2018 IPAC Visiting Graduate Fellowship

Program Description

The Infrared Processing and Analysis Center (IPAC) at Caltech announces the availability of six-month graduate student fellowships. The program is designed to allow students from other U.S. or international institutions to visit IPAC-Caltech and perform astronomical research in close association with an IPAC scientist. Eligible applicants are expected to have completed preliminary course work in their graduate program and be available for research during the period of the award. Funding from IPAC will be provided for a 6-month period via monthly stipends. Several students are expected to be accepted each year, subject to the availability of funding. Students are expected to be at IPAC during the duration of the Fellowship, nominally January to July, with some flexibility on the starting and ending dates.

Application Instructions

The call for 2018 applications is now closed.

  1. A brief letter of introduction indicating your interest in a particular research area connected with IPAC research. The letter should include the following information:
    • A brief description of your current status as a graduate student, including relevant course work completed.
    • A brief description of how your current research interests might match one of the IPAC research projects described below. Please rank the top 3 projects that you might like to participate in. 
    • Please list any research experience you may already have in astronomy. In addition, please tell us of any experience you may have with astronomical and image processing software packages (e. g. IRAF, IDL, HIPE, AIPS etc).
    • The approximate start-date of your visit (nominally early Jan 2016).
  2. A one-page CV.

In addition, we ask that a current professor or academic advisor familiar with the applicant’s work upload a letter of reference (PDF). This letter should also indicate that the applicant is available to visit IPAC during the proposed period, and address how well the visit would mesh with the applicant’s graduate education.

Research Projects

Short-timescale variability of blazars

Advisor: Ranga-Ram Chary

Blazars which are powered by supermassive black holes are responsible for the most energetic and violent GeV/TeV gamma-ray flares in the Universe. The canonical thinking is that these flares arise from Compton up-scattering of low-energy photons in the relativistic jet which is aligned along the line of sight. Short timescale variability analysis between the radio/millimeter intensity and the gamma-rays can help constrain the origin of the gamma-ray emission. The Planck mission data archived at IPAC provides a unique capability to measure the short (~day to week) timescale variability of gamma-ray blazars over nine frequencies. The student will download Planck data, generate light curves and spectral indices of blazars at various frequencies, compare with the measured intensity of Fermi/LAT gamma-ray emission and try to constrain the physical processes that occur prior to a gamma-ray flare. The duty cycle of flaring will also help obtain a better constraint on the contribution of blazars to the high energy extragalactic gamma-ray background.

Observing Galactic Superwinds with the Cosmic Web Imager

Advisor: Lee Armus

Galactic-scale winds, or 'Superwinds', driven by the collective effect of massive stars and supernovae, have been invoked as a source for the heating and metal-enrichment of both the intra-cluster and inter-galactic medium, as a critical factor in the evolution of galaxies, and as the source of the mass-metallicity relation for galaxies. It has also been suggested that superwinds are a natural and necessary transition phase in the evolution of powerful starbursts to QSOs, and that they may clear channels allowing ionizing photons to escape into the CGM. We are continuing a project to study the optical emission-line nebulae and outflowing superwinds in a sample of ~30 nearby starburst and infrared luminous galaxies using the Cosmic Web Imager - a wide-field, image slicing, optical integral field spectrograph on the Palomar Hale 200-inch Telescope. With an arcminute field of view and a resolution of R~5000, CWI enables detailed studies of an important piece of the local cosmic web. We are combining these CWI data with Keck/OSIRIS near-infrared LGS/AO observations and HST narrow-band imaging to trace the ionized atomic and warm molecular gas, as well as the escaping ionizing radiation, on scales ranging from 0.05 - 50 kpc. The student will participate in the observing, data reduction and analysis, and publication of the CWI and ancillary data.

