Thunch

Thunch is a weekly grad-student run seminar series which gives local and visiting scholars, especially students, the opportunity to present their research to a broad audience at the Princeton Department of Astrophysical Sciences. Potential visiting speakers should contact the Thunch Czars, Yue Pan and Zack Andalman.

When: Thursdays at 12 pm (food), 12:15 pm (talk)

Where: Peyton Hall, "grand central" (big room in the center of the building)

What: food + 45 minute talk and discussion

Rules

- Priority goes to students and to those who have not given a talk recently.

- Visiting speakers must be explicitly invited by the department.

- There is no money in Thunch to pay for travel, so visitors need another source of funding.

- Speakers must send a title and abstract to the Thunch Czars at least one week in advance of their talk.

Now you know everything you need to know about Thunch. Join us for good food and good science. Reach out to the Thunch Czars with any questions.

Spring 2024 Upcoming Speakers:

 

DateName and InstitutionTitleAbstract
February 8thBenjamin Remy (Princeton University)Generative modeling for weak lensing inverse problemsGravitational lensing, which is the effect of the distortion of distant galaxy images through the influence of massive matter densities in the line of sight, holds significant promise in addressing questions about dark matter and dark energy. It reflects the distribution of total matter of the Universe and is therefore a promising probe for cosmological models. In the case where these distortions are small, we call it the weak gravitational lensing regime, and a straightforward mapping exists between the matter distribution projected in the line of sight, called mass-map, and the measured lensing effect. However, when attempting to reconstruct dark matter mass-maps under conditions involving missing data and high noise corruption, this linear inverse problem becomes ill-posed and may lack a meaningful solution without additional prior knowledge. In this talk, I will present how to employ recent breakthroughs in the generative modeling literature that enable the modeling of complex distribution in high-dimensional spaces. We propose in particular a novel methodology to solve high-dimensional ill-posed inverse problems, characterizing the full posterior distribution of the problem. By learning the high dimensional prior from cosmological simulations, we demonstrate that we can reconstruct high-resolution 2D mass-maps alongside uncertainty quantification. Additionally, I will present a new method for cosmic shear estimation based on forward modeling of the observations at the pixel level. This represents a new paradigm for weak lensing measurement, as we no longer rely on galaxy shape measurements.
February 15thLauren Weiss (University of Notre Dame)The Origin of Earth: Lessons from Multi-Planet SystemsHow did we get here? Are we alone? A fundamental milestone in answering these questions is the origin of Earth, an event that we are just now contextualizing, thanks to the discovery of thousands of exoplanets. Using telescopes, my group is investigating how the thousands of known planetary systems formed, what they reveal about the origin of our solar system, and whether these distant worlds could harbor life. As part of our investigation, we are discovering Earth-sized planets with rocky surfaces amenable to life as we know it. Many of these worlds form in intriguing patterns of regular sizes and orbital spacing. These patterns, which we call “peas in a pod,” are mostly absent from the solar system, but resemble a theorized early generation of solar system planets thought to have been destroyed by Jupiter. Meanwhile, Earth’s extensive surface liquid water, which is a key aspect of its suitability for life, is attributed to the gravitational influence of Jupiter. How was Earth’s water delivered, and can we expect the small, rocky exoplanets we have discovered to be similarly endowed? By combing the cosmos to search for patterns in multi-planet systems and trace the role of Jupiter analogs in providing rocky planets with water, we hope to contextualize the origin of Earth and guide humanity’s next steps in the search for life.
February 22rdMartin Elvis (Harvard)Accelerating the Astro2020 Flagships with Super-Heavy Lift VehiclesThe Astro2020 Decadal Report recognized that 21st Century astrophysics required matched coverage across the electromagnetic spectrum. To create that coverage Astro2020 recommended a set of three Great Observatories: X-ray, IR/O/UV, and Mid-Far-IR.
Unfortunately, given the estimated costs and anticipated budget, the first of these (the IR/O/UV flagship) would not launch until 2045. This is a daunting timescale as even JWST with an anticipated lifetime of 20 years will likely be gone by then. Fortunately, there may be a means to speed up the Astro2020 program making use of a new generation of Super-Heavy Lift rockets: SLS, SpaceX Starship, and Blue Origin New Glenn. Each of these vehicles has far greater mass to orbit and payload volume than the present generation of launchers. As a result a broad design space is opened up. It may be possible to use mass and volume to cut the cost of the flagships by billions. Volume is particularly valuable for the IR/O/UV and Mid-Far-IR flagships to avoid the origami-like folding of large mirrors that was necessarily employed for JWST. Instead, the X-ray flagship mass seems to be more important. Discipline will be needed to restrain ourselves from mission creep, going beyond the already ambitious Astro2020 flagship capabilities. If we do, then major savings may cut years from the Astro2020 timeline.
 
