MIT Astrophysics Brown Bag Lunch Series: Fall 2016
Mondays at 12:05 PM in 37-187 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Michael McDonald and Paul Torrey.
Monday September 19 -- Two Talks!
Probing Exoplanet Atmospheres with Raman Scattering
Rayleigh scattering is an important source of opacity in the atmospheres of exoplanets at short optical and near-UV wavelengths. Raman scattering on molecules is an inelastic process related to Rayleigh scattering which leaves specific spectral signatures imprinted in the reflected light and the geometric albedo spectrum of the planet. These spectral features can be used to probe the atmospheres of exoplanets. The intensity of Raman features depends on the column of the atmosphere that the radiation passes through before being scattered into our line of sight and hence can provide information on the presence and altitude of clouds, even in atmospheres composed of heavy molecular species. The so-called Raman ghost lines could be used to spectroscopically identify the main scatterer in the atmosphere, even molecules like hydrogen or nitrogen, which do not show prominent spectral signatures in the optical wavelength range. If detected, these ghost lines could also provide information about the temperature of the atmosphere. In this talk I will present a feasibility analysis for detecting Raman spectral features in nearby exoplanets using the next-generation observational facilities. I will also discuss which stellar types are the most promising candidates for hosting exoplanets with strong Raman features in their albedo spectra.
Applications of Lensed Quasars’ Variability
Flux variability of lensed quasars can be used as a fun and fancy way to study the structure of galaxies and quasar accretion disks. These fluctuations are a combination of the intrinsic variability of the quasar, from which it is possible to measure time delays and determine the dark matter halo structure of the lens galaxy, and microlensing variability induced by the stars and compact objects in the lensing galaxy, from which we can measure accretion disk sizes,the mean mass of the stars in the lens galaxy, and the dark matter mass fraction near thelensed images. In my talk I will discuss where these cool measurements are leading us, and the current status of our target selection and follow up. As a treat, at the end of my talk I will advertise BINGO experiment (BAOs from Integrated Neutral Gas Observations), a project to build a novel radio telescope to map redshifted neutral hydrogen emission between z = 0.13 and 0.48.
Monday September 26 -- Two Talks!
Unraveling the History of the Milky Way
Understanding physical processes responsible for the formation and evolution of galaxies like the Milky Way is a fundamental problem in astrophysics. However, a key challenge is that the properties and orbits of the stars can only be observed at present: in order to understand what happened in the Milky Way at earlier epochs, one must explore “archeological” techniques. One idea, "chemical tagging”, aims to probe the history of the Milky Way via the unique imprint in chemical abundance space of long-disrupted star clusters. I will discuss the opportunities and challenges associated with chemical tagging, including a first constraint on the disrupted cluster mass function in the Milky Way. I will also describe a new set of tools for efficient fitting large quantities of stellar spectra and opportunities for extracting many stellar parameters from low-resolution data.
High-Redshift Astrophysics Using Every Photon
Large galaxy surveys have dramatically improved our understanding of the complex processes which govern gas dynamics and star formation in the nearby universe. However, we know far less about the most distant galaxies, as existing high-redshift observations can only detect the very brightest sources. Intensity mapping surveys provide a promising tool to access this poorly-studied population. By observing emission lines with low angular resolution, these surveys can make use of every photon in a target line to study faint emitters which are inaccessible using traditional techniques. With upcoming carbon monoxide experiments in mind, I will demonstrate how an intensity map can be used to measure the luminosity function of a galaxy population, and in turn how these measurements will allow us to place robust constraints on the cosmic star formation history. I will then show how cross-correlating CO isotopologue lines will make it possible to study gas dynamics within the earliest galaxies in unprecedented detail.
Monday October 3 -- Two Talks!
Simulating state transitions in black hole accretion flows
Galactic BH X-ray binaries often undergo state transitions during which their luminosities and spectrum significantly change. Accretion in distinct modes is also observed in AGN. In this talk I will review most recent progress in simulating transitions from optically thin to optically thick black hole accretion flows occurring during the onset of the outbursts, when the accretion rate on the BH increases.
