arXiv subjects

D. R. Ciardi

Publications and source records attributed to D. R. Ciardi.

68 records · Page 4Linked to original sources

arXiv2008

The NStED Exoplanet Transit Survey Service

The NASA Star and Exoplanet Database (NStED) is a general purpose stellar archive with the aim of providing support for NASA's planet finding and characterization goals, stellar astrophysics, and the planning of NASA and other space missions. There are two principal components of NStED: a database of (currently) 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high-precision photometric surveys for transiting exoplanets. We present a summary of the latter component: the NStED Exoplanet Transit Survey Service (NStED-ETSS), along with its content, functionality, tools, and user interface. NStED-ETSS currently serves data from the TrES Survey of the Kepler Field as well as dedicated photometric surveys of four stellar clusters. NStED-ETSS aims to serve both the surveys and the broader astronomical community by archiving these data and making them available in a homogeneous format. Examples of usability of ETSS include investigation of any time-variable phenomena in data sets not studied by the original survey team, application of different techniques or algorithms for planet transit detections, combination of data from different surveys for given objects, statistical studies, etc. NStED-ETSS can be accessed at \tt{http://nsted.ipac.caltech.edu}

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arXiv2007

The Palomar Testbed Interferometer Calibrator Catalog

The Palomar Testbed Interferometer (PTI) archive of observations between 1998 and 2005 is examined for objects appropriate for calibration of optical long-baseline interferometer observations - stars that are predictably point-like and single. Approximately 1,400 nights of data on 1,800 objects were examined for this investigation. We compare those observations to an intensively studied object that is a suitable calibrator, HD217014, and statistically compare each candidate calibrator to that object by computing both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis is that the frequency distribution of visibility data associated with calibrator stars differs from non-calibrator stars such as binary stars. Spectroscopic binaries resolved by PTI, objects known to be unsuitable for calibrator use, are similarly tested to establish detection limits of this approach. From this investigation, we find more than 350 observed stars suitable for use as calibrators (with an additional $\approx 140$ being rejected), corresponding to $\gtrsim 95%$ sky coverage for PTI. This approach is noteworthy in that it rigorously establishes calibration sources through a traceable, empirical methodology, leveraging the predictions of spectral energy distribution modeling but also verifying it with the rich body of PTI's on-sky observations.

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arXiv2007

Spitzer Space Telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars

We present Spitzer Space Telescope photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s photometry to construct the spectral energy distributions of these systems from the near- to mid-IR (1.235 - 8 microns). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modeled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 micron excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 micron flux density. While both model components can generate enough flux at 8 microns, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well-sampled to constrain such a complicated model. If the 8 micron flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.

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arXiv2005

First Results from the CHARA Array. III. Oblateness, Rotational Velocity and Gravity Darkening of Alderamin

We present observations of the A7IV-V star Alderamin ($α$ Cep, HR 8162, HD 203280) from the Georgia State University CHARA Array. These infrared interferometric angular size measurements indicate a non-circular projected disk brightness distribution for this known rapid rotator. The interferometric observations are modeled as arising from an elongated rigid atmosphere, with apparent polar and equatorial radii of $r_p=0.6753^{+0.0119}_{-0.0135}$ milliarcseconds (mas) and $r_e=0.8767^{+0.0293}_{-0.0183}$ mas, respectively, for a difference of $201\pm 32$ microarcseconds ($μ$as), and with an axial ratio of $r_e/r_p =1.298\pm 0.051$. Using the Hipparcos distance of $14.96\pm 0.11$ pc, these angular measures translate to $2.18\pm0.05$ and $2.82\pm0.10$ $R_\odot$. The inclination of Alderamin to the line of sight indicated by this modeling is effectively edge-on ($i=88.2^{+1.8}_{-13.3}$). The star has a true rotational velocity of $283\pm 10$ km/s ($\sim83$% of breakup velocity), and a polar temperature of roughly 8400 K. Significantly, a necessary aspect of this modeling is a determination of the gravity darkening coefficient, which at a value of $β=0.084^{+0.026}_{-0.049}$ is consistent with a convective photosphere, as expected for an A7IV-V star. Our detailed characterization of this object allows us to investigate various scenarios for the angular momentum history of Alderamin and the appropriateness of certain stellar evolution models.

