Astronomy Colloquia at Caltech for 2017-18

Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium.
Wine and cheese will be served in the Cahill Foyer from 5-5:30pm.

Month
Date
Speaker

Talk Title
SEPTEMBER 20

Speaker: Chris Kochanek
Institute: Ohio State
Host: M. Kasliwal

Dust Formation Is Not Voodoo

Dust formation is generally viewed as an act of magic or voodoo, to be viewed
with the same level of disbelief as invocations of rotation or magnetic fields.
But the formation of dust is actually a fairly simple problem that has been
obscured by invocations of the problems in classical nucleation theory. While
classical nucleation theory has many problems, they are not generally
relevant to observational problems in astrophysics.                                                 
When a transient source ejects mass, dust formation becomes inevitable if
the density is sufficiently high and the radiation environment is benign.
High density means that the condensible species forming the dust have a
significant collision rate.  A benign radiation environment means that
UV photons cannot destroy fragile, small grains before they can grow.
All the problems in nucleation theory boil down to modest and relatively
uninteresting shifts in when and where the dust forms.  Once
dust forms, the future evolution is well-specified and provides strong,
quantitative constraints on the ejecta.  When the dust optical depth is high,
the mid-IR emission constrains the luminosity while the evolution of
the optical emission constrains the ejecta mass.   As the optical
depth drops, the roles of the wavelengths reverses. I will discuss these
issues in the context of a range of astrophysical sources

27

Speaker: Jamie Bock
Institute: Caltech
Host: M. Kasliwal

SPHEREx:  An All-sky Infrared Spectral Survey Explorer Satellite

SPHEREx, a mission in NASA's Medium Explorer (MIDEX) program that was selected for a competitive Phase A in August 2017, is an all-sky survey satellite designed to address all three science goals in NASA's astrophysics division, with a single instrument, a wide-field spectral imager.  We will probe the physics of inflation by measuring non-Gaussianity by studying large-scale structure, surveying a large cosmological volume at low redshifts, complementing high-z surveys optimized to constrain dark energy. The origin of water and biogenic molecules will be investigated in all phases of planetary system formation - from molecular clouds to young stellar systems with protoplanetary disks - by measuring ice absorption spectra. We will chart the origin and history of galaxy formation through a deep survey mapping large-scale spatial power in two deep fields located near the ecliptic poles. Following in the tradition of all-sky missions such as IRAS, COBE and WISE, SPHEREx will be the first all-sky near-infrared spectral survey.  SPHEREx will create spectra (0.75 - 4.2 um at R = 40, and 4.2 - 5 um at R = 135) with high sensitivity using a cooled telescope with a wide field-of-view for large mapping speed.  During its two-year mission, SPHEREx will produce four complete all-sky maps that will serve as a rich archive for the astronomy community.  With over a billion detected galaxies, hundreds of millions of high-quality stellar and galactic spectra, and over a million ice absorption spectra, the archive will enable diverse scientific investigations including studies of young stellar systems, brown dwarfs, high-redshift quasars, galaxy clusters, the interstellar medium, asteroids and comets.  SPHEREx is a partnership between Caltech, JPL, Ball Aerospace, and the Korea Astronomy and Space Science Institute.

OCTOBER 4

Speaker: Juna Kollmeier
Institute: Carnegie
Host: J. Cohen

AS4:  Pioneering Panoptic Spectroscopy

I will describe the current plans for a Next Generation Sky Survey that will begin After SDSS-IV --- AS4.  AS4 will be an unprecedented all-sky spectroscopic survey of over six million objects. It is designed to decode the history of the Milky Way galaxy, trace the emergence of the chemical elements, reveal the inner workings of stars, and investigate the origin of planets. It will also create a contiguous spectroscopic map of the interstellar gas in the Milky Way and nearby galaxies that is 1,000 times larger than the state of the art, uncovering the self-regulation mechanisms of our Galactic ecosystem. It will pioneer systematic, spectroscopic monitoring across the whole sky, revealing changes on timescales from 20 minutes to 20 years.   The project is now developing new hardware to build on the SDSS-IV infrastructure, designing the detailed survey strategy, and is actively seeking to complete its consortium of institutional
and individual members.

