Columbia University Astronomy and Astrophysics

Astronomy C3646


Arlin Crotts
Professor of Astronomy
Phone number: 854-7899
Office: Pupin 1012

Here are a few possible observing projects, with more to follow:

Galaxies in the Kepler space telescope field
We will use existing images from the Palomar Transient Factory (using the 48-inch Oschin schmidt telescope on Mt. Palomar, California) to locate previously unidentified galaxies in the field of the Kepler space telescope and classify them by color and galaxy type. In January we will be able to propose to observe these with the Kepler and measure how the nuclei of galaxies vary in brightness (due to their central black holes, primarily) at levels that are much more sensitive than previously explored. In the meantime we can see how galaxy counts vary with Galactic Latitude, and measure if dimming due to galactic extinction by dust acts as predicted. These results can be supplemented for fainter galaxies by data to be obtained from the MDM 2.4-meter telescope.

Color-Magnitude Diagram of the Pleiades cluster
By measuring the brightness and color of a cluster of stars at the same distance we can measure their age and composition, along with other details. This involves taking a few images of the brightest stars in the Pleiades from the Pupin rooftop with the equipment there, and can be supplemented for fainter star with the MDM 2.4-meter telescope.

Spectroscopic ages of Pleiades cluster stars, with Marcel Agueros
A star's age is one of its most fundamental parameters but notoriously difficult to measure accurately for individal star. Andrew Skumanich proposed that rotation and chromospheric activity, a proxy for magnetic field strength, decrease at roughly the same rate for stars like the Sun between the ages of 100 and 500 Myr. The existence of a potential relationship between age, rotation, and activity has generated hope that measurements of rotation or activity can be used to infer the ages of field (non-cluster) stars. Unfortunately, the theoretical underpinnings of the age-rotation and age-activity relations are still poorly understood, and empirical measurements of these relations are somewhat limited. As a result, astronomers still struggle to answer that simplest of questions: how old is that star? We have a campaign with MDM to obtain spectra for stars of known ages for which we have attempted to measure rotation periods. Measurements of the Halpha emission lines in these spectra will allow us to calibrate the age-activity and rotation-activity relations, thereby improving our stellar chronometers. This project involves two phases: (1) reducing and analyzing existing MDM 1.3-meter spectra for stars in the Pleiades open cluster (roughly 120 Myr old), and (2) obtaining, reducing, and analyzing new MDM 2.4-meter spectra for Pleiades and Alpha Persei (35 Myr old) stars.

Mass-to-light ratio of different galaxy types
We learned four decades ago that galaxies are dominated by mass in a form we cannot see. This is expressed in the "mass-to-light" ratio, compared to that quantity for populations of stars. We can measure the mass by measuring the internal velocities of galaxies as a function of radius, primarily by the measuring the rotation velocity versus radius ("rotation curve"). The amount of light is measured from optical images, using different wavelengths to estimate the loss due to extinction of light by dust. This require images and spectra to be obtained at the MDM 2.4-meter telescope.

Variable Star Projects, with Joe Patterson
We receive photometric data daily from telescopes scattered around the world (the Center for Backyard Astrophysics: Most of this is time-series photometry -- typically 30 s integrations, all night long, of cataclysmic variables and other close binary stars. The over-arching goal is an understanding of how close binaries end their lives. But in the process we have been drawn into some other fascinating byways: (1) Precession of accretion disks. We now find nearly all of the world's supply of such things (about 100), and measure their periods; (2) Measurement of orbital period changes in close binaries. In most cases these are too small to measure in a few decades. But in the stars of highest mass loss rate, the supersoft binaries, we do measure them -- and hence obtain a direct measure of their evolution rate; (3) Discovery of accreting, rapidly rotating magnetic white dwarfs -- the DQ Herculis stars; (4) Discovery of "period bouncers" -- the oldest cataclysmic variable stars in the Galaxy; (5) Measurement of periods and masses in the AM CVn stars: double white dwarfs where the more massive is cannibalizing the less massive. With orbital periods of ~20 min and likely powered purely by gravitational radiation, these binaries are likely to be strongly detected by the coming generation of gravitational-wave detectors. They may become the principal calibrators of these detectors.

Optical Imaging and Spectroscopy of Cataclysmic Variable Stars and Other Compact Objects, with Jules Halpern
This involves existing images and spectra plus more data to be collected from the MDM 2.4-meter. We need to consult with Prof. Halpern for a better description.

Radio Interferometric Mapping, with Jacqueline van Gorkom
This involves existing radio telescope array data. We need to consult with Prof. van Gorkom for a better description.

X-Ray Imaging and Spectroscopy of Active Galactic Nuclei and Other Extragalactic Objects, with Frits Paerels
This involves existing X-ray satellite telescope data. We need to consult with Prof. Paerels for a better description.

2012 September 10