148 research outputs found
Constructing A Flexible Likelihood Function For Spectroscopic Inference
We present a modular, extensible likelihood framework for spectroscopic
inference based on synthetic model spectra. The subtraction of an imperfect
model from a continuously sampled spectrum introduces covariance between
adjacent datapoints (pixels) into the residual spectrum. For the high
signal-to-noise data with large spectral range that is commonly employed in
stellar astrophysics, that covariant structure can lead to dramatically
underestimated parameter uncertainties (and, in some cases, biases). We
construct a likelihood function that accounts for the structure of the
covariance matrix, utilizing the machinery of Gaussian process kernels. This
framework specifically address the common problem of mismatches in model
spectral line strengths (with respect to data) due to intrinsic model
imperfections (e.g., in the atomic/molecular databases or opacity
prescriptions) by developing a novel local covariance kernel formalism that
identifies and self-consistently downweights pathological spectral line
"outliers." By fitting many spectra in a hierarchical manner, these local
kernels provide a mechanism to learn about and build data-driven corrections to
synthetic spectral libraries. An open-source software implementation of this
approach is available at http://iancze.github.io/Starfish, including a
sophisticated probabilistic scheme for spectral interpolation when using model
libraries that are sparsely sampled in the stellar parameters. We demonstrate
some salient features of the framework by fitting the high resolution -band
spectrum of WASP-14, an F5 dwarf with a transiting exoplanet, and the moderate
resolution -band spectrum of Gliese 51, an M5 field dwarf.Comment: Accepted to ApJ. Incorporated referees' comments. New figures 1, 8,
10, 12, and 14. Supplemental website: http://iancze.github.io/Starfish
A Disk-based Dynamical Mass Estimate for the Young Binary AK Sco
We present spatially and spectrally resolved Atacama Large
Millimeter/submillimeter Array (ALMA) observations of gas and dust in the disk
orbiting the pre-main sequence binary AK Sco. By forward-modeling the disk
velocity field traced by CO J=2-1 line emission, we infer the mass of the
central binary, , a new dynamical measurement
that is independent of stellar evolutionary models. Assuming the disk and
binary are co-planar within 2{\deg}, this disk-based binary mass
measurement is in excellent agreement with constraints from radial velocity
monitoring of the combined stellar spectra. These ALMA results are also
compared with the standard approach of estimating masses from the location of
the binary in the Hertzsprung-Russell diagram, using several common pre-main
sequence model grids. These models predict stellar masses that are marginally
consistent with our dynamical measurement (at ), but are
systematically high (by 10%). These same models consistently predict an
age of Myr for AK Sco, in line with its membership in the Upper
Centaurus-Lupus association but surprisingly old for it to still host a
gas-rich disk. As ALMA accumulates comparable data for large samples of
pre-main sequence stars, the methodology employed here to extract a dynamical
mass from the disk rotation curve should prove extraordinarily useful for
efforts to characterize the fundamental parameters of early stellar evolution.Comment: Accepted to The Astrophysical Journa
The Intermediate Luminosity Optical Transient SN 2010da: The Progenitor, Eruption and Aftermath of a Peculiar Supergiant High-mass X-ray Binary
We present optical spectroscopy, ultraviolet to infrared imaging and X-ray
observations of the intermediate luminosity optical transient (ILOT) SN 2010da
in NGC 300 (d=1.86 Mpc) spanning from -6 to +6 years relative to the time of
outburst in 2010. Based on the light curve and multi-epoch SEDs of SN 2010da,
we conclude that the progenitor of SN 2010da is a ~10-12 Msol yellow supergiant
possibly transitioning into a blue loop phase. During outburst, SN 2010da had a
peak absolute magnitude of M<-10.4 mag, dimmer than other ILOTs and supernova
impostors. We detect multi-component hydrogen Balmer, Paschen, and Ca II
emission lines in our high-resolution spectra, which indicate a dusty and
complex circumstellar environment. Since the 2010 eruption, the star has
brightened by a factor of ~5 and remains highly variable in the optical.
