arXiv subjects

Melissa McClure

Publications and source records attributed to Melissa McClure.

27 records · Page 2Linked to original sources

arXiv2018

The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very Low Accretors

We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with H$α$ equivalent widths consistent with low or absent accretion and various degrees of excess flux in the mid-infrared. We aim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and to probe the accretion flows onto the star using H$α$ and He I $λ$10830 in spectra obtained at the Magellan and SOAR telescopes. At the critical age of 5 Myr, the targets are at different stages of disk evolution. One of our targets is clearly accreting, as shown by redshifted absorption at free-fall velocities in the He I line and wide wings in H$α$; however, a marginal detection of FUV H$_2$ suggests that little gas is present in the inner disk, although the spectral energy distribution indicates that small dust still remains close to the star. Another target is surrounded by a transitional disk, with an inner cavity in which little sub-micron dust remains. Still, the inner disk shows substantial amounts of gas, accreting onto the star at a probably low, but uncertain rate. The third target lacks both a He I line or FUV emission, consistent with no accretion or inner gas disk; its very weak IR excess is consistent with a debris disk. Different processes occurring in targets with ages close to the disk dispersal time suggest that the end of accretion phase is reached in diverse ways.

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arXiv2018

The Orbit of the Companion to HD 100453A: Binary-Driven Spiral Arms in a Protoplanetary Disk

HD 100453AB is a 10+/-2 Myr old binary whose protoplanetary disk was recently revealed to host a global two-armed spiral structure. Given the relatively small projected separation of the binary (1.05", or ~108 au), gravitational perturbations by the binary seemed to be a likely driving force behind the formation of the spiral arms. However, the orbit of these stars remained poorly understood, which prevented a proper treatment of the dynamical influence of the companion on the disk. We observed HD 100453AB between 2015-2017 utilizing extreme adaptive optics systems on the Very Large Telescope and Magellan Clay Telescope. We combined the astrometry from these observations with published data to constrain the parameters of the binary's orbit to a=1.06"+/-0.09", e=0.17+/-0.07, and i=32.5+/- 6.5 degrees. We utilized publicly available ALMA CO data to constrain the inclination of the disk to i~28 degrees, which is relatively co-planar with the orbit of the companion and consistent with previous estimates from scattered light images. Finally, we input these constraints into hydrodynamical and radiative transfer simulations to model the structural evolution of the disk. We find that the spiral structure and truncation of the circumprimary disk in HD 100453 are consistent with a companion-dirven origin. Furthermore, we find that the primary star's rotation, its outer disk, and the companion exhibit roughly the same direction of angular momentum, and thus the system likely formed from the same parent body of material.

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arXiv2016

Mass measurements in protoplanetary disks from hydrogen deuteride

The total gas mass of a protoplanetary disk is a fundamental, but poorly determined, quantity. A new technique \citep{bergin+13} has been demonstrated to assess directly the bulk molecular gas reservoir of molecular hydrogen using the HD J=1-0 line at 112 $μ$m. In this work we present a {\it Herschel} Space Observatory\footnote{Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.} survey of six additional T Tauri disks in the HD line. Line emission is detected at $>$3$σ$ significance in two cases: DM Tau and GM Aur. For the other four disks, we establish upper limits to the line flux. Using detailed disk structure and ray tracing models, we calculate the temperature structure and dust mass from modeling the observed spectral energy distributions, and include the effect of UV gas heating to determine the amount of gas required to fit the HD line. The range of gas masses are 1.0-4.7$\times10^{-2}$ for DM Tau and 2.5-20.4$\times10^{-2}$ for GM Aur. These values are larger than those found using CO for GM Aur, while the CO-derived gas mass for DM Tau is consistent with the lower end of our mass range. This suggests a CO chemical depletion from the gas phase of up to a factor of five for DM Tau and up to two orders of magnitude for GM Aur. We discuss how future analysis can narrow the mass ranges further.

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arXiv2016

Direct Imaging Discovery of a Jovian Exoplanet Within a Triple Star System

Direct imaging allows for the detection and characterization of exoplanets via their thermal emission. We report the discovery via imaging of a young Jovian planet in a triple star system and characterize its atmospheric properties through near-infrared spectroscopy. The semi-major axis of the planet is closer relative to that of its hierarchical triple star system than for any known exoplanet within a stellar binary or triple, making HD 131399 dynamically unlike any other known system. The location of HD 131399Ab on a wide orbit in a triple system demonstrates that massive planets may be found on long and possibly unstable orbits in multi-star systems. HD 131399Ab is one of the lowest mass (4+/-1 MJup) and coldest (850+/-50 K) exoplanets to have been directly imaged.

