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The mission of the Harvard-Smithsonian Center for Astrophysics (CfA) is to advance our knowledge and understanding of the universe through research and education in astronomy and astrophysics.
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2009 CfA Postdoc Science Symposium
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A day long event for the entire CfA community!
Phillips Auditorium
Wednesday Oct 14
9:30 am - 5:30 pm
Free coffee and snack breaks
Free lunch provided with RSVP to bpatla@cfa.harvard.edu by Oct 9th
Symposium schedule:
9:15-9:30: Poster Setup
9:30-10:15: Introduction
- Welcoming remarks by Charles Alcock
- Welcoming remarks by Christine Crowley
- CF orientation by Van McGlasson
- Library introduction by Michael Blake
- A working introduction to the CfA by current postdocs
10:15-10:30: Coffee Break and Poster Viewing
10:30-12:20: Science Session I: The Sun, solar system, lab experiments
12:20-13:20: Lunch (Free lunch provided with RSVP to bpatla@cfa.harvard.edu by Oct 9th)
13:20-13:25: Group Picture
13:25-15:15: Science Session II: Stellar, protostellar, galactic objects
15:15-15:40: Coffee Break and Poster Viewing II
15:40-17:30: Science Session III: The Milky Way, galaxies, and beyond
Postdoc Dinner Party (Post-doc significant others are welcome!!!) at the
Newtowne Grille Food & Spirits
Talk Abstracts
Title: Planetary observations with SMA
Abstract: Planetary observations with SMAMillimeter interferometry makes for an
excellent tool to study solar system planets physical properties. In particular,
thanks to the presence of strong rotational transitions of many species (CO, H2O,
SO2,...), this technique is helpful to retrieve properties of planetary atmospheres
such as temperature, density and dynamic profiles. This frequency range is also
adapted to measure thermal emission from outer solar system bodies, which is linked
to surface properties such as the albedo and emissivity. The spatial resolution that
is obtained with the Sub-Millimeter Array, Hawaii, makes it possible to resolve most
of the major solar system bodies at frequencies between 200 GHz and 690 GHz, and thus
to map those properties.A few examples of results obtained with the SMA will be
presented, in particular the first simultaneous mapping of SO and SO2 in Io's
atmosphere.
Title: Magnetic energy build-up and coronal brightness evolution
Abstract: We have investigated changes in the properties of the coronal magnetic
field in the context of different emission behaviors of coronal loops. Using
observations by the Transition Region and Coronal Explorer (TRACE), the Hinode/X-ray
Telescope (XRT), and the SoHO/Michelson Doppler Imager (MDI), NOAA active region 10963
has been analyzed in depth as to how different coronal signatures compare to inferred
coronal energy build-up. A Magnetic Charge Topology (MCT) model was used to establish
potential magnetic field connectivity of the surface magnetic flux distribution, and
a Minimum Current Corona (MCC) model was applied to quantify the energy build-up
along separator field lines. The results of the MCC analysis are compared to the
evolution of the coronal brightness, comparing areas of steady emission, very transient
emission, and temporal patterns of emission which imply coronal cooling.
Title: Survey of Quiescent Filament Channels at the Current Solar Minimum
Abstract: We present preliminary results on observations of filament channels
byHinode/XRT and STEREO/EUVI. The corresponding H-alpha filament informationis
provided by KSO and MLSO. The magnetic field information is provided bySOHO/MDI and SOLIS.
We study the correlation between filament channels onthe disk and cavities above the limb
as well as the X-ray and EUVstructures of filament channels. The studied filament channels
are dividedinto two types bases on the selection method: Type I channels (low- ormid-
latitude) are associated with long and continuous H-alpha filaments;while Type II channels
(mid- or high- latitude) are identified accordingto the cavities on the limb. Possible
corresponding cavities are found foronly 5 out of the 10 Type I filament channels, while
type II filamentchannels on the disk are identified for all of the studied 73 cavity pairs.