Furthermore, we detect SN 2010da in archival Swift and Chandra observations as
an ultraluminous X-ray source (L~6x10^{39} erg/s). We additionally attribute He
II 4686 Angstrom and coronal Fe emission lines in addition to a steady X-ray
luminosity of ~10^{37} erg/s to the presence of a compact companion.Comment: published; updated citations and other minor edit
BAFFLES: Bayesian Ages for Field Lower-mass Stars
Funding: R.D. acknowledges support from the Fonds de Recherche du QuĂ©bec. Supported by NSF grants AST-1411868 (E.L.N., B.M.), and AST-1518332 (R.J.D.R.). Supported by NASA grants NNX14AJ80G (E.L.N., B.M.), NNX15AC89G and NNX15AD95G (B.M., R.J.D.R.), 80NSSC17K0535 (B.M., E.L.N., R.J.D.R), and NASA Hubble Fellowship grant HST-HF2-51405.001-A (I.C.).Age is a fundamental parameter of stars, yet in many cases, ages of individual stars are presented without robust estimates of the uncertainty. We have developed a Bayesian framework, BAFFLES, to produce the age posterior for a star from its calcium emission strength (log(RâČHK)) or lithium abundance (Li EW) and B â V color. We empirically determine the likelihood functions for calcium and lithium as functions of age from literature measurements of stars in benchmark clusters with well-determined ages. We use a uniform prior on age, which reflects a uniform star formation rate. The age posteriors we derive for several test cases are consistent with literature ages found from other methods. BAFFLES represents a robust method to determine the age posterior probability distribution for any field star with 0.45 †B â V †0.9 and a measurement of RâČHK and/or 0.35 †B â V †1.9 and measured Li EW. We compile colors, RâČHK, and Li EW from over 2630 nearby field stars from the literature, and present the derived BAFFLES age posterior for each star.PostprintPeer reviewe
ALMA Measurements of Circumstellar Material in the GQ Lup System
We present ALMA observations of the GQ Lup system, a young Sun-like star with
a substellar mass companion in a wide-separation orbit. These observations of
870 m continuum and CO J=3-2 line emission with beam size
( AU) resolve the disk of dust and gas surrounding the primary star, GQ
Lup A, and provide deep limits on any circumplanetary disk surrounding the
companion, GQ Lup b. The circumprimary dust disk is compact with a FWHM of
AU, while the gas has a larger extent with a characteristic radius of
AU. By forward-modeling the velocity field of the circumprimary
disk based on the CO emission, we constrain the mass of GQ Lup A to be , where is a known distance, and
determine that we view the disk at an inclination angle of
and a position angle of . The
upper limit on the 870 m flux density of any circumplanetary
disk associated with GQ Lup b of mJy implies an upper limit on the dust
disk mass of for standard assumptions about optically thin
emission. We discuss proposed mechanisms for the formation of wide-separation
substellar companions given the non-detection of circumplanetary disks around
GQ Lup b and other similar systems.Comment: 11 pages, 4 figure
The Unusually Luminous Extragalactic Nova SN 2010U
We present observations of the unusual optical transient SN 2010U, including
spectra taken 1.03 days to 15.3 days after maximum light that identify it as a
fast and luminous Fe II type nova. Our multi-band light curve traces the fast
decline (t_2 = 3.5 days) from maximum light (M_V = -10.2 mag), placing SN 2010U
in the top 0.5% of the most luminous novae ever observed. We find typical
ejecta velocities of approximately 1100 km/s and that SN 2010U shares many
spectral and photometric characteristics with two other fast and luminous Fe II
type novae, including Nova LMC 1991 and M31N-2007-11d. For the extreme
luminosity of this nova, the maximum magnitude vs. rate of decline relationship
indicates a massive white dwarf progenitor with a low pre-outburst accretion
rate. However, this prediction is in conflict with emerging theories of nova
populations, which predict that luminous novae from massive white dwarfs should
preferentially exhibit an alternate spectral type (He/N) near maximum light.Comment: 16 pages, 16 figures. Submitted to the Astrophysical Journa
Detecting weak spectral lines in interferometric data through matched filtering
Funding: R.A.L. and J.H. gratefully acknowledge funding from National Science Foundation Graduate Research Fellowships (Grant No. DGE-1144152). R.A.L. also acknowledges funding from the NRAO Student Observing Support Program. K.I.Ă. acknowledges funding from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation. C.W. acknowledges financial support from the Netherlands Organisation for Scientific Research (NWO, grant 639.041.335) and start-up funds from the University of Leeds, UK.Modern radio interferometers enable observations of spectral lines with unprecedented spatial resolution and sensitivity. In spite of these technical advances, many lines of interest are still at best weakly detected and therefore necessitate detection and analysis techniques specialized for the low signal-to-noise ratio (S/N) regime. Matched filters can leverage knowledge of the source structure and kinematics to increase sensitivity of spectral line observations. Application of the filter in the native Fourier domain improves S/N while simultaneously avoiding the computational cost and ambiguities associated with imaging, making matched filtering a fast and robust method for weak spectral line detection. We demonstrate how an approximate matched filter can be constructed from a previously observed line or from a model of the source, and we show how this filter can be used to robustly infer a detection significance for weak spectral lines. When applied to ALMA Cycle 2 observations of CH3OH in the protoplanetary disk around TW Hya, the technique yields a â53% S/N boost over aperture-based spectral extraction methods, and we show that an even higher boost will be achieved for observations at higher spatial resolution. A Python-based open-source implementation of this technique is available under the MIT license at http://github.com/AstroChem/VISIBLE.Publisher PDFPeer reviewe