UKIRT microlensing survey: a pathfinder for WFIRST

Advisor: S. Calchi Novati

The microlensing team based in Pasadena (IPAC and JPL) is currently leading the first near-infrared (NIR) microlensing survey towards the Galactic Bulge, using the 3.8 UKIRT telescope on Mauna Kea, as a precursor of the WFIRST microlensing survey which will complete the statistical census of planetary systems in the Galaxy (Spergel et al, 2015). Two UKIRT surveys have already been conducted in 2015 and 2016, in conjunction and support with the Spitzer microlensing campaign and the K2C9 program (Henderson et al, 2016), with the main goal to combine ground and space-based data to characterize the lens systems through the measure of the microlensing parallax. In particular, together with Spitzer, UKIRT data have been key for the analysis of a massive remnant, possibly a black hole, in a well-separated binary in the Galactic Bulge (Shvartzvald et al 2015). UKIRT data from 2016, combined with Keck data, allowed the characterization of a microlensing exoplanet (Koshimoto et al, 2017). While continuing as support for optical ground-base surveys and the Spitzer program, the main goal of the currently ongoing UKIRT survey, (May to August 2017, and planned to continue in the following years), is to map the microlensing event rate across the highly extinguished region across the Galactic center, which is key for field selection and optimization of the forecoming WFIRST survey. Indeed, besides the detection and characterization of exoplanets, microlensing is a powerful tool for the understanding of the overall underlying Galactic structure, and the analysis in the NIR (in H and K bands) allows to probe, for the first time, the region right across the Galactic center. A pilot analysis reporting 5 microlensing events that were not detected by optical surveys due of the high extinction has already been carried out (Shvartzvald et al 2017). In this framework there are several avenues to pursue, according to the applicant's interests and previous experience: the development of the photometry pipeline; the analysis of specific interesting events; the development of a selection pipeline for microlensing signal with machine-learning tools.

Previous experience with microlensing analysis is preferred.

Directly Measuring the Radii of M Subdwarfs

Advisor: J. Davy Kirkpatrick

With the launch of WFIRST, subdwarfs –- F, G, K, and M stars with sub-solar metallicity –- will represent a significant fraction of Galactic bulge sources observed during the exoplanetary microlensing survey. When these sources are brightened by foreground lensing systems containing one or more exoplanets, their accurate characterization is an important component in determining the properties of the lensing system itself. Most of these exoplanet events will show “finite source” effects, for which knowing the angular radius of the subdwarf is critical. Yet, the angular sizes of subdwarfs are not well known, mainly because subdwarfs are rare in the solar neighborhood and have not seen the scrutiny that stars of higher metallicity have seen. We are in the process of obtaining full optical spectra of ~85 nearby subdwarfs ranging in type from K5 through M7 and in metallicity from [Fe/H] ~ -2.0 to -0.5. These spectra, when combined with Gaia parallaxes from the April 2018 Data Release and near-infrared photometry from 2MASS and WISE, will allow us to measure the bolometric luminosity for each object and to obtain Teff and metallicity using model fits to relative feature strengths. With these values in hand, we can then measure the radius R directly from the Stefan-Boltzmann law. In addition to providing characterization for WFIRST microlensing efforts, these observations will also provide the first accurate absolute magnitude versus color relations across late-K to mid-M subdwarf subclasses, establish brighter standards for the subdwarf spectral classification scheme, and possibly help uncover subdwarfs with unusually broad Na “D” lines that may be a heretofore unrecognized hallmark of extremely low metallicity. The student will be in charge of doing the model fits to the reduced spectra, calculating bolometric luminosities, measuring the radii, and most importantly writing up the final results in a first-author paper.

Archival IRAC Light Curves

Advisor: Jessica Krick

The Spitzer archive has to date well over 1000 high precision photometry light curve observations from the warm mission.  The IRAC team has uniformly reduced all of these light curves following standard practices, and we are now seeking help in validation and publication of the results.  Want to find out what is lurking in those beautifully reduced light curves and at the same time get experience working with analyzing Spitzer data? Transits, eclipses, phase curves, variability, etc! Depending on your interests (M stars, Brown Dwarfs, Hot Jupiters, etc.), this project would be to examine a subset of the reduced light curves, modeling the light curve parameters, validating against those light curves which have already been published, and publishing those which are not already in the literature.  It is not possible that everything interesting has already been published; these light curves are just waiting for a good brain to think about them.  

