Leisa K. Townsley

Senior Scientist
Department of Astronomy and Astrophysics, Penn State University

Ph.D. Physics, University of Wyoming, 1994

Contact Information:

Office: 405 Davey Lab
Mailing Address: 525 Davey Laboratory, University Park, PA 16802
Phone: (814) 863-7946
Fax: (814) 863-3399
email: townsley@astro.psu.edu

Research Interests:

Star formation, massive young stellar clusters, giant HII regions, X-ray astronomy instrumentation.

My CV gives further details, including a list of publications. 

My primary Penn State collaborators are Patrick Broos, Matthew Povich, and Eric Feigelson.

High Energy Astrophysics

I am a member of the Penn State High Energy Astrophysics Group and the Chandra X-ray Observatory  Advanced CCD Imaging Spectrometer (ACIS) Instrument Team.  The ACIS Principal Investigator is Penn State Professor Gordon Garmire.  The Chandra instrument teams provide expertise and support to the Chandra X-ray Center.  

ACIS Programmatics: 
Some of our current efforts on the ACIS Team at PSU include developing methods to analyze faint diffuse X-ray emission pervading fields of point sources, developing new techniques for photometry in extremely crowded ACIS fields, and reconstructing ACIS point source spectra and lightcurves that have been corrupted by photon pile-up.

Star Formation Science:

The Chandra Carina Complex Project (CCCP)