Filament channels are often observed as dark channels in X-rays and EUV. Shearedloops within
Type I channels can be seen in X-rays, but not in EUV. Wefind that the emission on the two
sides of the Type I filament channels isasymmetric in both X-rays and EUV. The eastern side
has curved brightfeatures while the western side has straight faint features.
Our interpretation is that the magnetic configuration is also asymmetric alongthe length of
the channel. The field lines in one polarity turn into theflux rope, while the field lines
from the other polarity are open orconnected to very distant sources. This asymmetric
emission is not oftenclearly identified in the type II filament channels.
Title: The Solar Dynamics Observatory Flare Detective
Abstract: We present the flare detective, a software module toautomatically detect and
characterize solar flares observed with theAtmospheric Imaging Assembly (AIA) telescopes
onboard the Solar DynamicsObservatory (SDO). The flare detective works in two steps.
First, flareswill be detected in EUV images by analysing lightcurves in macropixels.
At this time, only basic quantities such as time intervals,positions, and peak fluxes
will be determined. This will allow the module tokeep up with the extremely large size
of the incoming data stream and providenear real-time information for space weather
monitoring. Second, the flaredetective will be run again on the subset of images around
the time intervalswhere a flare has been already detected, where more sophisticated (and
slower)processing will be performed to better characterize the flare and providephysically
important parameters such as temperatures and emission measures,projected areas and
lightcurves in different channels. The events detected willbe made available to the H
eliophysics Knowledgebase (HEK) and also as a flarelist in text format accessible on the web.
This module is part of a largereffort to detect and track solar features and events that is
optimized to run onvery large datasets such as the ones provided by SDO.
Title: Global Effects on Magnetic Reconnection in a Laboratory Plasma Astrophysics
Experiment
Abstract: Magnetic reconnection is an inherently multiscale process in which smallscale
physics and global dynamics both play important roles. To addressthe interplay between
small and large scales, I present simulations ofmagnetic reconnection in a laboratory
plasma astrophyics experiment usingthe NIMROD extended MHD code. Geometric effects are
found to be moreimportant in determining the reconnection rate than the inclusion oftwo-fluid
effects. Communication between small and large scales isprimarily due to pressure gradients
that develop from a pileup ofreconnection exhaust which then feed back on the reconnection
process.Last, I present a model inspired by these results which describes steadymagnetic
reconnection with asymmetry in the outflow direction. This modelcan be applied to reconnection
events in the solar atmosphere and inplanetary magnetotails.
Title: Precision spectroscopy in molecular gases
Abtract: A cw fiber laser is stabilized to the P13 transition of ro-vibrational line (v1+v3)
of acetylene (12C2H2) inside the hollow core Kagome fiber. A frequency comb is then used to
measure the laser frequency to +-10 kHz. This work establishes that hollow core-optical fibers
are suitable for precision spectroscopy in molecular gases.
Title: Development of a test of Newton's law of gravitation at micrometer distances using
a superconducting spherical torsion balance
Abtract: To solve the theoretical problems associated with quantum gravity and the
unificationwith the other forces, many theories have been proposed such as string theory,
whichpredict violations of the inverse square law of gravity (ISL) at sub-millimeterdistances.
To search for such signatures at School of Physics of the University ofBirmingham, UK, we
developed an experiment based on a magnetically levitatedcryogenic torsion balance, the SSTB,
and a set of masses with modulated massdensity across their surfaces. The lateral force has to
be measured as one mass ismoved in front of the other with a micropositioner without any electrostatic
shieldinterposed between them. To optimize the experiment, the expected Newtonian andelectromagnetic
forces have been modeled and the sensitivity of the SSTB has beenstudied. During the first
experimental runs the masses were moved past each otherdown to a separation of 30 microns, and a
spurious signal (1.91x10^-11 \pm 4.7x10^-12 Nm)was measured at the spatial
periodicity of the density modulation. To fullycharacterize and reduce such signal, and to look
for new violations of the ISL ofgravity, the torque sensitivity of the SSTB, now as small as
\approx 10^-11 Nm/\sqrt{Hz}) at 30mHz, will be further improved in the next future.
