Engineering topics

Covey, K. R.

Publications and source records attributed to Covey, K. R..

4 records

DOE OSTI2021

2M17091769+3127589: A Mass-transfer Binary with an Extreme Mass Ratio

We present the orbital solution of a peculiar double-lined spectroscopic and eclipsing binary system, 2M17091769+3127589. This solution was obtained by a simultaneous fit of both APOGEE radial velocities and TESS and ASAS-SN light curves to determine masses and radii. This system consists of an M=0.256{sub -0.006}{sup +0.010} M {sub ⊙}, R=3.961{sub -0.032}{sup +0.049} R {sub ⊙} red giant and a hotter M=1.518{sub -0.031}{sup +0.057} M {sub ⊙}, R=2.608{sub -0.321}{sup +0.034} R {sub ⊙} subgiant. Modeling with the MESA evolutionary codes indicates that the system likely formed 5.26 Gyrs ago, with a M = 1.2 M {sub ⊙} primary that is now the system’s red giant and a M = 1.11 M {sub ⊙} secondary that is now a more massive subgiant. Due to Roche-lobe overflow as the primary ascends the red giant branch, the more evolved “primary” (i.e., originally the more massive star of the pair) is now only one sixth as massive as the “secondary.” Such a difference between the initial and the current mass ratio is one of the most extreme detected so far. Evolutionary modeling suggests the system is still engaged in mass transfer, at a rate of M-dot ~10{sup -9} M {sub ⊙}/yr, and it provides an example of a less evolved precursor to some of the systems that consist of white dwarfs and blue stragglers.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

A Differential Measurement of Circumstellar Extinction for AA Tau’s 2011 Dimming Event

AA Tau is a classical T Tauri star with a highly inclined, warped circumstellar disk. For decades, AA Tau exhibited photometric and spectroscopic variabilities that were successfully modeled as occultations of the primary star by circumstellar material. In 2011, AA Tau entered an extended faint state, presumably due to enhanced levels of circumstellar dust. We use two sets of contemporaneous optical–near-IR (NIR) spectra of AA Tau, obtained in 2008 and 2014 December, to directly measure the wavelength-dependent extinction associated with the dust enhancement driving AA Tau’s 2011 optical fade. Taken alone, AA Tau’s apparent optical–NIR increased extinction curve cannot be fit well with standard extinction laws. At optical wavelengths, AA Tau’s dimming event is consistent with predictions of common extinction models for an increase of A {sub V} = 2, but no such model reproduces AA Tau’s color–color excess at NIR wavelengths. We show that veiling emission accounts for the apparent anomalous NIR extinction: after including this veiling flux, AA Tau’s dimming event is consistent with a standard A {sub V} = 2 extinction law across the full optical–NIR range. We also report an increase in AA Tau’s mid-IR flux since its 2011 fade and suggest that an increase in the height of AA Tau’s inner disk is the most likely explanation for both the IR brightening and the additional extinction along the line of sight. In addition to informing our understanding of AA Tau, this analysis demonstrates that caution should be exercised when inferring extinction (and stellar parameters) from the NIR color–color excess of young stars with measurable NIR veiling fluxes.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2020

APOGEE Net: Improving the Derived Spectral Parameters for Young Stars through Deep Learning

Machine learning allows for efficient extraction of physical properties from stellar spectra that have been obtained by large surveys. The viability of machine-learning approaches has been demonstrated for spectra covering a variety of wavelengths and spectral resolutions, but most often for main-sequence (MS) or evolved stars, where reliable synthetic spectra provide labels and data for training. Spectral models of young stellar objects (YSOs) and low-mass MS stars are less well-matched to their empirical counterparts, however, posing barriers to previous approaches to classify spectra of such stars. In this work, we generate labels for YSOs and low-mass MS stars through their photometry. We then use these labels to train a deep convolutional neural network to predict logg, T {sub eff}, and Fe/H for stars with Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra in the DR14 data set. This “APOGEE Net” has produced reliable predictions of logg for YSOs, with uncertainties of within 0.1 dex and a good agreement with the structure indicated by pre-MS evolutionary tracks, and it correlates well with independently derived stellar radii. These values will be useful for studying pre-MS stellar populations to accurately diagnose membership and ages.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2013

Characterization of the Gaseous Companion k Andromedae B* New Keck and LBTI High-contrast Observations

Context. We previously reported the direct detection of a low mass companion at a projected separation of 55+/-2 astronomical units around the B9 type star kappa Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. Aims. We present new angular differential imaging (ADI) images of the system at 2.146 (K(sub s)), 3.776 (L'), 4.052 (NB 4.05) and 4.78 micrometers (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We aim to determine the near-infrared spectral energy distribution (SED) of the companion and use it to characterize the object. Methods. We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young/old objects and to a set of seven PHOENIX-based atmospheric models of cool objects accounting for the formation of dust. We used evolutionary models to derive mass estimates considering a wide range of plausible initial conditions. Finally, we used dedicated formation models to discuss the possible origin of the companion. Results. We derive a more accurate J = 15.86 +/- 0.21, H = 14.95 +/- 0.13, K(sub s) = 14.32 +/- 0.09 mag for kappa And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at K(sub s) and L' band. We derive NB 4.05 = 13.0 +/- 0.2 and M' = 13.3 +/- 0.3 mag and estimate Log(base 10)(L/solar luminosity) = −3.76 +/- 0.06. Atmospheric models yield T(sub eff) = 1900(+100/−200) K. They do not set constrains on the surface gravity. "Hot-start" evolutionary models predict masses of 14(+25/−2) Jupiter mass based on the luminosity and temperature estimates, and considering a conservative age range for the system (30(+120/−10) million years). "warm-start" evolutionary tracks constrain the mass to M greater than or equal to 11 Jupiter mass. Conclusions. The mass of kappa Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of kappa and b.

spectral energy distribution (SED)↗