The above project website is password-protected -- please contact me if you need access. 

~~~We have submitted 16 papers to ApJS for a Special Issue on the CCCP.~~~

The CCCP is a 1.2 Ms Cycle 9 Chandra Very Large Project to study the Great Nebula in Carina, consisting of 22 ACIS-I pointings and covering ~1.4 square degrees on the sky, to a depth of ~60 ks.  There are ~57 co-investigators divided into 7 science groups, with overall management provided by myself as Principal Investigator.

Science groups and group leaders: 

Data Products, Patrick Broos
Massive Stars, Marc Gagne' and Mike Corcoran
Revealed Stellar Populations, Eric Feigelson
Obscured Stellar Populations, Thierry Montmerle
Diffuse Emission, Leisa Townsley
Multiwavelength Studies, Nathan Smith
Global Synthesis, Sally Oey

The proposal abstract:
"The Great Nebula in Carina is a superb site to study the violent massive star formation and feedback that typifies giant HII regions and starburst galaxies. We propose to map the Carina star-forming complex with a mosaic of 20 new 60-ks ACIS-I pointings as a testbed for understanding recent and ongoing star formation and to probe its regions of bright diffuse X-ray emission. We will provide a catalog of multiwavelength properties of ~12,000 X-ray-selected stars. We will explore superbubble confinement, shocks, cloud evaporation, mass-loading of winds, ISM enrichment, and HII region energetics. We will also examine Carina as a surrogate environment for our Solar System's formation, where protoplanetary disks are bathed in harsh winds and radiation from nearby massive stars."

 My contributed talk on the CCCP from the Sept 2009 conference Chandra's First Decade of Discovery is available here as both a PDF file and video.

The Giant Extragalactic HII Region 30 Doradus 

30 Doradus was observed very early in the Chandra mission using the 17'x17' ACIS-I array, for just 20 ks.  Our two papers on those early data described the extensive diffuse X-ray emission across the field and the X-ray point sources. 

We re-observed 30 Dor with ACIS-I in Chandra Cycle 6, with a 70+30 ks GO+GTO program titled "30~Doradus:  Our Starburst Microscope," PI Townsley, co-I's E. Feigelson, P. Broos, K. Getman, M. Tsujimoto, B. Brandl, and G. Garmire.

The proposal abstract:
The Giant Extragalactic HII Region 30 Doradus in the LMC provides us with a unique view of the fundamental building block of the starburst phenomenon in galaxies.  It contains several extremely rich stellar clusters 1--10 million years old and a new generation of deeply embedded high-mass stars just now forming.  HST and Spitzer data show that the combined actions of stellar winds and supernovae have carved the ISM into an amazing display of arcs, shells, pillars, voids, and bubbles, ranging over spatial scales of 1--100 pc.  This is the GO contribution to a joint GTO+GO, 100-ksec observation of 30 Doradus.  We will exploit Chandra's fine spatial resolution to study the diffuse X-ray morphology on 1--10 pc scales and to study the highest-mass O and Wolf-Rayet stars that shape it.

M17, Our Closest Giant HII Region

This Chandra Large Project from Cycle 7 was another combination of GO and ACIS GTO programs (280 ks on the main cluster NGC 6618 + 92 ks on an eastern pointing) and added to the original 40-ks GTO observation of NGC 6618 from Cycle 2.  This was the first clear discovery of soft diffuse X-rays, probably from massive star winds, in the Chandra project (see Townsley et al. 2003).  The Cycle 2 data yielded ~900 X-ray point sources (Broos et al. 2007); the combined Cycle 2+7 data show ~2700 point sources and the extensive diffuse hot plasma shown in blue here (our Fire-breathing Dragon).

The Cycle 7 Large Project was called "Winds, OB Populations, and Young Stars:  New Science from M17," PI Townsley, co-I's P. Broos, Y-H. Chu, E. Churchwell, E. Feigelson, G. Garmire, K. Getman, N. Grosso, T. Montmerle, A. Tielens, M. Tsujimoto, and B. Whitney.

The proposal abstract:
We propose to use two ACIS-I pointings centered on the young, massive OB cluster in M17 (the Omega Nebula) and on its powerful X-ray outflow as a testbed for understanding recent and ongoing star formation in the M17 complex and its environmental impact.  This rich field is perfect for studying X-ray emission from high-mass as well as intermediate/low-mass stars and for comparing the X-ray luminosity function in a high-mass complex to that in the closer, less powerful Orion Nebula Cluster, recently established by a long Chandra exposure. M17 is ideal for exploring the properties of diffuse X-ray emission to elucidate wind/wind and wind/cloud shock physics in HII regions and for studying embedded stellar clusters and massive protostars forming as M17 interacts with its molecular cloud.

M17's O4-O4 Binary:  The Eyes of the Dragon

The two O4 stars at the center of NGC 6618, separated by 1.8", are clearly resolved in the Chandra data; both are bright X-ray sources and one of them is highly variable.  Their hard X-ray spectral components are unusual for massive stars. 

This image shows the ACIS event data binned up into an image with 0.25" pixels.  Point sources are marked by blue +'s.  Their extraction regions are outlined by red polygons.  Source-finding was performed on a maximum-likelihood reconstruction of this image, enabling us to find close pairs and faint sources in the PSF wings of brighter ones. 

