JULY 23 - 27, 2007

WEDNESDAY, JULY 25

12:30 pm: High Energy Astrophysics Lunch Talk. "The X-Ray Photoionized Wind of Cygnus X-1 during X-Ray High/Soft States," Dr. Saku Vrtilek, CfA. Pratt Conference Room.

Abstract: High-resolution ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Observations were taken at two epochs roughly one year apart; orbital phase ranges around phi_{orb} = 0 and 0.5 were covered at each epoch. We analyze the characteristics of a selection of P Cygni profiles and note, in particular, a strong dependence on orbital phase for the high ionization material: the profiles show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star.

We fit the P~Cygni profiles using the Sobolev with Exact Integration method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. Of the wind-formed lines, the Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L_x matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet. We test our model by comparing our predicted X-ray luminosity with contiguous observations by the RXTE ASM.

THURSDAY, JULY 26

11:00 am: Science Education Seminar and Open Discussion. Weekly sessions focus on current department programs, literature reviews, and recent trends in science education and assessment. Some sessions feature outside speakers or guests. All members of the CfA community are welcome. Please contact Bruce Ward at bward@cfa, if you wish to attend any session. Phillips Auditorium.

11:00 am: Institute for Theory and Computation (ITC) Special Seminar. "The Origin of the Galaxy Mass-Metallicity Relation and Implications for Galactic Flows," Kristian Finlator, University of Arizona. Pratt Conference Room.

Abstract: In recent years a number of observers have characterized the well-known luminosity-metallicity relation of galaxies with unprecedented precision. It has now become clear that (1) it is a manifestation of the more fundamental mass-metallicity relation; (2) this relation spans up to five decades in stellar mass at low redshift with extremely tight scatter; and (3) a similar relation exists at high-redshift. All of these observations indicate that a single, simple condition or process must govern the gas-phase metallicities of star-forming galaxies, yet a satisfying explanation that leads to quantitative agreement with observations has remained elusive. In this talk I will discuss the results of a comparison between the observed relation and cosmological hydrodynamic simulations incorporating several models for galactic outflows. I will show that (1) momentum-driven outflows are favored; and (2) gas-phase metallicities likely reflect an instantaneous equilibrium between the rates of enrichment due to star formation and dilution owing to fresh inflows. In this view, gas-phase metallicities are not a direct indicator of galaxies' star-formation histories. Instead, they probe the way in which galactic inflows and outflows vary with stellar mass. I will discuss how all of the characteristics of the observed relation (its slope, normalization, scatter, the observed flattening in massive galaxies at low redshift, and the evolution with cosmic time) can be understood in the context of this idea with a view towards highlighting what the observed mass-metallicity relation actually teaches us about galactic flows.

4:00 pm: Summer Colloquium Series. "Wavefront Sensors and Adaptive Optics," Deborah Woods, CfA. These talks are aimed at undergraduate summer students, but all CfA staff and visitors are welcome. Preceded by refreshments at 3:30 pm. For more information, contact Jonathan McDowell (jcm@cfa, 5-7176). Phillips Auditorium.

FRIDAY, JULY 27

11:30 am: Ph.D. Colloquium. "The Motions of the Magellanic Clouds and the Nature of Galactic Dark Matter," Nitya Kallivayalil. Phillips Auditorium.

Abstract: This dissertation develops two approaches to address Milky Way structure: one is the use of the Magellanic Clouds as a probe of properties of the halo, and the other is an investigation into the nature and location of the MACHO lenses. Specifically, I will 1) present a state-of-the-art measurement of the proper motion of the Magellanic Clouds using HST data.

This measurement has the required accuracy to constrain models of the Galactic dark halo as well as formation mechanisms for the Magellanic Stream. 2) Address with IRAC, on board Spitzer, the issue of whether the lenses belong to some population of dwarf stars in the disk that are too cool to have been detected with conventional telescopes thus far. I present the results, as well as predictions about the locations of the remaining lenses, based on the IRAC detection limits and our basis population.

12:30 pm: Radio and Geoastronomy Division Weekly Lunch Talk. "Survey of Dense Cores," Dr. Di Li, Jet Propulsion Laboratory. Room M-340, 160 Concord Avenue.

Abstract: The last couple of years have seen a dramatic increase in the volume of multiwavelength surveys of nearby star forming regions. The resolution and comprehensiveness of these surveys have allowed some new, systematic insights into the properties of dense molecular cores. I will introduce our work in studying dense cores in massive star forming regions. We identify samples of largely externally heated cores and find them to be in a core mass function (CMF) significantly flatter than the stellar initial mass function (IMF). This result suggests a time-evolving CMF. Fragmentation should be important in forming the IMF. The immediate future for systematic study of dense cores is bright, due to the expected arrival of large submillimeter surveys from space telescopes, such as Herschel and ground based instruments such as SCUBA2. I will introduce a new method to combine multiwavelength data to derive structures of dust cores. Our method will utilize the next generation of large surveys, especially several Herschel Key programs to form a physical core model in details never seen before.

SAO HUMAN RESOURCES NOTES

Leave Donations Requested

Due to a medical emergency, Donna Coletti of the Library has been approved as a leave donor recipient in the Leave Transfer Program. Any SAO Trust Fund employee who wishes to donate annual leave to Donna should complete a Leave Donor Form and send it to MS17.

Under the Leave Transfer Program, Trust Fund employees may only donate annual leave to other Trust Fund employees and Federal employees may donate annual leave only to other Federal employees. If you should have any questions or are not able to access the form, please contact Anne LeBlanc, ext. 5-7371.

Job Vacancies

An online listing of all current SAO job openings is available. For more information, please call Human Resources, ext. 5-7371, or view your nearest bulletin board.

A list of scientists currently visiting the CfA is available here.