February 29thDom Rowan (Ohio State University)Searching for Non-Interacting Black Holes in the Binary ZooCharacterizing the mass distribution of compact objects is essential for understanding the late-stage evolution of massive stars. The majority of black hole mass measurements come from X-ray binaries and gravitational wave mergers, but the majority of black holes in the Milky Way are expected to instead be either isolated, free-floating systems or in non-interacting binaries. The recent Gaia Focused Product Release contains radial velocity time-series for more than 9,000 long-period photometric variables. We search for binary systems with large radial velocity amplitudes to identify candidates with massive, unseen companions. Eight targets have large binary mass functions f(M) > 1 Msun, three of which are eclipsing binaries. The remaining five show evidence of ellipsoidal modulations. We fit spectroscopic orbit models to the Gaia radial velocities and fit the spectral energy distributions of three targets. For the two systems most likely to host dark companions, J0946 and J1640, we use PHOEBE to fit the ASAS-SN light curves and Gaia radial velocities. The derived companion masses are >3 Msun, but the high Galactic dust extinctions towards these objects limit our ability to rule out main sequence companions or subgiants hotter than the photometric primaries. These systems are similar to other stellar-mass black hole impostors, notably the Unicorn (V723 Mon) and the Giraffe (2M04123153+6738486). While it is possible that J1640 and J0946 are similar examples of stripped giant star binaries, high-resolution spectra can be used to determine the nature of their companions.
March 7thYubo Su (Princeton University)Merging Binary Black Holes with a Distant Tertiary Companion---Spin and Mass Ratio DependenciesThe LIGO/Virgo/KAGRA (LVK) collaboration has detected 90 compact object mergers to date, most of which are mergers of two black holes. Despite being a great triumph for relativists, these detections also pose a problem for astronomers: how do these binaries form? In the case of binaries consisting of two black holes (black hole binaries; BHBs), the simplest explanation is that they are formed from the large number of massive stellar binaries that we observe. However, this formation channel is both filled with uncertainties and struggles to reproduce certain features of the observed population of merging BHBs. As such, a zoo of alternative formation channels have been proposed, requiring anywhere from 3 gravitationally interacting objects to 10^8. Among these, the three-body channel is attractive for its plausibility (e.g. massive stellar triples are common) and its relative tractability ([semi]-analytical results can be obtained). In this informal talk, I will discuss the dynamics and some predictions of the three-body channel, including a potential application to the unresolved "q-chi_eff" anticorrelation in the LVK data.
March 21stPascal Marichalar (French National Center for Scientific Research)Reserving Mauna Kea for astronomy : a social and political history, from Kuiper to TMTIntrigued by the recent Thirty Meter Telescope protests on Mauna Kea in Hawai‘i, I launched on a historical study of how astronomy came to the mountain, based on the observatories’ own archives, most of which had never been studied. These documents show the colonial and social underpinnings of how astronomers “discovered” and conquered what Gerard Kuiper heralded in 1964 as “probably the best site in the world – I repeat – in the world, from which to study the Moon, the Planets, the Stars” : how vast tracts of lands were given away for $1 annual leases, how observatories took the place of the islands’ sugarcane plantations and ranches, how the world’s biggest mirrors collided with the environmental and Hawaiian movements. This case study raises the larger question of the responsibility of field scientists toward the sites and communities in which they work.
March 28thYuanhong Qu (University of Nevada, Las Vegas)Magnetospheric origin of Fast Radio BurstsFast Radio Bursts (FRBs) are the milliseconds-duration radio bursts mainly originating from cosmological distances. The high brightness temperatures (~10^{36} K) implies the intrinsic emission mechanisms must be coherent. Observations show that most FRBs are highly linear polarized and some have very high circular polarization degree, many FRBs have narrow frequency spectrum. Both polarization properties and narrow frequency bandwidth carry important information about radiation mechanisms and environment properties of FRB sources. In this talk, I will discuss three main questions regarding the physical mechanisms of FRBs by using observational results. The topics include “Where are FRBs produced?”, “Can strong FRB waves escape from the magnetosphere?” and “How are FRBs produced through possible coherent radiation mechanisms?”. I will provide arguments that FRBs likely originated from the magnetar magnetosphere, and discuss the production and propagation of FRBs. I will comment on the pros and cons of close-in (Pulsar-like) and far-away (GRB-like) radiation models based on the observational narrow bandwidth data. I will propose that the low frequency waves can be boosted to power FRBs via coherent inverse Compton scattering and discuss the observational implications.
April 4thNick Kokron (Princeton University)Structure formation at the juncture of simulations and perturbation theoryA key challenge for the next decade of survey cosmology is ensuring that the models for summary statistics they measure, such as galaxy clustering and lensing, are sufficiently accurate in light of the high degree of precision of these measurements. In this talk I will introduce some of the standard theoretical techniques the community has employed to extract cosmological information from galaxy surveys (from clustering to cosmic shear). These have historically belonged to two disparate classes of models -- pen-and-paper perturbation theory and supercomputer simulations of nonlinear structure formation. I will discuss pros and cons of these approaches, and discuss some of the work I carried out for my thesis at the intersection between these two techniques which makes them more powerful than the sum of their parts. 
April 11thKovi Rose (University of Sydney)The Wide-Field search for radio transients in the southern skyWe have entered the age of big data in astrophysics and radio astronomy is no exception. The Australian SKA Pathfinder (ASKAP) conducts wide-field radio surveys of the southern skies. With millions of radio-bright sources in the survey regions we are using new polarisation filtering and positional cross-matching, as well as more traditional variability metrics, to identify interesting astronomical transients. From stellar radio emission to late-time supernova re-brightening, I will speak about some of the different objects we are finding with ASKAP, including the detection of periodic bursts from an ultracool brown dwarf star.
April 18th (double talk)

