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

Nearby Young Stars and Young Moving Groups

The past two decades have seen dramatic progress in our knowledge of the population of stars of age $\lesssim$150 Myr that lie within $\sim$100 pc of the Sun. Most such stars are found in loose kinematic groups ("nearby young moving groups"; NYMGs). The proximity of NYMGs and their members facilitates studies of the X-ray properties of coeval groups of pre-main sequence (pre-MS) stars as well as of individual pre-MS systems. In this review, we focus on how NYMG X-ray studies provide unique insight into the early evolution of stellar magnetic activity, the X-ray signatures of accretion, and the irradiation and dissipation of protoplanetary disks by high-energy photons originating with their host pre-MS stars. We discuss the likely impacts of the next generation of X-ray observing facilities on these aspects of the study of NYMGs and their members.Comment: To appear in Springer's "Handbook of X-ray and Gamma-ray Astrophysics" (eds. A. Santangelo and C. Bambi), Section "The Sun, Stars & Planets" (eds. G. Micela & B. Stelzer

The ALMA Early Science View of FUor/EXor objects. IV. Misaligned Outflows in the Complex Star-forming Environment of V1647 Ori and McNeil's Nebula

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the star-forming environment surrounding V1647 Ori, an outbursting FUor/EXor pre-MS star. Dust continuum and the (J = 2 - 1) $^{12}$CO, $^{13}$CO, C$^{18}$O molecular emission lines were observed to characterize the V1647 Ori circumstellar disc and any large scale molecular features present. We detect continuum emission from the circumstellar disc and determine a radius r = 40 au, inclination i = 17$^{\circ}$$^{+6}_{-9}$ and total disc mass of M$_{\mathrm{disk}}$ of ~0.1 M$_{\odot}$. We do not identify any disc structures associated with nearby companions, massive planets or fragmentation. The molecular cloud environment surrounding V1647 Ori is both structured and complex. We confirm the presence of an excavated cavity north of V1647 Ori and have identified dense material at the base of the optical reflection nebula (McNeil's Nebula) that is actively shaping its surrounding environment. Two distinct outflows have been detected with dynamical ages of ~11,700 and 17,200 years. These outflows are misaligned suggesting disc precession over ~5500 years as a result of anisotropic accretion events is responsible. The collimated outflows exhibit velocities of ~2 km s$^{-1}$, similar in velocity to that of other FUor objects presented in this series but significantly slower than previous observations and model predictions. The V1647 Ori system is seemingly connected by an "arm" of material to a large unresolved structure located ~20$"$ to the west. The complex environment surrounding V1647 Ori suggests it is in the early stages of star formation which may relate to its classification as both an FUor and EXor type object.Comment: 18 pages, 14 figures, 4 tables; accepted for publication in MNRA

The ALMA Early Science View of FUor/EXor Objects - V. Continuum Disc Masses and Sizes

Low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion known as FUor and EXor outbursts. FUor objects are characterized by a sudden brightening of ∼5 mag at visible wavelengths within 1 yr and remain bright for decades. EXor objects have lower amplitude outbursts on shorter time-scales. Here we discuss a 1.3 mm Atacama Large Millimeter/submillimeter Array (ALMA) mini-survey of eight outbursting sources (three FUors, four EXors, and the borderline object V1647 Ori) in the Orion Molecular Cloud. While previous papers in this series discuss the remarkable molecular outflows observed in the three FUor objects and V1647 Ori, here we focus on the continuum data and the differences and similarities between the FUor and EXor populations. We find that FUor discs are significantly more massive (∼80–600 MJup) than the EXor objects (∼0.5–40 MJup). We also report that the EXor sources lack the prominent outflows seen in the FUor population. Even though our sample is small, the large differences in disc masses and outflow activity suggest that the two types of objects represent different evolutionary stages. The FUor sources seem to be rather compact (Rc \u3c 20–40 au) and to have a smaller characteristic radius for a given disc mass when compared to T Tauri stars. V1118 Ori, the only known close binary system in our sample, is shown to host a disc around each one of the stellar components. The disc around HBC 494 is asymmetric, hinting at a structure in the outer disc or the presence of a second disc