Understanding Star Formation and Black Hole Growth in Luminous Infrared Galaxies

Advisors: Lee Armus, Joe Mazzarella and the GOALS team

Luminous Infrared Galaxies (LIRGs) generate their enormous power through intense starbursts and the fueling of Active Galactic Nuclei (AGN). With the Great Observatories All-sky LIRG Survey (GOALS), we are characterizing a sample of over 200 low-redshift LIRGs across the electromagnetic spectrum (see http://goals.ipac.caltech.edu). A key part of GOALS is the study of how stars form and black holes grow under these extreme conditions, by observing the atomic  and molecular gas and star clusters at high resolution.  To this end, we have been  obtaining imaging and spectroscopic observations of LIRGs with HST, ALMA and Keck/AO  in the UV through the sub-mm. The successful candidate will work directly with the GOALS team to help understand the physical conditions driving the powerful starbursts and AGN in LIRGs, placing them in context with normal galaxies in the local Universe, and high-redshift dusty galaxies being studied now with ALMA. The student will also work with the team to plan for future observations of nearby and high-redshift LIRGs with JWST, as part of the ERS and GO proposal calls.

Using High Spectral Resolution Data to Characterize Exoplanets

Advisors: Chas Beichman and Marie Ygouf

Post-processing techniques developed for high-contrast imaging can be applied to data from high-spectral resolution Integral Field Spectrometers (IFSs) to characterize the atmosphere of the brightest exoplanets, e.g. HR8799b spectra obtained using Keck-OSIRIS [Bowler et al. 2010].  The main limitation of IFS observations toward exoplanets orbiting bright stars is the presence of speckles, which create systematic errors that are difficult to calibrate and remove. A strategy of speckle cancellation with high potential exploits the wavelength diversity offered by an IFS and is called Spectral Differential Imaging (SDI). Some empirical methods, based on a speckle intensity calibration in the focal plane, have been developed to subtract the speckle field from the image [Sparks et al. 2002, Soummer et al. 2012, Lafrenière et al. 2017]. However, in these methods crucial prior information inferred from the instrument is not used as an input for post-processing. The Medusae (Multispectral Exoplanet Detection Using Simultaneous Aberration Estimation) algorithm [Ygouf et al. 2013] is based on an analytical model of multispectral coronagraphic imaging and an inversion algorithm. The technique estimates jointly the instrumental aberrations and the object of interest, i.e. the exoplanets, in order to separate properly these two contributions. The inversion algorithm is based on a maximum-likelihood estimator, which measures the discrepancy between the multispectral data from an IFS and the imaging model. It is then possible to retrieve, from multispectral images, an estimation of the aberration map and of the position and flux of the planets at each wavelength and thus their spectra.

We expect that a visiting graduate student would work to improve and adapt the Medusae algorithm to non-coronagraphic IFS data. For this purpose, the student will test and validate the technique on both simulated data and on archival data of the Keck-OSIRIS instrument. This work is an essential step toward the application of this technique to other instruments such as the WFIRST coronagraph or future high-dispersion coronagraphs [Mawet et al. 2017].

Do young giant planets vary in brightness?
Assessing the photometric variation of the directly imaged HR 8799 planets.

Advisor: Tiffany Meshkat

It has long been assumed that planets maintain the same brightness over time. However, the presence of non-isotropic cloud structures in exoplanet atmospheres may cause variability in the planet brightness. Brown dwarfs have been shown to be variable. The nearest brown dwarf to Earth was found to be variable by 15% in certain wavelengths (Biller et al. 2013). This variability evolves on daily to weekly timescales (Gillon et al. 2013) and is likely due to changing cloud structures on these objects. Recent large scale surveys of brown dwarf variability with Spitzer reveal variability on from a few percent to > 50% (Heinze et al. 2013). We may therefore expect to see variability in young exoplanets, which share similar effective temperatures and are similar ages. Indeed the young planet HR 8799 b (Konopacky et al. 2013) shares a nearly identical spectrum with the most variable brown dwarf known (2M2139, Radigan et al. 2012). The goal of this project is to reanalyze the vast amount of archival data on the HR 8799 bcde planets and assess the photometric variability over several timescales. We expect that a visiting graduate student would have experience using python or a similar programming language. With the guidance of Dr. Meshkat, the graduate student will process raw data on this star from several telescopes, detect the planets, and asses the brightness and error budget for that measurement. The data reduction methods and amount of photometric variation of the planets will be analyzed by the student in a publication.