Jamila Pegues (Space Telescope Science Institute)


Kris Pardo (University of Southern California)

Pegues: Modeling Protoplanetary Disk Photochemistry around a Variety of Young Stars
 

 

Pardo: Gravitational-wave detection with photometric surveys

Pegues: Exoplanets have been discovered around a breathtaking variety of stars.  To constrain the chemical origins of these exoplanets, we study the chemical composition of their birthplaces: protoplanetary disks.  Over the past decade, modern interferometry has allowed us to probe volatile disk chemistry around stars across the young stellar distribution, from cool M-stars to bright Herbig Ae stars.  Now, studies are targeting larger samples of disks around a larger variety of stars than ever before. As our observations of planet-forming disks expand into every corner of the young stellar distribution, it is important that our astrochemical disk models, which are used to investigate and explain the chemistry that we observe, keep pace.  To this end, we discuss recent work in computing ultraviolet photochemistry for disks around a variety of young stars.  Ultraviolet photochemistry is an important component in any disk's chemical network, and is dependent on the central star.  Fully recalculating its coefficients for a given star, however, is a time-consuming process. Using a sample of young stellar spectra and templates for a variety of spectral types, as well as crafted astrochemical disk models, we evaluate methodologies for approximating these coefficients across a database of molecular species.  We investigate how these approximations are propagated through the models, and if/how they affect the modeled chemistry for commonly targeted molecular species.  Finally, we discuss the implications of these results for chemically modeling our growing database of planet-forming disks around young stars.