Analysis of the two O4 stars proved to be quite the circus:  one of them is constant and always piled up, the other is unpiled until it brightens, with the pile-up fraction changing as the source gets brighter (corrupting its lightcurve as well as its spectrum).  Annular spectral extraction is impossible due to surrounding sources and the fact that the region of overlap between the PSF wings of the two O4 stars is also variably piled up (so I call it the Bowtie of Death).  Using our ACIS CCD simulator and the MARX Chandra ray-tracing code, Patrick Broos built a tool to reconstruct an unpiled spectrum as a way of handling these two O4 stars.  Using the CCD simulator, this tool makes a spatio-spectral model of the two O4 stars to try to recover pile-up-free spectra and lightcurves.  We are testing this pile-up corrector on a number of Chandra targets.  X-ray study of these O4 stars is underway by Townsley, Broos, and our colleague Marc Gagne' from West Chester University.

  NGC 3576

Spontaneous or Triggered Formation of a Giant HII Region?

These two ACIS-I pointings were obtained as separate GO programs, NGC 3576 (the lower pointing) in Cycle 5, followed by the upper pointing in Cycle 9 to follow up the diffuse emission seen in the original dataset -- that proposal included joint Spitzer time (the Spitzer data are in the capable hands of Remy Indebetouw).  An added bonus in the Cycle 9 ACIS data was the discovery of a pulsar wind nebula around PSR J1112-6103 and hard diffuse emission pervading a large cavity outlined by heated dust, as seen in MSX, Spitzer, and now WISE data. 

We have recently completed the point source extraction and validation from these two pointings and find 1559 X-ray point sources -- a very large number for a partially-embedded complex at 2.8 kpc. 

For both observations I serve as PI; co-I's are P. Broos, Y-H. Chu, E. Feigelson, K. Getman, R. Indebetouw, and T. Montmerle.

The Cycle 9 proposal abstract:
Our understanding of massive star formation is uncertain at all levels, from individual stars to massive stellar clusters to OB associations.  Our first Chandra observation of the Galactic giant HII region NGC 3576 addressed one example of the first of these problems:  because hard X-rays penetrate even very large columns of obscuring material, we were able to pinpoint massive, young, embedded stars that remained undetected even at 3.5 microns, solving the mystery of NGC 3576's missing ionizing sources.  With a new ACIS-I pointing and the first Spitzer observation of this target, we will address the second two of these problems:  how massive clusters form and how they are related to the formation and evolution of the larger-scale, unbound populations known as OB associations.

The Embedded Massive Cluster Powering NGC 3576

These ACIS images show the ionizing cluster powering the NGC 3576 giant HII region.  They have 0.25" pixels and show soft X-rays (0.5-2 keV) in red, hard X-rays (2-8 keV) in green.  Point source extraction regions from ACIS Extract are shown as blue polygons.

The rightmost image is ~1.5' x 1.5' and shows the bulk of the cluster.  The near image (~30"x30") shows the deeply-embedded bright X-ray sources at the western edge of the cluster; many of them are closely-spaced pairs or multiples.  These are some of the massive stars responsible for the ionization of this region.  They are extraordinarily hard X-ray emitters -- the only reason we detect them behind such large absorbing columns.

The Cycle 5 proposal abstract:
We propose a 60-ksec ACIS observation of the Galactic giant HII region NGC 3576, to image and confirm a soft X-ray bubble suggested by ROSAT data and likely due to flows created by the winds of OB stars powering the HII region.  This observation will also reveal the massive stellar engine powering NGC 3576, shredding the molecular cloud from which it formed, and triggering a new generation of massive star formation. This target bears many similarities to the edge-on `blister' HII region M 17 (the Omega Nebula), where such soft X-ray flows were first confidently detected by Chandra.  We hope to establish whether these wind-blown bubbles, filled with hot X-ray-emitting gas, are ubiquitous in blister HII regions (given O stars of sufficient wind power) or unique to M 17 for some reason.

My Other Chandra Targets and Projects

The examples above are just a few of the massive star-forming regions for which I have obtained Chandra data.  Some of my other projects are listed below.

Multiwavelength Studies

I am co-PI (with Eric Feigelson) on a multi-year NSF Astrophysics project to link Chandra observations of young stellar clusters with international groups developing theories and models of star formation and evolution.

I am co-I (PI Eric Feigelson) on a multi-year NASA ADP project to study 20 young stellar clusters with Chandra, XMM, Spitzer, and 2MASS.  We are working with several groups around the world to include high-quality ground-based imaging and spectroscopy in this effort.

I am working with NSF Postdoctoral Fellow Matthew Povich and University of Wisconsin Professor Ed Churchwell to obtain near-IR imaging of northern young stellar clusters with the WHIRC camera on WIYN.

The Advanced CCD Imaging Spectrometer (ACIS)


ACIS is used for most Chandra observations.  We typically observe with the 2x2-CCD ACIS-I array, covering ~17'x17' on the sky.  I recently gathered up a short sampler of some ACIS Instrument Team science activities and software projects.

The Penn State ACIS Team has developed data and science analysis tools and recipes, many of which are publicly available (others may be supplied upon request because they change often).  Please visit Tools for ACIS Data Analysis to see what's available.  Note that most of this software is in IDL and was written by Patrick Broos.  For an overview, please see Pat's recent ApJ paper, Broos et al. 2010.

