arXiv subjects

Ann Marie Cody

Publications and source records attributed to Ann Marie Cody.

71 records · Page 4Linked to original sources

arXiv2015

CSI 2264: Probing the inner disks of AA Tau-like systems in NGC 2264

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 behavior 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 color 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 analyze 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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arXiv2015

CSI 2264: Characterizing Young Stars in NGC 2264 with Short-Duration, Periodic Flux Dips in their Light Curves

We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical {\em CoRoT} light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian co-rotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3<P<11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard "disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous {\em Spitzer} IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel-flow accretion columns arising near the inner disk wall.

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arXiv2014

Simulated Performance of Timescale Metrics for Aperiodic Light Curves

Aperiodic variability is a characteristic feature of young stars, massive stars, and active galactic nuclei. With the recent proliferation of time domain surveys, it is increasingly essential to develop methods to quantify and analyze aperiodic variability. We develop three timescale metrics that have been little used in astronomy -- Δm-Δt plots, peak-finding, and Gaussian process regression -- and present simulations comparing their effectiveness across a range of aperiodic light curve shapes, characteristic timescales, observing cadences, and signal to noise ratios. We find that Gaussian process regression is easily confused by noise and by irregular sampling, even when the model being fit reflects the process underlying the light curve, but that Δm-Δt plots and peak-finding can coarsely characterize timescales across a broad region of parameter space. We make public the software we used for our simulations, both in the spirit of open research and to allow others to carry out analogous simulations for their own observing programs.

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arXiv2014

A Pulsation Search Among Young Brown Dwarfs and Very Low Mass Stars

In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters $σ$ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the non-detection, we did uncover a rich set of variability behavior- both periodic and aperiodic- on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.

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arXiv2014

Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry

We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in order to directly probe the accretion process from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40% are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha, UV and IR excesses). The remaining non-accreting members define the (photospheric+chromospheric) reference UV emission level over which flux excess is detected and measured. We revise the membership status of cluster members based on UV accretion signatures and report a new population of 50 CTTS candidates. A large range of UV excess is measured for the CTTS population, varying from a few 0.1 to 3 mag. We convert these values to accretion luminosities and obtain mass accretion rates ranging from 1e-10 to 1e-7 Mo/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6sigma correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for upper limits, yields M_acc $\propto$ M^{1.4+/-0.3}. At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, much smaller than the observed spread. We suggest that a non-negligible age spread across the cluster may effectively contribute to the observed spread in accretion rates at a given mass. In addition, different accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-flow accretion) may be associated to different M_acc regimes.

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arXiv2014

CSI 2264: Simultaneous optical and infrared light curves of young disk-bearing stars in NGC 2264 with CoRoT and Spitzer-- evidence for multiple origins of variability

We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30-day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes, and assess the fractional representation within each class. The largest category (>20%) are optical "dippers" having discrete fading events lasting ~1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with H$α$ emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk.

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arXiv2014

CSI 2264: Characterizing Accretion-Burst Dominated Light Curves for Young Stars in NGC 2264

Based on more than four weeks of continuous high cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high quality, multi-wavelength light curves for young stellar objects (YSOs) whose optical variability is dominated by short duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief -- several hour to one day -- brightenings at optical and near-infrared (IR) wavelengths with amplitudes generally in the range 5-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a thirty day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u-g vs. g-r color-color diagram with the largest UV excesses. These stars also have large Halpha equivalent widths, and either centrally peaked, lumpy Halpha emission profiles or profiles with blue-shifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Amongst the stars with the largest UV excesses or largest Halpha equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

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arXiv2013

CoRoT 223992193: A new, low-mass, pre-main sequence eclipsing binary with evidence of a circumbinary disk

We present the discovery of CoRoT 223992193, a double-lined, detached eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the CoRoT space mission during a 23-day observation of the 3 Myr old NGC 2264 star-forming region. Using multi-epoch optical and near-IR follow-up spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William Herschel Telescope we obtain a full orbital solution and derive the fundamental parameters of both stars by modelling the light curve and radial velocity data. The orbit is circular and has a period of $3.8745745 \pm 0.0000014$ days. The masses and radii of the two stars are $0.67 \pm 0.01$ and $0.495 \pm 0.007$ $M_{\odot}$ and $1.30 \pm 0.04$ and $1.11 ~^{+0.04}_{-0.05}$ $R_{\odot}$, respectively. This system is a useful test of evolutionary models of young low-mass stars, as it lies in a region of parameter space where observational constraints are scarce; comparison with these models indicates an apparent age of $\sim$3.5-6 Myr. The systemic velocity is within $1σ$ of the cluster value which, along with the presence of lithium absorption, strongly indicates cluster membership. The CoRoT light curve also contains large-amplitude, rapidly evolving out-of-eclipse variations, which are difficult to explain using starspots alone. The system's spectral energy distribution reveals a mid-infrared excess, which we model as thermal emission from a small amount of dust located in the inner cavity of a circumbinary disk. In turn, this opens up the possibility that some of the out-of-eclipse variability could be due to occultations of the central stars by material located at the inner edge or in the central cavity of the circumbinary disk.