 

Pardo: Gravitational waves have offered us a whole new way of looking at our Universe. So far, we have seen them in the ~10-100 Hz range, and, most recently, in the nanohertz regime. However, there are parts of the frequency space that are currently not covered by any future or planned observations. I will explain how we can use upcoming photometric surveys to bridge the gaps in the spectrum through relative astrometric measurements. Similar to the pulsar timing array measurements, these astrometric measurements rely on the coherent spacetime distortions produced by gravitational waves at Earth. These induce coherent, apparent stellar position changes on the sky. Upcoming photometric surveys will have excellent relative astrometry and timing resolution, which will make them perfect for detecting gravitational waves in the microhertz regime. I will discuss this measurement scheme, as well as our concrete steps to develop ideal estimators, and our work to mitigate systematics in real data.

April 25thJackie Faherty (American Museum of Natural History)Brown Dwarfs in the Era of JWST Brown Dwarfs are objects with masses that straddle “planet” and star classifications.  They are defined by an inability to sustain stable Hydrogen burning.  Their spectral energy distributions morph with time as they cool from objects as hot as stars to those as cool as Jupiter.  The atmospheres of brown dwarfs contain a potpourri of molecules and exotic condensate cloud materials that are extremely relevant to planetary science investigations (both solar system and objects discovered beyond the Sun).  In this talk I will discuss the importance of brown dwarfs in the context of how and where they overlap with exoplanet studies.  I will particularly focus on brown dwarfs that challenge our understanding of the line between a companion planet and an isolated object.  Specifically on the collection of brown dwarfs dubbed “Super Jupiters” that have masses, temperatures, and atmosphere features nearly identical to what is seen or what is expected in exoplanet studies.   I will show newly obtained data from JWST cycle 1 and 2 on nearby brown dwarfs and how this data is redefining our understanding of the diversity in the atmospheres of cold worlds.
May 2ndTrung Ha (University of North Texas)Segmentation of Current Sheets in Magnetized Plasma Turbulence with Computer VisionComputer vision and machine learning tools offer an exciting new way to automate analyzing and categorizing information from complex computer simulations. In my talk, I will discuss our current efforts in designing new computer vision tools for the automatic segmentation of large, high-resolution plasma turbulence simulations. In particular, there is a great interest in the astrophysical community to identify and characterize so-called current sheets -- 2D sheet-like structures of intense current flow near accreting black holes. The formation of sheets and subsequent magnetic reconnections due to the tearing of these sheets contribute to the local heating in plasma and serve as a mechanism for non-thermal particle acceleration. Our current sheet identification framework is based on a self-organizing map (SOM) algorithm. SOMs are an unsupervised machine-learning technique that is particularly efficient in clustering and classifying high-dimensional big data. In my talk, I will describe in detail our new implementation of a JIT-accelerated SOM algorithm and usage of the SOM method to efficiently cluster plasma simulations.
May 16thStephane Werner (Durham University)Intracluster Light in the Core of z~2 Galaxy Proto-clustersIntracluster light is thought to originate from stars that were ripped away from their parent galaxies by gravitational tides and galaxy interactions during the build up of the cluster. The stars from such interactions will accumulate over time, so semi-analytic models suggest that the abundance of intracluster stars is negligible in young proto-clusters at 𝑧 ∼ 2 and grows to around a quarter of the stellar mass in the oldest, most mature clusters. In contrast to these theoretical expectations, we report on the detection of intracluster light within two protoclusters at 𝑧 = 2 using deep HST images. We use the colour of the intracluster light to estimate its mass-to-light ratio in annuli around the brightest cluster galaxies (BCG), up to a radius of 100 kpc. We find that 54 ± 5% and 71 ± 3% of the stellar mass in these regions is located more than 10 kpc away from the BCGs in the two proto-clusters. This low concentration is similar to BCGs in lower redshift clusters, and distinct from other massive proto-cluster galaxies. This suggests that intracluster stars are already present within the core 100 kpc of proto-clusters. We compare these observations to the Hydrangea hydrodynamical galaxy cluster simulations and find that intracluster stars are predicted to be a generic feature of group-sized halos at 𝑧 = 2. These intracluster stars will gradually move further away from the BCG as the proto-cluster assembles into a cluster.