11
Speaker: Eve J. Lee
Institute: Caltech
Host: Phil Hopkins

Planets Close-in and Far-out

The data-rich Kepler mission provided an unprecedented view of the demographics of planetary systems. Close to the star (orbital periods shorter than ~100 days), super-Earths (~1--4 Rearth) and Earth-sized planets dominate. These small planets are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. I will demonstrate that both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be reproduced if planets form in situ in disks that are truncated by their host star magnetospheres at co-rotation. Planets can be brought from disk edges to ultra-short (<1 day) periods by asynchronous equilibrium tides raised on their stars. Close-in super-Earths are massive enough to trigger runaway gas accretion, yet they accreted atmospheres that weigh only a few percent of their total mass, keeping their size below that of the Neptune. This puzzle is solved if super-Earths formed late, in the inner cavities of transitional disks. Over a wide range of nebular depletion histories, super-Earths can robustly build their ~1% by mass envelopes. Super-puffs present the inverse problem of being too voluminous for their small masses. I will show that super-puffs most easily acquire their thick atmospheres as dust-free, rapidly cooling worlds outside 1 AU, and then migrate in just after super-Earths appear. Small planets may remain ubiquitous out to large orbital distances. The variety of debris disk morphologies revealed by scattered light images can be explained by viewing an eccentric disk, secularly forced by a planet of just a few Earth masses, from different observing angles. The farthest reaches of planetary systems may be perturbed by eccentric super-Earths.

18
Speaker: Robyn Ellyn Sanderson
Institute: Caltech
Host: Phil Hopkins

Insights into dark matter from the stellar halos of galaxies

Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should in principle lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the distant outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New surveys of Milky Way stars like Gaia, alongside next-generation instruments and giant telescopes, will for the first time provide accurate positions, velocities, and abundances for large numbers of stars in faint tidal streams: remnants of tidally-disrupted satellite galaxies that trace out the mass distribution in the distant reaches of galaxy halos. I will show how state-of-the-art simulations play a crucial role in interpreting and analyzing this wealth of new information about stellar halos, and how stellar halo observations over the next decade will characterize the dark matter distribution in galaxies, test theories of the nature of dark matter, and illuminate the role of dark matter in galaxy formation.


25
Speaker: Meredith MacGregor
Institute: Harvard
Host: D. Mawet

Debris Disks as Probes of Planetary System Evolution

At least 20% of nearby main sequence stars are surrounded by disks of dusty material resulting from the collisional erosion of planetesimals, larger bodies similar to asteroids and comets in our own Solar System.  The resulting dust can be observed via scattered light at visible to near-infrared wavelengths or thermal emission at mid-infrared to millimeter wavelengths.  Since the dust-producing planetesimals are expected to persist in stable regions like belts and resonances, the locations, morphologies, and physical properties of dust in these debris disks provide probes of planet formation and subsequent dynamical evolution.  Observations at millimeter wavelengths are especially critical to our understanding of these systems, since the large grains that dominate emission at these long wavelengths do not travel far from their origin and therefore reliably trace the underlying planetesimal distribution.  I will present ongoing work that uses observations of the angularly resolved brightness distribution and the spectral dependence of the flux density to constrain both the structure and grain size distribution of nearby debris disks.  In particular, I will show new ALMA observations that place constraints on the position, width, surface density gradient, and any asymmetric structure of several well-known debris disks (including the Fomalhaut system).  Together these results provide an exciting foundation to investigate the dynamical evolution of planetary systems through multi-wavelength observations of debris disks.

NOVEMBER
1
Speaker: Peter Jonker
Institute:  SRON-NL
Host: S. Phinney

Stellar-mass black holes: X-ray binaries vs LIGO/Virgo?

The existence of stellar-mass black holes is now well established but the recent LIGO/Virgo results suggest that the black hole mass distribution of stellar-massblack holes is more complex than that derived so far from measurements usingX-ray binaries. I will explain how dynamical mass measurements have been obtained for the black holes in these single-lined spectroscopic binaries anddiscuss several biases that may help explain the differences between masses from LIGO/Virgo events and X-ray binaries. The goals are to investigate how black holes forms and, in particular, to investigate if intermediate-mass black holes exist. Intermediate-mass black holes may well be necessary to explain the presence of super-massive black holes when the universe was less than 1 Gyr old.I will finish by showing results from our recent attempts to find intermediate-mass black holes using geometrical constraints.