Rates of Tidal Disruption Events: Both Too High and Too Low
When an unlucky star passes too close to a supermassive black hole, it is torn apart in a tidal disruption event (TDE). Half the stellar debris remains bound to the black hole, and as it returns to pericenter it begins to accrete at super Eddington rates, powering a multiwavelength flare comparable in luminosity to the brightest supernovae. As our sample size of observed TDEs has increased, two opposing event rate dilemmas have emerged. In typical galaxies, the observationally inferred rate of TDEs is 1-2 orders of magnitude below theoretically predicted rates that are computed using the Fokker-Planck approximation for collisional stellar dynamics. However, TDEs are overproduced in E+A galaxies, a rare post-starburst galaxy subtype that makes up ~0.1% of local universe galaxies yet hosts ~50% of all TDEs. I will discuss my ongoing observational and theoretical work to resolve these parallel rate discrepancies. Preliminary evidence suggests that the underproduction of TDEs in standard galaxies may be linked to a bimodality in the TDE luminosity function, while the overproduction of TDEs in E+As may be due to central stellar overdensities produced in the starburst that
made the E+A.
Monday October 10 -- No talk (MIT Holiday)
Monday October 17 -- Two Talks!
Effects of Tides on Milky Way Dwarf Satellite Galaxies
Using detailed observations of the Local Group to study wide-ranging questions in galaxy formation and dark matter physics – has become a rich field over the past decade. In this talk, I will present frameworks that address some of these questions by combining high-precision stellar kinematic measurements with state-of-art cosmological N-body simulations. I will demonstrate that the properties of dark matter subhalo of individual satellite galaxies implied by stellar kinematic data can be linked to the galaxy evolution mechanisms such as infall time and the gravitational tidal interaction with Milky Way potential. In the cold dark matter (CDM) scenario, some dwarf galaxies explicitly require to be shaped under significant gravitational tidal forces, which will leave imprints on their stellar distribution and kinematics. I will discuss how these features could serve as a test to the nature of dark matter or stellar feedback strength. I will also discuss how we can study the tidally “disturbed” or even “destroyed” satellite galaxies as building blocks to our Milky Way stellar halo by understanding the properties of their progenitors and observation limit imposed by current and future surveys.
Probing Compact Dark Matter
Johns Hopkins University
It has been recently argued that if part of the dark matter are compact objects, such as primordial black holes, they would form binaries and emit gravitational waves like the ones observed by LIGO. I will discuss how FRBs (fast radio bursts) can be used to determine the amount of dark matter in compact objects, providing an alternative constraint to their abundance.
Monday October 24 -- Two Talks!
UV and X-Ray Emission from Exoplanet Host Stars: Results of the MUSCLES HST Survey and Photochemistry in Exoplanet Atmospheres
University of Colorado, Boulder
Exoplanets evolve in the radiation environment created by their host stars. Of particular interest is the UV and X-ray emission produced in the chromospheres and coronae of cooler stars such as M dwarfs. I will summarize the results of our HST MUSCLES Treasury Survey, consisting of complete UV spectra of 7 M-type dwarf stars and 4 K-type dwarf stars, together with coordinated X-ray and optical observations of these stars. The spectral energy distributions of the M stars are very different from G-type stars like the Sun with much stronger far-UV, extreme-UV, and X-ray emission compared to the optical emission. The far-UV emission, in particular the very bright Lyman-alpha line, photodissociates H20, CO2, and CH4 in the outer atmospheres of exoplanets, and the extreme-UV and X-ray emission is the energy source for exoplanet mass loss. I will describe how we correct the observed Lyman-alpha emission from interstellar absorption and present our new model of the chromosphere and corona of the M1.5 V host star GJ 832.