Fall 2023:

September 7th: Andrew Saydjari (Harvard)
September 14th: Samantha Wu (CalTech)
September 21st: Viraj Karambelkar (CalTech)     
September 28th: Fan Zou (Penn State)   
October 5th: Zhuhai Li (CalTech)
October 12th: Prof. Yue Shen (University of Illinois Urbana-Champaign)                 
October 19th: Mor Rozner (Israel Institute of Technology)                          
October 26th: Shangjia Zhang  (University of Nevada)
November 2nd: Teodor Grosu
November 9th:  Hsiang-Chih Hwang (IAS)                           
November 16th: Kishore Patra (Berkeley)
November 30th: Princeton Graduate Students

Spring 2023:

February 16th: Chia-Yu Hu (University of Florida)
February 23rd: Dan Foreman-Mackey (CCA)
March 2nd: Lyla Jung (ANU)
March 16th: Vicente Valenzuela-Villaseca (Princeton University)
March 30th: Roohi Dalal (Princeton)
April 6th: David Velasco (Princeton)
April 13th: Zili Shen (Yale)
April 20th: Sabrina Appel (Rutgers)
May 4th: Ewine van Dishoeck Leiden (Leiden Observatory, the Netherlands)
May 11th: Charlotte Ward (Princeton University)

Fall 2022:

September 15th: Aritra Ghosh (Graduate Student, Yale University)
September 22nd: Thales Gutcke (NASA Hubble Fellow and Lyman Spitzer, Jr. Postdoctoral Fellow, Princeton)
September 29th: Fengwu Sun (Graduate Student, University of Arizona)
October 6th: Yubo Su (Lyman Spitzer Jr. Postdoctoral Fellow, Princeton)
October 13th: Yinhao Wu (Graduate Student, Leicester University)
October 20th: Fall break
October 27th: Lizhong Zhang (Graduate Student, University of California, Santa Barbara)
November 3rd: Oliver Zier (Graduate Student, Max Planck Institute for Astrophysics, Garching, Germany)
November 10th: Chang-Goo Kim (Post-Doctoral Associate Research Scholar, Princeton)
November 17th: Tsun Hin Navin Tsung (Graduate Student, University of California, Santa Barbara)
December 1st: Ore Gottlieb (Rothschild Fellow, CIERA Postdoctoral Fellow, Northwest University)
December 8th: Sihao Cheng (Postdoc Member at the Institute for Advanced Study)

Spring 2022:


February 3rd: Benjamin Crinquand (Post-Doctoral Associate Research Scholar, Princeton)
February 10th: Matthew Coleman (Post-Doctoral Associate Research Scholar, Princeton)
February 17th: Riddhi Bandyopadhyay (Post-Doctoral Associate Research Scholar, Princeton)
February 24th: Alex Gagliano (Pre-Doctoral Fellow, CCA Flatiron)
March 10th: Igor Andreoni (Postdoctoral Fellow, Joint Space-Science Institute)
March 17th: Sihao Cheng (Postdoctoral Fellow, Johns Hopkins University)
March 24th: Keith Hawkins (Assistant Professor, University of Texas at Austin)
March 31th: Frank van den Bosch (Professor of Theoretical Astrophysics, Yale)
April 7th: Mor Rozner (Graduate Student, Technion - Israel Institute of Technology)
April 14th: Sam Yee (Graduate Student, Princeton)

Click here for the latest schedule information

 

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