Using machine learning to model the galaxy population in cosmology surveys

Advisor: P. Capak

Our team has been developing machine learning and big data techniques to improve data extraction from future large cosmological surveys.  We are seeking students who are interested in cosmology, galaxy evolution, or computer science to either carry out astronomical research with these techniques or to develop new computational methods.  The group lead P. Capak has access to the COSMOS (http://cosmos.astro.caltech.edu), SPLASH (http://splash.caltech.edu), and Hyper-Suprime-Cam (http://hsc.mtk.nao.ac.jp/ssp/) surveys which could be used for the studies as well as being a member of the Euclid (http://www.euclid.caltech.edu) and WFIRST (https://wfirst.ipac.caltech.edu) cosmology teams.  The initial motivation of this work was to improve systematics control in photometric redshift estimates which are essential to constrain the dark matter equation of state using weak lensing.  We have now extended this work to include  modeling the physical parameters (mass, age, star formation rate, etc) of galaxies, defining the selection function of rare galaxy populations (such as high-redshift galaxies) and optimally selecting galaxies for spectroscopic follow-up. We are open to a wide range of potential research topics in this area.

The Spitzer-Euclid-WFIRST Legacy Survey and The Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH)

Advisor: P. Capak

We are seeking students to work on high-redshift galaxy studies with two large Spitzer surveys.  The Spitzer-Euclid-WFIRST Legacy Survey which covers 10 square degrees in each of the Chandra Deep Field South, and the North Ecliptic Poles and the Spitzer Large Area Survey with Hyper-Suprime-Cam which is covering 1.8 square degrees in each of COSMOS and the SXDS.  We are particularly interested in students who would like to work on galaxy clustering studies to link galaxies to their dark matter haloes across cosmic time.

The Complete Calibration of the Color Redshift Relation (C3R2) survey

Advisor: P. Capak

We are mid-way through a ~40 night survey using the Keck, VLT, and GTC to carry out a color complete survey of galaxies to R,I,Z < 25 (http://c3r2.ipac.caltech.edu).  This will be the first truly representative spectroscopic survey of the entire galaxy population to these faint magnitudes, covering the full range of observed galaxy colors in the 0.3-2.5um wavelength range.  The primary goal is to calibrate photometric redshifts for cosmology, however the survey will also provide a wealth of high-quality spectra for galaxy evolution studies.  We are seeking students interested in participating in the survey and conducting research with the line diagnostics in the spectra or in developing new photometric redshift techniques.

A Spectroscopic Survey of Cool White Dwarfs

Advisor: Sergio Fajardo-Acosta

While studying high proper motion objects in the AllWISE and AllWISE2 surveys (Kirkpatrick et al. 2014, ApJ, 783, 122; 2016, ApJS, 224, 36), we re-discovered an object with unusual 2MASS and WISE colors, WISEA 0615-1247, originally thought to be a subdwarf. Upon spectroscopic follow-up in the optical and near-infrared, we discovered that the object is a binary of a late-type dwarf and a cool white dwarf (Fajardo-Acosta et al. 2016, ApJ, 832, 62). The cool white dwarf exhibits metals in its atmosphere, of unusual relative abundances. It is thought that, under ordinary circumstances, metals in these objects are the result of pollution of their atmospheres with accreted material from extant debris disks. Our discovery points out to possible alternative scenarios as the source of these metals.

We are carrying out optical and near infrared spectroscopy of other cool white dwarfs, using the Double Spectrograph at the Hale Telescope at Palomar Observatory, and the SpeX near-infrared spectrograph at the NASA Infrared Telescope Facility, among others. The goal of these observations is to understand the variety of spectral energy distributions seen in these stars, what their Galactic distribution is, and how often debris disk material is responsible for their atmospheric composition. Of our selected targets, many of which with high proper motions, some are multiple, and therefore we will obtain spectra of late-type dwarf, brown dwarf, and subdwarf companions. These observations may uncover objects of peculiar characteristics, as was the case of WISE 0615-1247.

We expect the graduate student to assist with reduction and analysis of our spectroscopic observations, and with interpretation and publication of results. Since our survey involves various kinds of objects, such as low-mass companions to cool white dwarfs, the graduate student may have some flexibility on the area of research. Spectra of brown dwarf companions, for example, may give a wealth of information on the characteristics of these systems.

The ideal graduate student for this program would have a basic knowledge of white dwarf theory, and familiarity with optical and near-infrared spectroscopy. Knowledge of brown dwarf theory is definitely a plus. Familiarity with programming languages such as IDL and IRAF, the IRSA catalog search tools, and imaging datasets such as WISE/NEOWISE and 2MASS is recommended.