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arXiv2005

Crystalline Silicate Emission in the Protostellar Binary Serpens--SVS20

We present spatially resolved mid-infrared spectroscopy of the class I/flat-spectrum protostellar binary system SVS20 in the Serpens cloud core. The spectra were obtained with the mid-infrared instrument T-ReCS on Gemini-South. SVS20-South, the more luminous of the two sources, exhibits a mid-infrared emission spectrum peaking near 11.3 \micron, while SVS20-North exhibits a shallow amorphous silicate absorption spectrum with a peak optical depth of $τ\sim 0.3$. After removal of the the line-of-sight extinction by the molecular common envelope, the ``protostar-only'' spectra are found to be dominated by strong amorphous olivine emission peaking near 10 \micron. We also find evidence for emission from crystalline forsterite and enstatite associated with both SVS20-S and SVS20-N. The presence of crystalline silicate in such a young binary system indicates that the grain processing found in more evolved HAeBe and T Tauri pre-main sequence stars likely begins at a relatively young evolutionary stage, while mass accretion is still ongoing.

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arXiv2005

Simultaneous X-ray and Optical Observations of EX Hydrae

The intermediate polar, EX Hydrae, was the object of a large simultaneous multiwavelength observational campaign during 2000 May - June. Here we present the Rossi X-ray Timing Explorer photometry and optical photometry and spectroscopy from ground-based observatories obtained as part of this campaign. Balmer line radial velocities and Doppler maps provide evidence for an extended bulge along the outer edge of the accretion disk and some form of extended/overflowing material originating from the hot spot. In addition, the optical binary eclipse possesses an extended egress shoulder, an indication that an additional source (other than the white dwarf) is coming out of eclipse. We also compare the X-ray and optical results with the results obtained from the EUV and UV observations from the multiwavelength data set.

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arXiv2004

Observations and modeling of the inner disk region of T Tauri stars

We present observations of four T Tauri stars using long baseline infrared interferometry from the Palomar Testbed Interferometer. The target sources, T Tau N, SU Aur, RY Tau and DR Tau, are all known to be surrounded by dusty circumstellar disks. The observations directly trace the inner regions (< 1 AU) of the disk and can be used to constrain the physical properties of this material. For three of the sources observed, the infrared emission is clearly resolved. We first use geometric models to characterize the emission region size, which ranges from 0.04 to 0.3 AU in radius. We then use Monte Carlo radiation transfer models of accretion disks to jointly model the spectral energy distribution and the interferometric observations with disk models including accretion and scattering. With these models, we are able to reproduce the data set with extended emission arising from structures larger than 10 milliarcseconds contributing less than 6% of the K band emission, consistent with there being little or no envelope remaining for these Class II sources (d log(lambda*F_lambda)/d log(lambda) ~ -2 to 0 in the infrared). The radiation transfer models have inner radii for the dust similar to the geometric models; however, for RY Tau emission from gas within the inner dust radius contributes significantly to the model flux and visibility at infrared wavelengths. The main conclusion of our modeling is that emission from inner gas disks (between the magnetic truncation radius and the dust destruction radius) can be a significant component in the inner disk flux for sources with large inner dust radii.

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arXiv2001

Simultaneous EUV and IR Observations of the Eclipsing Polar HU Aqr

We present simultaneous EUV and infrared (J,K) observations of the polar HU Aqr obtained during August 1998 when the star was in a high mass accretion state. EUV and IR light curves and EUV spectra are presented and compared with previous observations. The accretion region on the white dwarf has increased in temperature (124,000K to 240,000K) and radius (0.04 R$_{WD}$ to 0.06 R$_{WD} $) compared with previous EUV observations made during low mass accretion states. The EUV and IR photometric observations are shown to have a similar appearance as a function o f orbital phase. The EUV photometry shows rapid changes and provides evidence for mass accretion via blobs. The high state IR light curves present an asymmetric double-humped shape with J=14.8 and K=14.1. We applied an ellipsoidal model fit to the observations and the result indicates that the cause of the modulated shape is both due to ellipsoidal variations from the Roche Lobe filling secondary star and a complex flux combination probably dominated at all orbital phases by cyclotron emission. The source of maximum cyclotron emission appears to be in the accretion column high above the white dwarf surface.

