Multiwavelength Observations of Young Stars and Their Circumstellar Disks

Observational studies of star and planet formation represent a fundamental means to understand the origins of exoplanetary systems and our own solar system. In this dissertation I present a multiwavelength approach to study a wide range of stages of the evolution of protostars and pre-main sequence (pre-MS) stars. I have investigated, via an infrared and multi-epoch X-ray study of the L1630 dark cloud, whether and how X-ray variability in young stellar objects is related to protostellar evolutionary state. Among a subsample of 52 X-ray-emitting young stars in L1630, I find that the earliest stages of young stellar evolution show the highest levels of X-ray variability, indicative of high levels of magnetic activity during star formation. I also find evidence of active magnetospheric accretion in three L1630 stars with circumstellar disks. I also present multiwavelength analyses of several phenomenologically interesting young stars at different stages of pre-MS stellar evolution. I investigate, via contemporaneous X-ray and optical/IR observations, the nearby, pre-main sequence star/disk systems T Chamaeleontis (T Cha) and TWA 30A and 30B. I analyze X-ray and optical emission signatures of accretion and magnetic activity in T Cha, and infer the X-ray absorbing properties of its circumstellar disk. Like T Cha, each component of the wide binary TWA 30A and 30B is viewed through a highly inclined circumstellar disk. Both TWA 30A and 30B display large near-IR variability, suggestive of (respectively) variable obscuration of the stellar photosphere and a possible disk-rim warp. In the case of T Cha and TWA 30A, I find a correlation of optical/near-IR and X-ray extinction consistent with that of photospheric obscuration from an intervening disk clump

Evidence for Variable, Correlated X-ray and Optical/IR Extinction toward the Nearby, Pre-main Sequence Binary TWA 30

We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR spectroscopic observations of the nearby ($D \approx 42$ pc), low-mass (mid-M) binary system TWA 30A and 30B. The components of this wide (separation $\sim$3400 AU) binary are notable for their nearly edge-on disk viewing geometries, high levels of variability, and evidence for collimated stellar outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011 June and July, accompanied (respectively) by IRTF SpeX (near-IR) and VLT XSHOOTER (visible/near-IR) spectroscopy obtained within $\sim$20 hours of the X-ray observations. TWA 30A was detected in both XMM-Newton observations at relatively faint intrinsic X-ray luminosities ($L_{X}$$\sim$$8\times10^{27}$ $erg$ $s^{-1}$) compared to stars of similar mass and age . The intrinsic (0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor 20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected, and we infer an upper limit of ($L_{X}$ $\lesssim$ 3.0 $\times$ $10^{27}$ erg s$^{-1}$). We measured a large change in visual extinction toward TWA 30A (from $A_V \approx 14.9$ to $A_V \approx 4.7$) between the two 2011 observing epochs, and we find evidence for a corresponding significant decrease in X-ray absorbing column ($N_H$). The apparent correlated change in $A_V$ and $N_H$ is suggestive of variable obscuration of the stellar photosphere by disk material composed of both gas and dust. However, in both observations, the inferred $N_{H}$ to $A_{V}$ ratio is lower than that typical of the ISM, suggesting that the disk is either depleted of gas or is deficient in metals in the gas phase.Comment: 10 pages, 7 figures, Accepted for publication in MNRA

The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity

We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of HBC 515, a system consisting of multiple young stellar objects (YSOs). The five members of HBC 515 represent a remarkably diverse array of YSOs, ranging from the low-mass Class I/II protostar HBC 515B, through Class II and transition disk objects (HBC 515D and C, respectively), to the "diskless", intermediate- mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes that all five components are X-ray sources, with HBC 515A - a subarcsecond-separation binary that is partially resolved by Chandra - being the dominant X-ray source. We detect an X-ray flare associated with HBC 515B. In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission source; a second, weaker mm continuum source is coincident with the position of the transition disk object HBC 515C. These results strongly support the protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars" that are binaries and have shed their disks at very early stages of pre-MS evolution. The coexistence of two such disparate objects within a single, presumably coeval multiple YSO system highlights the influence of pre- MS star mass, binarity, and X-ray luminosity in regulating the lifetimes of circumstellar, planet-forming disks and the timescales of star-disk interactions.Comment: Accepted for publication in A&A; 11 pages, 5 figure

The ALMA Early Science View of FUor/EXor objects. III. The Slow and Wide Outflow of V883 Ori

We present Atacama Large Millimeter/ sub-millimeter Array (ALMA) observations of V883 Ori, an FU Ori object. We describe the molecular outflow and envelope of the system based on the $^{12}$CO and $^{13}$CO emissions, which together trace a bipolar molecular outflow. The C$^{18}$O emission traces the rotational motion of the circumstellar disk. From the $^{12}$CO blue-shifted emission, we estimate a wide opening angle of $\sim$ 150$^{^{\circ}}$ for the outflow cavities. Also, we find that the outflow is very slow (characteristic velocity of only 0.65 km~s$^{-1}$), which is unique for an FU Ori object. We calculate the kinematic properties of the outflow in the standard manner using the $^{12}$CO and $^{13}$CO emissions. In addition, we present a P Cygni profile observed in the high-resolution optical spectrum, evidence of a wind driven by the accretion and being the cause for the particular morphology of the outflows. We discuss the implications of our findings and the rise of these slow outflows during and/or after the formation of a rotationally supported disk.Comment: 12 pages, 7 figures, 2 tables. Accepte

Adsorption selectivity of CO2 over CH4, N2 and H2 in melamine-resorcinol-formaldehyde xerogels

Sorptive selectivity of melamine-resorcinol-formaldehyde xerogels, towards CO2, CH4, N2 and H2, is reported, where all systems demonstrate potential for selective adsorption of CO2 from corresponding binary gas mixtures. Selected gas mixtures represent important gas separation applications found in industry, i.e. CO2 removal from power plant flue gases (CO2-N2), sour gas sweetening (CO2-CH4), and separation of species in the water-gas shift reaction (CO2-H2). All materials tested exhibit microporous character, enhancing adsorption of small molecules, however, it is the inclusion of a nitrogen-rich material into the gel matrix that results in enhanced selectivities for these systems. Despite the porous character of the gels, under the test conditions used to simulate industrial parameters, all three balance gases, i.e. H2, N2 and CH4, showed low affinities for the xerogels, while CO2 adsorption was notably higher and increased with the inclusion and increased concentration of melamine. Ideal Adsorbed Solution Theory was used to demonstrate significant differences in adsorption uptake, especially for CO2-CH4, and high selectivities for CO2 over N2. In all cases, selected xerogels exhibited industrially relevant adsorption timescales for CO2 over competitor gases, demonstrating the potential of these materials for the selective adsorption of CO2 from process streams