* Astronomy

Eyes on the Sky: Christmas Tree Cluster (NGC 2264)

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Title: CSI 2264: Probing the inner disks of AA Tau-like systems in NGC 2264
Author: Pauline T. McGinnis, Silvia H. P. Alencar, Marcelo M. Guimaraes, Alana P. Sousa, John Stauffer, Jerome Bouvier, Luisa Rebull, Nathalia N. J. Fonseca, Laura Venuti, Lynne Hillenbrand, Ann Marie Cody, Paula S. Teixeira, Suzanne Aigrain, Fabio Favata, Gabor Furesz, Fred J. Vrba, Ettore Flaccomio, Neil Turner, Jorge Filipe Gameiro, Catherine Dougados, William Herbst, Maria Morales-Calderon, Giusi Micela

The classical T Tauri star AA Tau presented photometric variability attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined magnetosphere. Previous studies of NGC 2264 have shown that similar photometric behaviour is common among CTTS. 
The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their variability. We investigate veiling variability in their spectra and u-r colour variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We compare infrared and optical light curves to analyse the dust responsible for the occultations. AA Tau-like variability is transient on a timescale of a few years. We ascribe it to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains. 
The inner disk warp scenario is consistent with observations for all but one periodic star in our sample. AA Tau-like systems comprise 14% of CTTS observed in NGC 2264, though this increases to 35% among systems of mass 0.7M_sun<M<2.0M_sun. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp, possibly because of a change in magnetic field configurations among stars of lower mass.

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Title: Testing Disk-Locking in NGC 2264
Authors: P. Wilson Cauley, Christopher M. Johns-Krull, Catrina M. Hamilton, Kelly Lockhart

We test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model (Shu et al. 1994) which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age~1-2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess, finding good agreement. In addition, we use our accretion parameters to determine the relationship between accretion and H-beta luminosity, again finding good agreement with previously published results; however, we also find that care must be used when applying this relationship due to strong chromospheric emission in young stars which can lead to erroneous results in some cases.

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Title: Spitzer observations of NGC2264: The nature of the disk population
Authors: P. S. Teixeira, C. J. Lada, M. Marengo, E. A. Lada

NGC2264 is a young cluster with a rich circumstellar disk population which makes it an ideal target for studying the evolution of stellar clusters. Our goal is to study its star formation history and to analyse the primordial disk evolution of its members. The study presented is based on data obtained with Spitzer IRAC and MIPS, combined with deep NIR ground-based FLAMINGOS imaging and previously published optical data. We build NIR dust extinction maps of the molecular cloud associated with the cluster, and determine it to have a mass of 2.1x10^3Msun above an Av of 7mag. Using a differential K_s-band luminosity function of the cluster, we estimate the size of its population to be 1436±242 members. The star formation efficiency is ~25%. We identify the disk population: (i) optically thick inner disks, (ii) anaemic inner disks, and (iii) disks with inner holes, or transition disks. We analyse the spatial distribution of these sources and find that sources with thick disks segregate into sub-clusterings, whereas sources with anaemic disks do not. Furthermore, sources with anaemic disks are found to be unembedded (Av<3mag), whereas the clustered sources with thick disks are still embedded within the parental cloud. NGC2264 has undergone more than one star-forming event, where the anaemic and extincted thick disk population appear to have formed in separate episodes. We also find tentative evidence of triggered star-formation in the Fox Fur Nebula. In terms of disk evolution, our findings support the emerging disk evolution paradigm of two distinct evolutionary paths for primordial optically thick disks: a homologous one where the disk emission decreases uniformly at NIR and MIR wavelengths, and a radially differential one where the emission from the inner region of the disk decreases more rapidly than from the outer region (forming transition disks).

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