Boundary Conditions for Abiogenesis: Constraints on the UV Environment Relevant to Prebiotic Chemistry on Young Planets
Recent laboratory studies (e.g., Ritson et al 2012, Patel et al 2015) suggest UV light may have played a critical role in the synthesis of molecules relevant to abiogenesis (prebiotic chemistry), such as RNA. UV light interacts with prebiotic chemistry in ways that are sensitive to wavelength and fluence level (e.g., Ranjan & Sasselov 2016a). Here, we use radiative transfer models to constrain the UV environment on Earth in the era of abiogenesis (~3.9 Ga). We exhaustively explore the available surface fluence as a function of surface conditions and atmospheric composition, exploring all plausible areas of parameter space. We convolve the resultant spectra against action spectra for prebiotically relevant processes to estimate the effect of these variables on prebiotic chemistry. Some workers have also suggested the possibility of abiogenesis on Mars and subsequent panspermic transfer to Earth (e.g, Benner+2015). Therefore, we also explore plausible UV spectral fluences on Mars at 3.9 Ga to evaluate its hospitability for prebiotic chemistry (Ranjan, Wordsworth & Sasselov 2016). Lastly, we calculate the UV fluences on exoplanets orbiting M-dwarfs, and evaluate the implications for the hospitability of these worlds for abiogenesis events.
Monday October 31 -- Two Talks!
A modern approach to fitting galactic broadband photometry
We present a new model framework for fitting the broadband photometry of galaxies, based on the Flexible Stellar Populations Synthesis (FSPS) code. Our default model, Prospector-Gold, includes dust attenuation and re-radiation, a flexible attenuation curve, nebular emission, stellar metallicity, and a 6-component non-parametric star formation history. The parameter space is sampled via MCMC techniques within the Prospector inference framework to account for the considerable degeneracies inherent in fitting broadband photometry. A common concern with modeling stellar populations is that different models predict significant differences in parameters of interest, such as star formation rates, stellar masses, and dust attenuation, while fitting the observed photometry equally well. We address this issue by modeling a sample of 129 local galaxies with UV to far-IR photometry and high quality aperture-matched spectra. We test the accuracy of the model fit, which is fit only to the broadband photometry, by comparing the predicted strength of key diagnostic spectral features, such as L(Hα) and the 4000 Angstrom break, to the measured values from the spectra. Our model predicts the Hα luminosities with a scatter of ∼0.16 dex and an offset of 0.08 dex across a wide range of morphological types and stellar masses. This agreement is remarkable, as the Hα luminosity is dependent on accurate star formation rates, dust attenuation, and stellar metallicities. This small scatter implies that galaxy star formation rates do not strongly vary over 100 Myr timescales. The model also accurately predicts the dust-sensitive Balmer decrements, spectroscopic stellar metallicities, and the stellar absorption features Dn4000 and Hδ, which are sensitive to the age of the stellar population. To our knowledge these are the first models that provide star formation rates from broadband photometry with uncertainties <50%.
The Past, Present, and Future of Planetary Systems
We are using the Kepler space telescope in its new K2 mission to study the past, present, and future of planetary systems -- that is, the history of how planets might form and migrate, their present-day characteristics, and the ultimate fate of planetary systems. I will discuss what we have learned, in particular from the discovery of a hot Jupiter with close planetary companions, planets orbiting nearby bright stars, and a disintegrating minor planet transiting a white dwarf.
Monday November 7 -- Two Talks!
Structures, Masses, and Composition of High Redshift Star-Forming Galaxies
Recent photometric and spectroscopic surveys have paved the way for detailed studies of galaxy evolution from when galaxies were most rapidly forming stars (z~2) to the current epoch. The combination of deep multi-band photometry and spectra has extended our understanding of the composition, structure, and masses of early star-forming galaxies. In particular, spectroscopic surveys provide key insights into galaxy masses and structures through their internal kinematics. Additionally, these surveys constrain the properties of dust in early galaxies, which is needed to infer accurate star formation rates. In this talk, I will present observations of the dust content, internal kinematics, and masses of star-forming galaxies at z~1.5-2.3, and how these properties change over time. I will also discuss how mock observations of cosmological simulations aid in the interpretation of structural and kinematic properties of distant galaxies and how they compare to late-time galaxy properties.