Title: Pre-transitional Disks: The Missing Link for Planet Formation in Disks?
Abstract: In their initial stages of formation planets should interact with theaccretion
disk surrounding the newborn star, clearing the materialaround themselves and leaving behind
an observational signature in theform of gaps in the primordial disk. Stars with inwardly
truncated diskshave been detected with Spitzer IRS spectra and are now labeled as "transitional
disks." While planet formation can create the inner holein these disks, other formation mechanisms
such as photoevaporation orthe magneto-rotational instability can account for this type of
clearingas well. Our analysis of IRAC broad-band photometry and IRS spectra withirradiated
accretion disk models has now isolated the earliest stages ofplanet formation in the disk.
This new class of "pre-transitional disks"has an inner optically thick disk separated from
an outer opticallythick disk by an optically thin gap. Physical mechanisms that have
beenpresented to explain disk clearing in transitional disks can now betested with this
new class of disk; forming planets emerge as alikely explanation.
Title: 2D MHD Simulations of Star Formation and a Comparison to Observations in
the Perseus Molecular Cloud
Abstract: The environments surrounding dense, star-forming cores can now be fully characterized
through large (degree) scale surveys of dust continuum, molecular lines, and extinction, revealing
the column density, temperature, density, and kinematics. As well, computational speed now allows
for an exploration of parameter space by numeric simulators of star formation.I will present
analysis of a suite of 2D local turbulent magneto-hydrodynamic simulations spanning a range of
initialconditions (magnetic field strengths and mach numbers). We compare our 'observations' of
these simulations, in particular linewidths and centroid velocities of both dense star-forming cores
and the surrounding lower density material, with our molecular lineobservations of dense cores, their
envelopes, and larger structureswithin the Perseus molecular cloud. Even strongly magnetic simulations
have difficulty reproducing the observed highly turbulent motions on the largest scale of the molecular
cloud while simultaneously showing the quiescent (nearly thermal) motions seen on the scales dense
cores within their local environment and smaller. This may implythe need to account for global
effects of the cloud and turbulentdriving on the largest scales.
Title: The Effects of Radiative Transfer on Low-Mass Star Formation
Abstract: Forming stars emit a substantial amount of radiation into their natalenviornment, an
effect that is often neglected by simulations of starformation. I will present gravito-radiation-
hydrodynamics simulations oflow-mass stars forming in a turbulent molecular cloud. I will
comparethe distribution of stellar masses, accretion rates, and gas temperaturesin the cases with
and without radiative transfer. I will demonstrate thatradiative feedback has a profound effect on
accretion, multiplicity, andmass by reducing the number of stars formed and the total rate at
which gas turns into stars. Although protostellar heating is mainlyconfined to the dense core
regions, I find that it is sufficient tosuppress disk fragmentation that would otherwise result
in very low-masscompanions or brown dwarfs.
Title: On the triple origin of blue stragglers
Abstract: On the triple origin of blue stragglersBlue straggler stars (BSSs) are stars
observed to be hotter and bluer than other stars with the same luminosity in their environment.
As such they appear to be much younger than the rest of the stellar population. Two main channels
have been suggested to produce such stars: (1) collisions between stars in clusters or (2) mass
transfer between, or merger of, the components of primordial short-period binaries. Here we
suggest a third scenario, in which the progenitors of BSSs are formed in primordial (or
dynamically formed) hierarchical triple stars. In such configurations, the dynamical evolution
of the triples through the Kozai mechanism and tidal friction can induce the formation of very
close inner binaries. Angular momentum loss in a magnetized wind or stellar evolutioncould then
lead to the merger of these binaries (or to mass transfer between them)and produce BSSs in binary
(or triple) systems. We study this mechanism and its implications and show that it could naturally
explain many of the characteristics of the BSS population in clusters, most notably the large
binary fraction of long-period BSS binaries; their unique period-eccentricity distribution (with
typical periods > 700 days); and the typical location of these BSSs in the color-magnitude diagram,
far from the cluster turnoff point of their host clusters. We suggest that this scenario has a
major (possibly dominant) role in the formation of BSSs in open clusters and give specific
predictions for the BSSs population formed in this manner.
