Research Interests

Some Cool On-Going Projects

A Novel Method to Find Young Planets Most of the planets orbiting stars other than our sun were identified by the small radial velocity shifts they induce on the star they orbit. Unfortunately, this technique does not work as well for young stars, which typically have lots of star spots that can give the impression of radial velocity shifts at optical wavelengths. To overcome this, John Bailey (a UAH undergraduate) and myself are using a novel method to minimize this "noise" by observing at infrared wavelengths where the contrast between the star spot and the photosphere is significantly diminished. This reduces the apparent radial velocity noise sufficiently to achieve ~50 m/s precisions, which would allow us to identify most known extra-solar planets if orbiting a young low mass star. We are using this technique primarily with NIRSPEC on Keck, but have also tried the experiment on Gemini (with collaborators Blake, Charbonneau, Doppmann) and have engineering time to try this with NASA's IRTF.

An image of the sun at visible wavelengths from July 20 2004 showing several star spots. Young stars have many more of these, and they are typically much bigger.

Determining Fundamental Properties of So-Called Protostars During the earliest stages of star formation, a young star is still deeply embedded within the dust and gas from which it is forming. This makes them very faint and difficult to observe and study. To overcome this we used the world's largest optical telescope, the Keck 10-m telescope, to observe these faint stars. To our surprise, we have found that many of these so called protostars resemble their older T Tauri counter-parts. Some may in fact be T Tauri stars that have been misclassified as protostars because of edge-on orientations or large line-of-sight extinctions (see White & Hillenbrand 2004; White et al 2007).

An image of a young star taken with the NICMOS camera aboard the Hubble Space Telescope. We suspect many so-called protostars are actually T Tauri stars that have been misclassified, in some cases because of an edge-on disk orientation.

... And What Powers Herbig-Haro Objects? Many young stars support very powerful jets that blast gaseous material away at one-half million miles per hour (ie. 200 km/s). The most dramatic of these jets are Herbig-Haro Objects (named after G. Herbig and G. Haro who independently discovered these structures in the early 1950s); they are regions of shocked gas caused by the jet. Although the mechanism that powers these jets is not well understood, it is generally believed to be the result of accretion from a circumstellar disk. The large mass outflow rates (suggesting a large mass accretion rage) and the embedded nature of the central star has led many astronomers to suggest that these are protostars in the main phase of mass accretion. However, very little is known of the basic stellar properties of these stars. Given our success in studying protostars in Taurus (see above), I completed an All Northern Sky Survey of Herbig-Haro Energy Sources to determine if they really are protostars, and to study the processes involved in the main accretion phase. New Spitzer Space Telescope observations may help us answer this!

An image of Herbig-Haro 34, obtained with the Hubble Space Telescope. The system is 1,500 light- years away and in the vicinity of the Orion Nebula, a nearby star birth region.

A Lithium-Depleted Classical T Tauri Star?!?! The most universally accepted criterion for establishing extreme stellar youth is a non-depleted Lithium abundance. We have identified the first instance of a lithium depleted young star that is still under-going accretion (ie a classical T Tauri star). This calls into quesion the usefulness of Lithium as litmus test for youth, and suggests that current memberships lists of star forming regions may be biased. In the case of this star, we suggest that the circumstellar accretion disk has survived for >10 million because of its close companion which tidally inhibits, though does not prevent accretion (see White & Hillenbrand 2005). Interferometric techniques which can spatially resolve circumbinary disks, in combination with spectroscopically determined orbits for these binary systems will allow us to test this hypothesis.

Portions of the Keck/HIRES spectrum of St 34 showing the strong, broad Ha emission profile (left panel) and 2 temperature sensitive region (right panels). The best fit synthetic spectroscopic binary, composed of 2 M3 dwarf stars, and the M3 T Tauri star TWA 8a are also shown for comparison. St 34 shows no lithium in its spectrum.

A Multiplicity Survey from Solar to Substellar Masses at 3 Million Years As it turns out, a little over half (57%) of stars like our sun (G stars) are in binary systems. This fraction decreases signficantly for lower mass stars (~35%; M dwarfs) and brown dwarfs (~20%). The declining frequency of binary stars could either be because low mass binaries are more easily disrupted (their gravitational binding energy is less) or because the form less often. To help distinguish between the possibilities, Andrea Ghez and I have conducted a multiplicity survey of very low mass stars and brown dwarfs shortly after their formation (which means there has been less time for binaries to be disrupted). This survey was done with the Keck Telescope using a technique called speckle imaging. The results, in combination with previous surveys at higher masses, allow us for the first time to investigate the binary fraction from solar to sub-stellar masses at a very young age. The similarities in both the binary frequency and binary star properties of young and old stars suggest that the lowest mass star form as binaries less often than higher mass stars.

An example of speckle imaging reconstruction (from Patience et al. 1998). The left fringe pattern displays the calibrated Fourier amplitudes, and the right fringe pattern displays the Fourier phases. Using an inverse Fourier transform, a diffraction-limited image of the binary is produced.

How Do Brown Dwarfs Form? ... is the wrong question that everyone is asking. There is no reason to expect the conditions necessary for dynamical collapse, fragmentation, etc. will have anything to do with the whether the central temperatures in the object formed will be large enough to fuse Hydrogen. The right question to ask is "How does the formation process depend upon mass?" Towards this end, Gabor Basri and I carried out a pioneer spectroscopic study of very low mass stars and brown dwarfs at T Tauri ages and found that most in fact form like solar mass stars, but have shorter disk lifetimes (White & Basri 2003). This is also supported by our discovery of several Class I-like brown dwarfs (White & Hillenbrand 2004). To extend the multiplicity properties to even lower mass than speckle imaging will allow (see above), Adam Kraus, a graduate student at Caltech, and I used the Hubble Space Telescope to image 34 T Tauri age brown dwarfs (Kraus et al. 2005, 2006). Jeff Burchfield (a UAH graduate student) and I are investigating the velocity dispersion as a function of mass in the Upper Scorpius OB Association (believe it or not, this simple experiment has never been extended to the substellar regime). Finally, in collaboration with Gail Schaeffer and Mike Simon at Stony Brook, we are surveying the lowest mass stars and brown dwarfs at millimeter wavelengths to search for circumstellar disks.

A brown dwarf binary in Upper Sco identified in the Hubble Space Telescope / Advanced Camera for Surveys image.

Spectra of Stars Observed with Spitzer Working with Lynne Hillenbrand (the Deputy PI on the FEPS Spitzer Legacy Program) and Jared Gabor (a Caltech undergraduate, now a graduate student at Arizona), we spectroscopically observed over 400 stars using the Echelle Spectrograph on the Palomar 60" telescope; most of these stars were observed with the Spitzer Space in search of warm circumstellar material. The stellar properties extractable from these observations (e.g. mass, surface gravity, rotation rate) permit a more accurate assessment of how circumstellar material evolves with time, which is important for understand how it may accretionally coagulate to form planets. The work also identified a handful of "yet unappreciated" nearby young stars (see White et al. 2007).

Some useful observing lists:

Spectral Type Standards
Dwarf Colors (F-M)