolution of galaxies.

Despite the large reservoir of dense molecular gas in the CMZ that rivals starburst regions in external galaxies, its average star formation efficiency is more than a factor of 10 lower than in the Milky Way disk. What is the underlying cause of such a low star formation? Strong turbulence and Galactic shear may raise the density threshold and suppress star formation in the CMZ. However, turbulence alone cannot account for variations of star formation among clouds in the CMZ. Do magnetic fields play a role in suppressing overall star formation in the CMZ? Solving these puzzles will inform our understanding of star formation in different physical environments and has broad implications to starburst phenomena in galaxies.

This project will analyze observations from two large programs from ALMA and VLA respectively, which mosaic the CMZ in continuum and a set of molecular line transitions. The project is augmented by magnetic field data of the CMZ from SOFIA HAWC+ and for individual CMZ clouds from ALMA. The findings promise to offer critical insight to the role of magnetic fields on star formation in the CMZ.

References:

Funding: Yes, partially

Fund Length: One year