8
Speaker: Krista Smith
Institute: KIPAC Stanford
Host: G. Hallinan

A New Regime of Optical Variability in AGN: the Kepler Light Curves

The optical light curves of AGN provide a unique window into the conditions and behavior within the accretion disk. The development of a specialized pipeline for AGN science with the unparalleled photometry of exoplanet-hunting satellites allows us to explore new optical variability phenomena. Among the insights from these new light curves are bimodal flux distributions, power spectral slopes that depend on luminosity, characteristic variability timescales, and more. Such data provide an opportunity for direct comparison with X-ray light curves, and promise to inform models of both accretion physics and the relationship between X-ray and optical emitting regions in the central engine. These data will be critical in learning how to interpret AGN light curves from upcoming large variability surveys like LSST. Finally, exoplanet mission data have enormous future promise for a multifaceted understanding of accretion processes, including blazar jets and quasi-periodic oscillations.

15

Speaker: Lia Corrales
Institute: Univ of Wisconsin
Host: M. Kasliwal

An X-ray View of the Dusty Universe

A significant fraction of the heavy elements produced by stars spend some time in the interstellar medium as dust grains.  These heavy metal transporters influence gas cooling during star formation, eventually becoming the seeds for planet formation.  Observations of X-ray bright Galactic compact objects can yield key insights to the mineralogy and evolution of dust grains in the Milky Way.  With high resolution X-ray spectroscopy, we can directly measure the state of metals and the mineral composition of dust in the interstellar medium.  In addition, dust scattering produces a diffuse halo image around bright X-ray objects, revealing information about dust grain sizes and their spatial distribution.  I will review the most recent exciting dust scattering discoveries, which draw on multi-wavelength observations. I will show how X-ray studies of the ISM are important for interpreting accretion by compact objects and the supermassive black hole at the center of our galaxy. Finally, I will discuss open questions regarding our X-ray view of the dusty Universe that can be addressed with future X-ray observatories.

29
Speaker: Paola Caselli
Institute: MPI
Host: N. Scoville

Astrochemistry at the dawn of star and planet formation in the ALMA Era 

Molecules are unique tracers of the dynamical and chemical evolution of star and planet forming regions. Thus, astrochemistry is crucial to test theories and shed light on our origins. Stars and planetary systems in our Galaxy form within dense (n(H2) ~ 100,000 cm-3) and cold (T ~ 10 K) fragments of interstellar molecular clouds, called pre-stellar cores. Important chemical processes take place at this early stage, such as isotope fractionation, production of complex organic molecules and growth of thick icy mantles onto dust grains, where water and organics are stored, and which boost dust coagulation. These processes can affect later phases of star and planet formation, which can now be traced with powerful interferometers such as ALMA and NOEMA. In this talk I shall review the chemical and physical structure of pre-stellar and protostellar cores, as well as theoretical work on prototostellar disk formation and early evolution. Links to protoplanetary disks and our Solar System will be made.


JANUARY 3

Speaker:
Institute:
Host:

No Colloquium

10

Speaker: Erik Petigura
Institute:  Caltech
Host: D. Mawet

A Bird's Eye View of Extrasolar Planets

One startling result from the Kepler mission was that nearly every Sun-like star has a planet between the size of Earth and Neptune. Given the lack of such planets orbiting the Sun, Kepler has demonstrated that the Solar System is not a typical outcome of planet formation, in at least that one key respect. Therefore, to build a complete understanding of the processes that form planets, we must look to extrasolar planets. I will present some new insights into the physics of planet formation, made possible by advances in the exoplanet census and our understanding of planet host stars. This bird's eye view sheds light on where planets form, the speed at which they are assembled, and how they are sculpted by high-energy radiation from their host stars.