Calculating Occurrence Rates with K2

Advisor: Jessie Christiansen

Measuring the occurrence rate of extrasolar planets is one of the most fundamental constraints on our understanding of planets throughout the Galaxy. By studying planet populations across a wide parameter space in stellar age, type, metallicity, and multiplicity, we can inform planet formation, migration and evolution theories. In the post-Kepler era, our collaboration has been compiling catalogues of planet candidates from the K2 mission (see, e.g. Crossfield et al. 2016), and has expended considerable effort in characterizing the resulting planetary systems through a large follow-up program. We are now interested in investigating the underlying planet population; in particular, since K2 has observed nearly an order of magnitude more M-dwarfs than Kepler, we aim to constrain occurrence rates both more precisely and with more granularity across the broad M-dwarf parameter range than previously. We also aim to take advantage of the wide variety of stellar environments sampled by the community-driven K2 mission to estimate the frequency of planets orbiting stars with a range of metallicities and ages. We expect that a visiting graduate student would work on and publish an aspect of the occurrence rate calculations that they were interested/experienced in, such as: examining biases between planet host and non-planet host stars; coordinating and performing follow-up observations of candidates with high-resolution imaging or reconnaissance spectra; or developing and implementing algorithms to automate planet candidate selection for generating a uniform, repeatable, quantifiable final planet candidate catalogue.

Characterizing Young Companion Candidates from the PTF Orion Survey

Advisor: Julian Van Eyken

The PTF Orion survey photometric time-series survey ran during the winters of 2009 and 2010 as part of the Palomar Transient Factory (PTF), with the primary goal of searching for extremely young transiting planets in the young 25 Ori association (van Eyken et al. 2011). The large dataset, comprising some ~110,000 light-curves, also provided for a variety of other science, including searching for young binaries and new T-Tauri stars. Since stars at such young ages tend to be very active, finding companion signals among the intrinsic stellar noise can be challenging; properly interpreting the data from the resulting candidate systems can be even more so. As a result, there are currently relatively few known examples available to constrain theoretical formation and evolution models. One planet candidate identified in the data, PTFO 8-8695 (van Eyken et al. 2012), has since been the subject of follow-up by several groups, and remains under investigation. More recently, a previous participant in the IPAC Visiting Graduate Student program made a complete classification of all the variable light curves in the dataset, identifying among other things a number of new planet and binary candidates (Bustamante et al., submitted).

In early 2016, we obtained medium resolution (R~5,000-7,000) spectroscopic radial-velocity follow-up data for several candidate pre-main sequence binary stars from the PTF Orion survey, along with spectra for some of the candidate young planets and classical T-Tauri stars. The primary aim of this project is to reduce and analyze those data, characterize the respective systems as far as possible, and place them within the context of current research, with an eye to publishing the resulting findings.

Microlensing Parallax for Exoplanets Towards the Bulge

Advisor: Sean Carey

The Spitzer team is leading a microlensing observational campaign towards the Galactic Bulge following up microlensing events alerted by ground-based surveys. The key feature of this project is the possibility to measure the microlensing parallax thanks to the simultaneous observation of the same microlensing event from two distant (d > 1.5 au) observers (from Earth and from Spitzer). The main scientific driver of the project is to build the Galactic distribution of exoplanets (Calchi Novati et al, 2015a, Zhu et al 2017). Besides three exoplanets (Udalski et al 2015, Street et al 2016, Shvartzvald et al 2017), several additional interesting events (binary systems, high magnification events, single lens with finite source effects) have also been analyzed based on the first three years (2014, 2015 and 2016) of data (Yee et al 2015a, Zhu et al 2015a,2016, Shvartzvald et al 2016a,b, Bozza et al, 2016, Poleski et al 2016, Han et al 2016, 2017, Chung et al 2017). In addition, relevant work is being done for the analysis of the data (Calchi Novati et al 2015b) and to assess the statistical meaning of the observations (Yee et al 2015b, Zhu et al 2015b). The microlensing campaign is planned to continue during the two years of the ``Spitzer Beyond'' phase (2017-2018). In this framework there are several avenues to pursue, according to the applicant's interests and previous experience: planning for the observational campaign; optimizing the photometry of Spitzer data; characterization of microlensing events; evaluation of the detection efficiency.