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arXiv2013

New Uses for the Kepler Telescope: A Survey of the Ecliptic Plane For Transiting Planets and Star Formation

With the loss of two reaction wheels, the period of Kepler's ultra-high precision photometric performance is at an end. Yet Kepler retains unique capabilities impossible to replicate from the ground or with existing or future space missions. This White Paper calls for the use of Kepler to conduct a survey in the ecliptic plane to search for planet transits around stars at high galactic latitudes and to study star forming regions to investigate physics of very young stars not studied by Kepler in its prime mission. Even with reduced photometric precision, Kepler's 1 m aperture will enable it to survey faint M stars to find ice giants and Super Earths in Habitable Zone orbits.

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arXiv2013

Precise High-Cadence Time Series of Five Variable Young Stars in Auriga with MOST

To explore young star variability on a large range of timescales, we have used the MOST satellite to obtain 24 days of continuous, sub-minute cadence, high-precision optical photometry on a field of classical and weak-lined T Tauri stars (TTS) in the Taurus-Auriga star formation complex. Observations of AB Aurigae, SU Aurigae, V396 Aurigae, V397 Aurigae, and HD 31305 reveal brightness fluctuations at the 1-10% level on timescales of hours to weeks. We have further assessed the variability properties with Fourier, wavelet, and autocorrelation techniques, identifying one significant period per star. We present spot models in an attempt to fit the periodicities, but find that we cannot fully account for the observed variability. Rather, all stars exhibit a mixture of periodic and aperiodic behavior, with the latter dominating stochastically on timescales less than several days. After removal of the main periodicity, periodograms for each light curve display power law trends consistent with those seen for other young accreting stars. Several of our targets exhibited unusual variability patterns not anticipated by prior studies, and we propose that this behavior originates with the circumstellar disks. The MOST observations underscore the need for investigation of TTS light variations on a wide range of timescales in order to elucidate the physical processes responsible; we provide guidelines for future time series observations.

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arXiv2011

Young brown dwarfs at high cadence: Warm Spitzer time series monitoring of very low mass Sigma Orionis cluster members

The continuous temporal coverage and high photometric precision afforded by space observatories has opened up new opportunities for the study of variability processes in young stellar cluster members. Of particular interest is the phenomenon of deuterium-burning pulsation in brown dwarfs and very-low-mass stars, whose existence on 1-4 hours timescales has been proposed but not yet borne out by observations. To investigate short-timescale variability in young, low-mass objects, we carried out high-precision, high-cadence time series monitoring with the Warm Spitzer mission on 14 low mass stars and brown dwarfs in the ~3 Myr Sigma Orionis cluster. The flux in many of our raw light curves is strongly correlated with sub-pixel position and can vary systematically as much as 10%. We present a new approach to disentangle true stellar variability from this "pixel-phase effect," which is more pronounced in Warm Spitzer observations as compared to the cryogenic mission. The light curves after correction reveal that most of the sample is devoid of variability down to the few-millimagnitude level, on the minute to day timescales probed. However, one exceptional brown dwarf displays erratic brightness changes at the 10-15% level, suggestive of variable obscuration by dusty material. The uninterrupted 24-hour datastream and sub-1% photometric precision enables limits on pulsation in the near-infrared. If this phenomenon is present in our light curves, then its amplitude must lie below 2-3 millimagnitudes. In addition, we present three field eclipsing binaries and one pulsator for which optical ground-based data is also available.

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arXiv2010

Precision photometric monitoring of very low mass Sigma Orionis cluster members: variability and rotation at a few Myr

We present high-precision photometry on 107 variable low-mass stars and brown dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band photometric monitoring within two fields, encompassing 153 confirmed or candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot. We are sensitive to brightness changes on time scales from 10 minutes to two weeks with amplitudes as low as 0.004 magnitudes, and find variability on these time scales in nearly 70% of cluster members. We identify both periodic and aperiodic modes of variability, as well as semi-periodic rapid fading events that are not accounted for by the standard explanations of rotational modulation of surface features or accretion. We have incorporated both optical and infrared color data to uncover trends in variability with mass and circumstellar disks. While the data confirm that the lowest-mass objects (M<0.2M_\odot) rotate more rapidly than the 0.2-0.5 M_\odot members, they do not support a direct connection between rotation rate and the presence of a disk. Finally, we speculate on the origin of irregular variability in cluster members with no evidence for disks or accretion.

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arXiv2009

Pulsation powered by deuterium burning in brown bwarfs and very-low-mass stars

Pulsation powered by deuterium burning in brown dwarfs and very low mass stars has been put forth (Palla & Baraffe 2005) as a novel probe of the interiors of these objects in the 1-15 Myr age range. Previous observations have hinted at variability on the expected timescales of a few hours, suggesting but not confirming that the phenomenon is at work in young brown dwarfs. We have recently carried out a dedicated campaign to search for this putative class of pulsators among known low-mass members of five young star clusters. Our survey achieves sensitivity to periodic oscillations with photometric amplitudes down to several millimagnitudes. We present the census of variability over timescales ranging from minutes to days and discuss the current prospects for pulsation as a tool in the study of young, objects near the substellar boundary. As a byproduct, this work provides new insights into the distribution of stellar rotation periods at young ages via the detection of variability due to cool surface spots.