Massive Black Hole Binary Mergers and their Gravitational Waves
Gravitational Waves (GW) from stellar-mass BH binaries have recently been observed by LIGO, but GW from their supermassive counterparts have remained elusive. Recent upper limits from Pulsar Timing Arrays (PTA) have excluded significant portions of the predicted parameter space. Most previous studies, however, have assumed that most or all Massive Black Hole (MBH) Binaries merge effectively and quickly. I will present results derived—for the first time—from cosmological, hydrodynamic simulations with self-consistently coevolved populations of MBH particles. We perform post-processing simulations of the MBH merger process, using realistic galactic environments, including models of dynamical friction, stellar scattering, gas drag from a circumbinary disk, and GW emission—with no assumptions of merger fractions or timescales. We find that despite only the most massive systems merging effectively (and still on gigayear timescales), the GW Background is only just below current detection limits with PTA. Our models suggest that PTA should make detections within the next decade, and will provide information about MBH binary populations, environments, and even eccentricities. I’ll also briefly discuss prospects for observations of dual-AGN, and the possible importance of MBH triples in the merger process.
Monday November 14 -- Two Talks!
The Destructive Birth of Massive Stars and Massive Star Clusters
Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject with their intense radiation fields and stellar winds into the interstellar medium (ISM) dwarfs the contribution by their vastly more numerous low-mass cousins. These mechanisms can halt accretion onto massive stars and limit star formation in massive star clusters (MSCs), which can host thousands of massive stars. For stellar winds, I discuss how we can use observations to constrain a range of kinetic energy loss channels for the shock-heated gas from stellar winds in MSCs. I demonstrate that the kinetic energy injected by stellar winds in MSCs is not a significant contributor to stellar feedback for young MSCs. I argue instead that radiation pressure is likely the dominant feedback mechanism in massive star and MSC formation. Therefore detailed simulation of their formation requires an accurate treatment of radiation. For this purpose, I have developed a new, highly accurate hybrid radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new method, I performed a suite of three-dimensional radiation-hydrodynamic simulations of the formation of massive stars and MSCs. For individual massive stellar systems, I find that mass is channeled to the massive stellar system via gravitational and Rayleigh-Taylor instabilities. I will also present a simulation of the formation of a MSC from the collapse of a dense, turbulent, magnetized million solar mass molecular cloud. I find that the influence of the magnetic pressure and radiative feedback slows down star formation. These early results suggest that the combined effect of turbulence, magnetic pressure, and radiative feedback from massive stars is responsible for the observed low star formation efficiencies in molecular clouds.
Characterizing Dust Attenuation in Local Star Forming Galaxies
The presence of dust in galaxies can significantly alter their observed spectral energy distribution. If not properly accounted for, this effect can lead to incorrect values of derived physical quantities such as the star formation rate, stellar mass, and photometric redshift. Virtually all studies of star forming galaxies, both local and distant, make use the attenuation curve derived from a small sample of local starburst galaxies to correct for the effects of dust. However, it is not clear how reasonable the general application of this curve is for more typical star forming galaxies. In this talk, I will present results from a study utilizing ~10000 local (z<0.1) star forming galaxies to identify the extent to which attenuation varies as a function of galactic physical properties.
Monday November 21 -- Two Talks!
Hidden Sector Atomic Dark Matter: Implications for Small-scale Structure
I will discuss the atomic physics and the astrophysical implications of a model in which the dark matter is the analog of hydrogen in a secluded sector. The self interactions between dark matter particles include both elastic scatterings as well as inelastic processes due to a hyperfine transition. The self-interaction cross sections are computed by numerically solving the coupled Schrodinger equations for this system. The velocity-dependence of the self-interaction cross sections produces the low dark matter density cores seen in spiral galaxies while maintaining consistency with constraints from observations of galaxy clusters. Significant cooling losses may occur due to inelastic excitations to the hyperfine state and subsequent decays (up to about 10% of the collisional heating rate) in this region of parameter space, with implications for the evolution of low mass halos and early growth of black holes. Finally, the minimum halo mass is in the range of 10^3 to 10^7 solar masses for viable regions of parameter space, which is significantly larger than the typical predictions for weakly-interacting dark matter models.