Title: Gas driven supermassive black hole binaries
Abstract: Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a
common by-product of major galaxy mergers. We use simple disk models for the circumbinary gas
and for the binary-disk interaction to follow the orbital decay of SMBHBs through physically
distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior
to the GW-driven phase, the viscous decay is analogous to type-IIplanetary migration.In this
talk, I will discuss possible observational implications of the gas driven decay. A population
of periodically variable active galactic nuclei may be sufficiently common to be detectable
during these stages. A statistical sample of these objects can be used to test the decay
mechanism for these binaries. I will also demonstrate that the GW background is significantly
modified for pulsar timing arrays in the most sensitive nHz frequency range.
Title: Measuring black hole spin on stellar and supermassive scales
Abstract: The primary focus of my postdoctoral research is BH spin: what is itsdistribution
between and among different source populations, how does itaffect the inflow and outflow of
matter, energy and angular momentum inthe system, and what role does it play in the temporal
evolution of thesystem as a whole? My interest is in both Galactic black holes (GBH)as well
as supermassive black holes in active galactic nuclei (AGN).Knowing the distribution of natal
spins among stellar mass black holes isvital to our understanding of how these objects form
and evolve overcosmic time. Also, knowing how/whether physical scale plays a role inspin
evolution will illuminate possible links between the AGN and GBHblack hole populations, and
will help to assess whether accretion episodesor mergers are primarily responsible for the
growth and evolution in AGNwith high black hole spins. I approach these questions from
anobservational perspective, fitting detailed X-ray spectra of the accretiondisk with models
that take into account both relativistic iron lineemission and, when necessary, complex
intrinsic absorption.
Title: Thermal Equilibia of Magnetically Supported Black Hole Accretion Disks
Abstract: Galactic black hole candidates are known to exhibit transitionsbetween different
X-ray spectral states.I focus on the "bright/hard state" during the "bright/slow transition"
(e.g., Belloni et al. 2006;Gierlinski and Newton 2006; Miyakawa et al. 2008). The X-ray
spectrum in the bright/hard state is roughly described by ahard power-lawwith a (moderately)
high energy cutoff like in the low/hard state.However, the luminosity is "bright" (10 - 30
percent of the Eddingtonluminosity)and the cutoff energy anti-correlates with the luminosity
anddecreases to about 50 keV.These feature cannot be explained by the conventional model of
blackhole accretion disks (e.g., the ADAF/RIAF model).I will present new thermal equilibrium
solutions incorporating magnetic fieldson the basis of the results of global three-dimensional
magnetohydrodynamic simulations.In these solutions, the magnetic pressure is dominant and the
magneticheating balances the radiative cooling. We find that these solutions exist above the
maximum mass accretionrate for the ADAF/RIAF solution,and that the temperature anti-correlates
with the mass accretion rate. Therefore, these solutions can explain the bright/hard state.
Title: Dwarf galaxies as building blocks of the Milky Way
Abstract: The chemical evolution of the Galaxy and the early Universe is a keytopic in modern
astrophysics. The most metal-poor Galactic halo starsare now frequently used in an attempt to
reconstruct the onset of thechemical and dynamical formation processes of the Galaxy. The same
isalso possible for other systems such as dwarf galaxies. Metal-poorstars, once also identified
in the dwarfs, thus become vital probesfor near-field cosmology. Until recently, no extremely
metal-poorstars ([Fe/H]<-3) were known in any of these satellites. I willpresent new results
for several stars with [Fe/H]<-3 in the newultra-faint galaxies, as well as the classical dwarf
Sculptor(obtained with Magellan/MIKE). We find evidence that the chemicalsignature of all these
stars strongly resembles that of old Milky Wayhalo field stars. For the first time, this
agreement strongly supportsthat systems like the surviving dwarfs were building blocks of the
Milky Way's low-metallicity outer halo. This opens a new window tostudy galaxy formation through
stellar chemistry.