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arXiv2001

Molecular Excitation and Differential Gas-Phase Depletions in the IC 5146 Dark Cloud

We present a combined near-infrared and molecular-line study of 25' x 8' area in the Northern streamer of the IC 5146 cloud. Using the technique pioneered by Lada et al 1994, we construct a Gaussian smoothed map of the infrared extinction with the same resolution as the molecular line observations in order to examine correlations of integrated intensities and molecular abundances with extinction for C17O, C34S, and N2H+. We find that over a visual extinction range of 0 to 40 magnitudes, there is good evidence for the presence of differential gas-phase depletions in the densest portions of IC 5146. Both CO and CS exhibit a statistically significant (factor of ~3) abundance reduction near Av ~ 12 magnitudes while, in direct contrast, at the highest extinctions, Av > 10 magnitudes, N2H+ appears relatively undepleted. Moreover, for Av < 4 magnitudes there exists little or no N2H+. This pattern of depletions is consistent with the predictions of chemical theory.

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arXiv2000

Investigating the structure of the accretion disk in WZ Sge from multi-wave-band, time-resolved spectroscopic observations: Paper II

We present our second paper describing multi-wave-band, time-resolved spectroscopy of WZ Sge. We analyze the evolution of both optical and IR emission lines throughout the orbital period and find evidence, in the Balmer lines, for an optically thin accretion disk and an optically thick hot-spot. Optical and IR emission lines are used to compute radial velocity curves. Fits to our radial velocity measurements give an internally inconsistent set of values for K1, gamma, and the phase of red-to-blue crossing. We present a probable explanation for these discrepancies and provide evidence for similar behaviour in other short orbital period dwarf-novae. Selected optical and IR spectra are measured to determine the accretion disk radii. Values for the disk radii are found to be strongly dependent on the assumed WD mass and binary orbital inclination. However, the separation of the peaks in the optical emission line (i.e. an indication of the outer disk radius) has been found to be constant during all phases of the super-cycle period over the last 40 years.

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arXiv2000

Infrared interferometric observations of young stellar objects

We present infrared observations of four young stellar objects using the Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147 and SU Aur, the 2.2 micron emission is resolved at PTI's nominal fringe spacing of 4 milliarcsec (mas), while the emission region of AB Aur is over-resolved on this scale. We fit the observations with simple circumstellar material distributions and compare our data to the predictions of accretion disk models inferred from spectral energy distributions. We find that the infrared emission region is tenths of AU in size for T Tau and SU Aur and ~1 AU for MWC 147.

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arXiv2000

Infrared spectroscopy of cataclysmic variables: III. Dwarf novae below the period gap and novalike variables

We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the novalike variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2-M6. The spectra of the dwarf novae are dominated by emission lines of HI and HeI. The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths which are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10-30% for the contribution of the secondary star to the observed K-band flux. In conjunction with the K-band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems. The spectra of the novalike variables are dominated by broad, single-peaked emission lines of HI and HeI - even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits NaI, CaI and 12CO absorption features consistent with a M0V secondary contributing 65% of the observed K-band flux, we find no evidence for the secondary star in any of the novalike variables. The implications of this result are discussed.

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arXiv1999

J-band spectroscopy of cataclysmic variables

We present time resolved J-band (1.025 - 1.340 micron) infrared spectra of the short-period dwarf novae (DNe) WZ Sge, VY Aqr and single spectra of the short-period DN EF Peg and the novalike variable PX And. There is some evidence in the spectra of VY Aqr and EF Peg that we have detected the secondary star, both in the continuum slope and also through the possible presence of spectral features. The spectra of WZ Sge and PX And, on the other hand, show no evidence for the secondary star, with upper limits for its contribution to the J-band light of 10% and 20% respectively. The spectral type of the secondary in WZ Sge is constrained to be later than M7.5V. Using skew mapping we have been able to derive a value for the radial velocity semi-amplitude of the secondary star in VY Aqr of Kr = (320 +/- 70) km/s, which in conjunction with Kw from Thorstensen & Taylor (1997) gives a mass ratio of q = 0.15 +/- 0.04.

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arXiv1998

A New Cataclysmic Variable RX J0757.0+6306: Candidate for the Shortest Period Intermediate Polar

A new cataclysmic variable is identified as the optical counterpart of the faint and hard X-ray source RX J0757.0+6306 discovered during the ROSAT all-sky survey. Strong double-peaked emission lines bear evidence of an accretion disc via an S-wave which varies with a period of 81 +/- 5 min. We identify this period as the orbital period of the binary system. CCD photometry reveals an additional period of 8.52 +/- 0.15 min. which was stable over four nights. We suggest that RX J0757.0+6306 is possibly an intermediate polar, but we cannot exclude the possibility that it is a member of the SU UMa group of dwarf novae.

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