17

Speaker: Jennifer Barnes
Institute: Columbia
Host: G. Hallinan

Welcome to the multi-messenger era: a report on the first binary neutron star merger detection

On August 17th, the gravitational wave detectors LIGO and Virgo observed for the first time the signature of a binary neutron star merger. Roughly two seconds later, the Fermi satellite detected a short gamma-ray burst whose location was consistent with the position of the gravitational wave source. These signals triggered an electromagnetic follow-up campaign by dozens of groups around the world, who quickly identified an electromagnetic counterpart, which was observed over the next several weeks at energies ranging from the x-ray to the radio. These observations allowed astronomers to construct a detailed picture of an event that had previously been studied only theoretically, and to test key theories about the nature of neutron star mergers. Among these is whether mergers are the astrophysical site of r-process nucleosynthesis, which produces roughly half of elements heavier than iron. I will give 
​​
 an overview of the electromagnetic observations of this system, with an emphasis on the optical and infrared emission (the "kilonova") powered by the radioactive decay of elements synthesized in the merger. I will outline how recent theoretical advances allowed us to interpret kilonova observations and decode signs of heavy element production.

24

Speaker: Scottt Tremaine
Institute: Princeton
Host: S. Phinney

Comets and the outer fringes of the solar system

Comets have inspired awe since prehistoric times, but their nature and origin have only been investigated in the last few decades. The statistical analysis of a few thousand comets with well-determined orbits implies that there are two distinct sources of comets: the Oort cloud,
containing over 100 billion comets at 5,000 to 50,000 times the Earth-Sun distance; and the Kuiper belt outside Neptune's orbit. I will review our current understanding of the formation of the Oort cloud and Kuiper belt, our successes and failures in explaining the properties of comets and their orbits, and the relation between comets and possible undiscovered planets in the outer solar system.

31

Speaker: Carl Rodriguez
Institute: MIT
Host: M. Kasliwal

Forging binary black holes in dense star clusters

 Since the first detection two years ago, gravitational waves have promised to revolutionize our understanding of astrophysics.  But to understand what the gravitational waves are telling us, we need to understand how these relativistic systems formed in the first place.

I will describe how binary black holes form in the cores of dense star clusters by simple, Newtonian gravitational interactions.  I will demonstrate how these dynamically-formed binary black holes can easily explain most of the signals that LIGO has detected so far, and what distinguishes them from similar systems formed by the evolution of binary stars.  Finally, I will discuss how we can potentially discriminate between different formation scenarios, and what this can tell us about astrophysics.


FEBRUARY
7

Speaker: Jessica Werk
Institute: Univ Washington
Host: P. Hopkins

Circumgalactic Matter Matters for Galaxy Evolution

The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape the assembly and evolution of galaxies. It most likely contains enough material to harbor most of the metals lost in galaxy winds and to sustain star-formation for billions of years.  Owing to the vastly improved capabilities in space-based UV spectroscopy with the installation of HST/COS, observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. In this talk, I will describe observational constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state of gas 10 - 200 kpc from galaxiesí stars. I will conclude by introducing several exciting new techniques for resolving the gaseous structures in the CGM, and by posing unanswered questions about the CGM that will be addressed with future survey data and hydrodynamic simulations in a cosmological context.

14

Speaker: Richard Ellis
Institute: Univ College London
Host: P. Hopkins

TBD

21

Speaker: Marc Pinsonneault
Institute: Ohio State
Host: L. Hillenbrand



28

Speaker:
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MARCH
7

Speaker: Cara Battersby
Institute: U. Conn
Host: P. Hopkins

TBD

14

Speaker: Rachel Shuchter Bezanson
Institute: Univ of Pittsburgh
Host: Phil Hopkins

TBD

21

Speaker: Maryam Modjaz
Institute: NYU
Host: M. Kasliwal

TBD

28

Speaker: Alex Szalay
Institute: John Hopkins
Host: P. Hopkins

Kingsley Colloquium

APRIL
4

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11

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18

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25

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MAY
2

Speaker: Bjorn Emonts
Institute: NRAO, Charlottesville
Host: Phil Appleton

The Cold Molecular Medium around Distant Galaxies: Light up the Darkness!
9

Speaker: Mike Liu
Institute: Univ Hawaii
Host: D. Mawet

TBD

16

Speaker: Scott Gaudi
Institute: Ohio State
Host:Dimitri Mawet

TBD

23

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30

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JUN
6

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Information for Speakers

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