The rest-UV and rest-optical spectra of high-redshift galaxies: clues regarding star-formation history and chemical enrichment in the early Universe
Galaxies at the peak of cosmic star formation (z~2-3) exhibit significantly higher star formation rates and gas fractions at fixed stellar mass than nearby galaxies. Further, z~2-3 galaxies are also distinct in terms of their nebular spectra, reflecting important differences not only in the physical conditions of their interstellar medium (e.g., electron density and gas-phase metallicity), but also in the details of their massive stellar populations, especially their ionizing radiation fields. Thus, jointly observing galaxies' HII regions (in the rest-UV and rest-optical) and massive stars (in the rest-UV) is central to constructing a framework for understanding the differences between z~2-3 and z~0 star-forming galaxies and for self-consistently explaining the trends observed in the high-redshift population. I will present results from the Keck Baryonic Structure Survey (KBSS), which uniquely combines observations of the rest-optical spectra of ~400 individual galaxies (obtained with Keck/MOSFIRE) with deep rest-UV spectroscopy of many of the same galaxies (obtained with Keck/LRIS). These measurements represent one of the most comprehensive spectroscopic studies of z~2-3 galaxies to date and, when compared with predictions from photoionization models, offer compelling evidence that high-redshift galaxies exhibit chemical abundance patterns distinct from local galaxies, likely as the result of systematic differences in their star-formation histories.
Monday November 28 -- no talk scheduled
Monday December 5
Characterizing Kepler’s Multiplanet Systems
While NASA's Kepler mission has identified thousands of planets and measured their periods and radii through transit observations. Planet mass and eccentricity measurements, in addition to periods and radii, are required to constrain planets’ compositions and dynamical histories. Unfortunately, for many Kepler planets such measurements are not feasible or require intensive follow-up observations. In multi-planet systems, however, mutual gravitational perturbations cause variations in the planets' times of transit that can be used to recover dynamical information about the planetary system. However, inferring planet masses and orbits from these transit timing variations (TTVs) is a difficult inverse problem with a large parameter space and multiple degeneracies. I will describe our approach to TTV inversion that combines Markov chain Monte Carlo simulations with insights from simplified analytic formulae. I will present mass and eccentricity measurements from a study of 145 planets and discuss the implications of these measurements for planets’ compositions and dynamics.
NOTE DATE AND TIME: Thursday December 12 @ 2pm
Orbital Dynamics In the Habitable Zone
San Francisco State University
A key component of characterizing multi-planet exosystems is testing the orbital stability based on the observed properties. Such characterization not only tests the validity of how observations are interpreted but can also place additional constraints upon the properties of the detected planets. In this talk I will describe my recent work on investigating the dynamical stability of various planetary systems, most particularly those with planets that lie within the Habitable Zone. I will show the results of studying four systems with extreme eccentricities in which the two known planets exchange angular momentum through secular oscillations of their eccentricities. I will further discuss the orbital architectures for two systems, HD 5319 and HD 7924, that show signs of close planetary encounters for which the scenario of coplanar orbits results in their rapid destabilization. An exhaustive scan of parameter space via dynamical simulations reveals specific mutual inclinations between the two outer planets in each system that allow for stable configurations over long timescales. I will present the results of dynamical simulations that constrain the location of additional terrestrial planets in the Habitable Zone of the Kepler-68 and Proxima Centauri systems. Finally, I will discuss the dynamical stability of the Wolf 1061 system and the potential impact of the evolving eccentricity of the Habitable Zone planet on surface conditions.