Title: Nuclear Star Clusters and Black Holes
Abstract: Massive star clusters are found at the centers of a majority of spiral and elliptical
galaxies with masses similar to or smaller than Milky Way. Recent studies have shown that these
nuclear cluster masses scale with galaxy or bulge mass in the same way as supermassive black holes,
and thus seem connected to the overall evolution of their host galaxy. The link between black holes
and nuclear star clusters is largely unknown, but we have found that they appear to frequently co-habitate.
We have assembled a wide range of observations of the nearest nuclear star clusters to understand their
formation and the process of mass accretion on parsec scales at the centers of galaxies. Using adaptive
optics assisted integral-field spectroscopy, we resolve the kinematics and structure of the nuclear star
clusters, measure their masses and constrain the mass of any coincident black holes.
Title: Stacking Analysis of RSR Spectrums: Properties of a Local ULIRG population
Abstract: Redshift Search Receiver (RSR), a ultra-wideband spectrometer, is a facility instrument
that has been being built for the Large Millimeter Telescope (LMT). It covers the entire 3mm
atmospheric window from 75 to 111 GHz instantaneously. While the LMT is under its final construction,
the RSR has been commissioned on the Five College Radio Astronomy Observatory (FCRAO) 14m. As one of
the early science programs, we carried out a CO survey of 29 local ULIRGs using the RSR. In this talk,
I will present the results from the RSR spectrum stacking analysis of 27 CO detected ULIRGs. I will
discuss properties of the local ULIRG population (e.g. star formation rate, molecular abundance)
inferred from the coadded RSR spectrum.
Title: The radiogalaxies and quasars conspiracy
Abstract: In 1974 Fanaroff & Riley divided the extended radio sources into two structural classes,
on the basis of their radio morphology and power.For several years we have been collecting basic
parameters forextragalactic jets in quasars and radio galaxiesto look for an extension of the
classification criterion,based on their radio and X-rays properties.The fact that different
processeshave been proposed to describe their X-ray radiation,(synchrotron vs inverse Compton emission)
appeared as a good indication of a new classification scheme.However, comparing the radio-to-X-ray
propertiesof the extragalactic jets, several aspects on their naturebecame unexpectedly unclear.
Title: Low frequency results for the AGNs
Abstract: We have studied several radio sources at low radio frequencies using GMRT.Our prime motivation to
study these fields is to detect faint radioemission at very low frequencies due to low energy electrons.
Our results provide evidence that the spectra of low-surface-brightnessfeatures are flatter than the spectra
of high-surface-brightness featuresin several radio galaxies. In addition, the low frequency radio
imagesshow morphologies that are similar to the morphologies at highfrequencies. This suggests that
low-frequency synchrotron emission fades(nearly) as rapidly as high-frequency synchrotron emission. These
results suggest that the simple picture of spectral electron ageing needs revisionor we need to re-examine
the formation mechanism of such sources.In the talk, I will present the images and statistics, and will
discussthe relevance of these results and the role of low-frequency radioinstruments in exploring several
unknowns.
Title: Recent progress on quasar studies based on SDSS samples
Abstract: Large optical survey programs such as the SDSS have provided ample data for the statistical
investigations of quasars. I will talk about recent progress on quasar studies based on SDSS samples,
with focuses on quasar clustering and Eddington ratio distributions.
Title: The problem of foregrounds for observations of the cosmological 21cm line
Abstract: Foreground contamination is the most serious problem formeasurements of the cosmological 21
cm line. I will review the problem andpresent observational results which characterize foreground
properties atvarious Galactic latitudes. I will discuss their implications for the dataanalysis and
the observational strategies of cosmological 21cm lineobservations.
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