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arXiv2008

Fundamental Properties of Low-Mass Stars and Brown Dwarfs

Precise measurements of the fundamental properties of low-mass stars and brown dwarfs are key to understanding the physics underlying their formation and evolution. While there has been great progress over the last decade in studying the bulk spectrophotometric properties of low-mass objects, direct determination of their masses, radii, and temperatures have been very sparse. Thus, theoretical predictions of low-mass evolution and ultracool atmospheres remain to be rigorously tested. The situation is alarming given that such models are widely used, from the determination of the low-mass end of the initial mass function to the characterization of exoplanets. An increasing number of mass, radius, and age determinations are placing critical constraints on the physics of low-mass objects. A wide variety of approaches are being pursued, including eclipsing binary studies, astrometric-spectroscopic orbital solutions, interferometry, and characterization of benchmark systems. In parallel, many more systems suitable for concerted study are now being found, thanks to new capabilities spanning both the very widest (all-sky surveys) and very narrowest (diffraction-limited adaptive optics) areas of the sky. This Cool Stars 15 splinter session highlighted the current successes and limitations of this rapidly growing area of precision astrophysics.

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arXiv2005

Chemical Composition in the Globular Cluster M71 from Keck/HIRES Spectra of Turn-Off Stars

We have made observations with the Keck I telescope and HIRES at a resolution of $\sim$45,000 of five nearly identical stars at the turn-off of the metal-rich globular cluster M 71. Our mean Fe abundance, [Fe/H]=-0.80 +-0.02, is in excellent agreement with previous cluster determinations from both giants and near-turnoff stars. There is no clear evidence for any star-to-star abundance differences or correlations in our sample. Abundance ratios of the Fe-peak elements (Cr, Ni) are similar to Fe. The turn-off stars in M71 have remarkably consistent enhancements of 0.2 - 0.3 dex in [Si/Fe], [Ca/Fe] and [Ti/Fe] -- like the red giants. Our [Mg/Fe] ratio is somewhat lower than that suggested by other studies. We compare our mean abundances for the five M 71 stars with field stars of similar [Fe/H] -- 8 with halo kinematics and 17 with disk kinematics. The abundances of the alpha-fusion products (Mg, Si, Ca, Ti) agree with both samples, but seem a closer match to the disk stars. The Mg abundance in M71 is at the lower edge of the disk and halo samples. The neutron-capture elements, Y and Ba, are enhanced relative to solar in the M71 turn-off stars. Our ratio [Ba/Fe] is similar to that of the halo field stars but a factor of two above that for the disk field stars. The important [Ba/Y] ratio is significantly lower than M71 giant values. The Na content of the M71 turn-off stars is remarkably similar to that in the disk field stars, but more than a factor of two higher than the halo field star sample. We find [Na/Fe] = +0.14 $\pm$0.04 with a spread less than half of that found in the red giants in M71. Excluding Mg, the lack of intracluster $α$-element variations (turn-off vs giants) suggests the polluting material arose in a more traditional $s$-process environment such as AGB stars.

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arXiv2004

Stellar Evolution with Enriched Surface Convection Zones I. General Effects of Planet Consumption

Abundance analyses of stars with planets have revealed that their metallicities are enhanced relative to field stars. Such a trend was originally suggested to be due to accretion of iron-rich planetary material. Based on this assumption, we have developed a stellar evolution code to model stars with non-uniform metallicity distributions. We have calculated ``polluted'' stellar evolution tracks for stars with M=0.9-1.2 M_sun. Our models encompass a range of initial metal content from Z=0.01 to 0.03, and include metallicity enhancements within the stellar convection zone corresponding to Delta-Z=0.005-0.03. We find that the primary effects of metal enhancement on stellar structure and evolution are expansion of the convection zone and downward shift of effective temperature. In addition, we have computed the surface metallicities expected for stars of different mass for fixed quantities of pollution; there appears to be no correlation with present observational data on the metallicities of stars known to harbor planets.

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arXiv2003

Observations of the High-redshift Galaxy B2 0902+343 at 92cm

We present 92cm observations of the high-redshift galaxy B2 0902+343 (z=3.40) with the Westerbork Synthesis Radio Telescope (WSRT). Twenty-one cm HI is detected in absorption, at a depth of 15.9 mJy (=~6σ) and rest frame velocity width 120 km/s. We also report a null detection of OH emission or absorption at the 1665/1667 MHz rest-frame transitions. Based on our spectral sensitivity, we derive an upper limit on OH maser luminosity of log(L/Lsun)<5.2. In addition, we consider the possibility of absorption, and estimate a maximum column depth N(OH) of 1x10^15 cm^-2, corresponding to at most 2x10^5-9x10^6 Msun of OH in the galaxy. The implications of these results are discussed in the context of current efforts to extend the OH megamaser luminosity function and galactic merger rate to high redshift.

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