A coplanar circumbinary protoplanetary disk in the TWA 3 triple M dwarf system
Funding: This project has received funding from the European Unionâs Horizon 2020 research and innovation program under the Marie SkĆodowska-Curie grant agreement No 210021. E.C. acknowledges NASA grants 80NSSC19K0506 and NNX15AD95G/NEXSS.We present sensitive ALMA observations of TWA 3, a nearby, young (âŒ10 Myr) hierarchical system composed of three pre-main-sequence M3âM4.5 stars. For the first time, we detected 12CO and 13CO J = 2â1 emissions from the circumbinary protoplanetary disk around TWA 3A. We jointly fit the protoplanetary disk velocity field, stellar astrometric positions, and stellar radial velocities to infer the architecture of the system. The Aa and Ab stars (0.29 ± 0.01 Mâ and 0.24 ± 0.01 Mâ, respectively) comprising the tight (P = 35 days) eccentric (e = 0.63 ± 0.01) spectroscopic binary are coplanar with their circumbinary disk (misalignment <6° with 68% confidence), similar to other short-period binary systems. From models of the spectral energy distribution, we found the inner radius of the circumbinary disk (rinner = 0.50â0.75 au) to be consistent with theoretical predictions of dynamical truncation rcav/ainner â 3. The outer orbit of the tertiary star B (0.40 ± 0.28 Mâ, a ⌠65 ± 18 au, e = 0.3 ± 0.2) is not as well constrained as the inner orbit; however, orbits coplanar with the A system are still preferred (misalignment < 20°). To better understand the influence of the B orbit on the TWA 3A circumbinary disk, we performed SPH simulations of the system and found that the outer edge of the gas disk (router = 8.5 ± 0.2 au) is most consistent with truncation from a coplanar, circular, or moderately eccentric orbit, supporting the preference from the joint orbital fit.Publisher PDFPeer reviewe
A coplanar circumbinary protoplanetary disk in the TWA 3 triple M dwarf system
We present sensitive ALMA observations of TWA 3, a nearby, young (10
Myr) hierarchical system composed of three pre-main sequence M3--M4.5 stars.
For the first time, we detected CO and CO =2-1 emission
from the circumbinary protoplanetary disk around TWA 3A. We jointly fit the
protoplanetary disk velocity field, stellar astrometric positions, and stellar
radial velocities to infer the architecture of the system. The Aa and Ab stars
( and , respectively) comprising
the tight ( days) eccentric () spectroscopic binary are
coplanar with their circumbinary disk (misalignment with 68%
confidence), similar to other short-period binary systems. From models of the
spectral energy distribution, we found the inner radius of the circumbinary
disk ( au) to be consistent with theoretical
predictions of dynamical truncation . The outer orbit of the tertiary star B (, au, ) is not as well constrained as the inner orbit,
however, orbits coplanar with the A system are still preferred (misalignment ). To better understand the influence of the B orbit on the TWA 3A
circumbinary disk, we performed SPH simulations of the system and found that
the outer edge of the gas disk ( au) is most
consistent with truncation from a coplanar, circular or moderately eccentric
orbit, supporting the preference from the joint orbital fit.Comment: 19 pages, 9 figures, accepted to Ap
Disentangling Time-series Spectra with Gaussian Processes: Applications to Radial Velocity Analysis
Measurements of radial velocity variations from the spectroscopic monitoring of stars and their companions are essential for a broad swath of astrophysics; these measurements provide access to the fundamental physical properties that dictate all phases of stellar evolution and facilitate the quantitative study of planetary systems. The conversion of those measurements into both constraints on the orbital architecture and individual component spectra can be a serious challenge, however, especially for extreme flux ratio systems and observations with relatively low sensitivity. Gaussian processes define sampling distributions of flexible, continuous functions that are well-motivated for modeling stellar spectra, enabling proficient searches for companion lines in time-series spectra. We introduce a new technique for spectral disentangling, where the posterior distributions of the orbital parameters and intrinsic, rest-frame stellar spectra are explored simultaneously without needing to invoke cross-correlation templates. To demonstrate its potential, this technique is deployed on red-optical time-series spectra of the mid-M-dwarf binary LP661-13. We report orbital parameters with improved precision compared to traditional radial velocity analysis and successfully reconstruct the primary and secondary spectra. We discuss potential applications for other stellar and exoplanet radial velocity techniques and extensions to time-variable spectra. The code used in this analysis is freely available as an open-source Python package
- âŠ