Software engineer Patrick Broos and I have developed Monte Carlo simulations in IDL to understand the interactions of photons and particles with different types of X-ray CCD devices. Using these simulations, we developed a technique to derive subpixel positions of astronomical targets using the distribution of multiple-pixel events. For some photon energies, this may result in a 10-fold improvement in source position estimation for ACIS. We are also using the IDL simulation code to model photon pile-up from bright point sources.  In our ACIS calibration efforts over the years, we used the CCD simulator to explore the corruption of photon events by particle events, to derive the best split-event brightness threshold, and as a basis for a model of the entire ACIS instrument response.

The PSU ACIS CCD Simulator, developed by Patrick Broos and Leisa Townsley and written in IDL, is available from our Simulator Webpage.  A detailed description of our methods is available in Townsley et al. 2002, NIMPA, 486, 716.  

Charge Transfer Inefficiency

The back-illuminated (BI) chips in ACIS (S1 and S3) always exhibited marked charge transfer inefficiency (CTI) due to their manufacturing process.  We worked on amelioration techniques for this kind of CTI for several years before the Chandra launch.  At the beginning of on-orbit operations the front-illuminated (FI) ACIS chips were damaged by radiation, so now they also exhibit CTI, to a more severe degree than the BI chips.

Patrick Broos and I subsequently developed a technique to recover event energies and grades from CTI-corrupted data.  Our IDL code is available and there is an ApJ Letter (Townsley et al. 2000) giving a brief description of our method.  More details and results are on our old CTI Corrector Webpage.  A detailed description of our methods is available in Townsley et al. 2002, NIMPA, 486, 751.

UV/Optical Blocking Filter Calibration

As part of the ACIS Team's pre-launch calibration efforts, in 1995-1997 we spatially and spectrally mapped the transmissivity of the ACIS UV/Optical Blocking Filters (OBFs) at the University of Wisconsin Synchrotron Radiation Center (SRC) as part of their flight calibration. I managed a team of scientists, engineers, and technicians at Penn State and MIT and worked with high-energy physicist James MacKay and the SRC staff to design and carry out suites of measurements on several sets of these aluminized Lexan and Polyimide unsupported thin films (made by Luxel Corporation) that block UV and visual radiation.  We developed specialized procedures for handling this extremely delicate space hardware and constructed a Class 100 cleanroom over the SRC beamline to adhere to NASA requirements for X-ray mission flight units.  Our transmissivity uniformity measurements provided the basic criterion for choosing the filters to be installed in the ACIS camera, just in front of the focal plane, to ensure that only X-radiation is detected by the CCDs. This calibration effort involved five major institutions and produced the most detailed spatial maps ever obtained on such films.  The OBFs are single-point failures for the ACIS camera (if they rupture Chandra+ACIS turns into a really bad UV/visual telescope).

ACIS Spectroscopy Array Filter, actual size ~1"x6"
           273 eV                  522 eV                 775 eV  
                                                                                                                           273 eV                                                   522 eV                                                        775 eV
                                                                                                                                                             ACIS Imaging Array Filter, actual size ~2"x2"

Some Conference Presentations:

The Chandra Carina Complex Project
Contributed talk at "Chandra’s First Decade of Discovery," 22-25 September 2009, Boston, MA

Remarkable X-ray Emission from the Young O4-O4 Binary in M17
Poster at Contifest, a workshop honoring Peter Conti:  "Hot Massive Stars... A Lifetime of Influence," 12-15 October 2008, Lowell Observatory

X-ray observations of massive star formation and feedback
Contributed talk at the 2008 ALMA Workshop "Transformational Science with ALMA:  the Birth and Feedback of Massive Stars, Within and Beyond the Galaxy," 25-27 September 2008, Charlottesville, VA

Chandra's X-ray View of Massive Star-forming Regions
Poster at "Eight Years of Science with Chandra" Symposium, 23-24 October 2007, Huntsville, AL

An X-ray Tour of Massive Star-forming Regions with Chandra
Contributed talk at the 2006 May Symposium, "Massive Stars: From Pop III and GRBs to the Milky Way," 8-11 May 2006, STScI

Parsec-scale X-ray Flows in High Mass Star Forming Regions
Contributed talk at IAU Symposium 227, "Massive Star Birth:  A Crossroads of Astrophysics,"  16-20 May 2005, Acireale, Italy

Chandra/ACIS Spectra of the 30 Doradus Star Forming Region
Poster from the January 2001 AAS meeting in San Diego.  It reviews some of our preliminary spectral analysis of the ACIS
GTO observation of 30 Doradus and updates our work on CTI and response matrices for ACIS front-illuminated CCDs.

Chandra/ACIS Observations of 30 Doradus
Poster from the January 2000 AAS meeting in Atlanta.  It shows some of the early images of the ACIS data on 30 Doradus and
outlines our early attempts to correct CTI on ACIS front-illuminated CCDs.

Image Releases and other Public Outreach:

Our friends in the Chandra X-ray Center E/PO group have done a great job over the years of publicizing my research

Web page by Leisa Townsley ( townsley@astro.psu.edu )
Department of Astronomy and Astrophysics
Penn State University