Research at the Centre for Astrophysics:

Overview:
My research here at the Harvard Smithsonian is in the Optical and Infrared astronomy group.

I work with Tom Megeath, Scott Wolk and Lori Allen on the study of disks around low mass stars in young stellar clusters, observing in the Infrared, Optical, and X-ray regimes. This allows us to identify young stellar objects by their infrared excess (emission from warm disks) and elevated X-ray activity (young stars are more X-ray active than stars like the Sun.)

Some Detail:

The proposed research will utilise the capabilities of the Spitzer Space Telescope in conjunction with ground based near-IR and optical observations to study the properties of circumstellar disks in young stellar clusters. A statistical study will be undertaken to compare the various properties of these stars with the evolution of their disks. For the first time, the ages of the stars will be available to characterise the substantial evolution of the disk over the first 3 Myrs. This will lead to a better understanding of nebular disk dynamics and spatial structure, and the formation of planets both in our own Solar System and around other stars.

Stars are formed from clouds of gas and dust, and they remain surrounded by planet forming disks for about the first ten million years. Dust scatters and absorbs visible light, and then reemits it in the infrared. This makes young stars hard to study at optical wavelengths. Infrared observations allow study of young stellar clusters that are less than 3 Myr old and retain their circumstellar disks. The proposed research will utilise this capability to observe the early life cycle of stars: from their formation in the stellar nurseries of molecular clouds, to the evolution of the disk of dust surrounding them into early planetary systems.

Most stars form as part of a cluster and studies of their disks will reveal how they are affected by interactions with neighbouring stars in the cluster.

This research will further the study of stellar formation, disk evolution and the early stages of planetary formation.

In our recent paper , we present Spitzer and Chandra observations of the nearby (~260 pc) embedded stellar cluster in the Serpens Cloud Core.

We observed, using Spitzer's IRAC and MIPS instruments, in six wavelength bands from 3 to 70 um, to detect thermal emission from circumstellar disks and protostellar envelopes, and to classify stars using colour-colour diagrams and spectral energy distributions (SEDs). These data are combined with Chandra observations to examine the effects of circumstellar disks on stellar X-ray properties. Young diskless stars were also identified from their increased X-ray emission.

We have identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17 transition disk, and 21 class III stars; 60 of which exhibit X-ray emission.

Our primary results are the following:

1.) ten protostars detected previously in the sub-millimeter are detected at lambda < 24 um, seven at lambda < 8um,

2.) the protostars are more closely grouped than more evolved YSOs (median separation : ~0.024 pc), and

3.) the luminosity and temperature of the X-ray emitting plasma around these YSOs does not show any significant dependence on evolutionary class.

We combine the infrared derived values of AK and X-ray values of NH for 8 class III objects and find that the column density of hydrogen gas per magnitude of extinctions is less than half the standard interstellar value, for AK > 1. This may be the result of grain growth through coagulation and/or the accretion of volatiles in the Serpens cloud core.