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arXiv2016

A Herschel view of protoplanetary disks in the $σ$ Ori cluster

We present new Herschel PACS observations of 32 T Tauri stars in the young ($\sim$3 Myr) $σ$ Ori cluster. Most of our objects are K & M stars with large excesses at 24 $μ$m. We used irradiated accretion disk models of D'Alessio et al. (2006) to compare their spectral energy distributions with our observational data. We arrive at the following six conclusions. (i) The observed disks are consistent with irradiated accretion disks systems. (ii) Most of our objects (60%) can be explained by significant dust depletion from the upper disk layers. (iii) Similarly, 61% of our objects can be modeled with large disk sizes ($\rm R_{\rm d} \geq$ 100 AU). (iv) The masses of our disks range between 0.03 to 39 $\rm M_{Jup}$, where 35% of our objects have disk masses lower than 1 Jupiter. Although these are lower limits, high mass ($>$ 0.05 M$_{\odot}$) disks, which are present e.g, in Taurus, are missing. (v) By assuming a uniform distribution of objects around the brightest stars at the center of the cluster, we found that 80% of our disks are exposed to external FUV radiation of $300 \leq G_{0} \leq 1000$, which can be strong enough to photoevaporate the outer edges of the closer disks. (vi) Within 0.6 pc from $σ$ Ori we found forbidden emission lines of [NII] in the spectrum of one of our large disk (SO662), but no emission in any of our small ones. This suggests that this object may be an example of a photoevaporating disk.

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arXiv2014

The evolution of accretion in young stellar objects: Strong accretors at 3 - 10 Myr

While the rate of accretion onto T Tauri stars is predicted to decline with age, objects with strong accretion have been detected up to ages of 10 Myr. We analyze a sample of these old accretors identified by having a significant $U$ band excess and infrared emission from a circumstellar disk. Objects were selected from the ~3 Myr sigma Ori, 4-6 Myr Orion OB1b and 7-10 Myr Orion OB1a star forming associations. We use high resolution spectra from the Magellan Inamori Kyocera Echelle to estimate the veiling of absorption lines and calculate extinction for our T Tauri sample. We also use observations, obtained with the Magellan Echellette and in a few cases the SWIFT Ultraviolet and Optical Telescope, to estimate the excess produced in the accretion shock, which is then fit with accretion shock models to estimate the accretion rate. We find that even objects as old as 10 Myr may have high accretion rates, up to ~10^-8 msun/ yr. These objects cannot be explained by viscous evolution models, which would deplete the disk in shorter timescales, unless the initial disk mass is very high, a situation which is unstable. We show that the infrared spectral energy distribution of one object, CVSO 206, does not reveal evidence of significant dust evolution, which would be expected during the 10 Myr lifetime. We compare this object to predictions from photoevaporation and planet formation models and suggest that neither of these processes have had a strong impact on the disk of CVSO 206.

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arXiv2011

NUV Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), as well as optical and near infrared spectroscopic observations. The two stars have similar levels of near UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Halpha and He I 10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g. U band excess luminosity or the Ca II triplet line luminosity are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 x 10^-10 solar masses/ year for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near infrared, indicating that substantial dust remains. Fluorescent H_2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.

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arXiv2009

Far-Ultraviolet H2 Emission from Circumstellar Disks

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of \h2 to study the evolution of the gas in the inner disk. We find that the \h2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting \h2 in the disk, the fact that the non-accreting sources show no \h2 emission implies that the \h2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 Å, we estimate that the column density of \h2 left in the inner regions of the debris disks in our sample is less than ~ 3x10^-6 g cm^-2, nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.

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arXiv2007

Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer

Here we present the Spitzer IRS spectrum of CS Cha, a member of the ~2 Myr old Chamaeleon star-forming region, which reveals an optically thick circumstellar disk truncated at ~43 AU, the largest hole modeled in a transitional disk to date. Within this inner hole, ~5x10^-5 lunar masses of dust are located in a small optically thin inner region which extends from 0.1 to 1 AU. In addition, the disk of CS Cha has bigger grain sizes and more settling than the previously modeled transitional disks DM Tau, GM Aur, and CoKu Tau/4, suggesting that CS Cha is in a more advanced state of dust evolution. The Spitzer IRS spectrum also shows [Ne II] 12.81 micron fine-structure emission with a luminosity of 1.3x10^29 ergs s^-1, indicating that optically thin gas is present in this ~43 AU hole, in agreement with H_alpha measurements and a UV excess which indicate that CS Cha is still accreting 1.2x10^-8 M_sun yr^-1. We do not find a correlation of the [Ne II] flux with L_X, however, there is a possible correlation with mass accretion rate, which if confirmed would suggest that EUV fluxes due to accretion are the main agent for formation of the [Ne II] line.

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