Detection of Low Mass-ratio Stellar Binary Systems

O- and B-type stars are often found in binary systems, but the low binary mass-ratio regime is relatively unexplored due to observational difficulties. Binary systems with low mass-ratios may have formed through fragmentation of the circumstellar disk rather than molecular cloud core fragmen- tation. We describe a new technique sensitive to G- and K-type companions to early B stars, a mass-ratio of roughly 0.1, using high-resolution, high signal-to-noise spectra. We apply this technique to a sample of archived VLT/CRIRES observations of nearby B-stars in the CO bandhead near 2300 nm. While there are no unambiguous binary detections in our sample, we identify HIP 92855 and HIP 26713 as binary candidates warranting follow-up observations. We use our non-detections to determine upper limits to the frequency of FGK stars orbiting early B-type primaries.Comment: 13 pages, 7 figures Accepted to the Astrophysical Journa

Correcting For Telluric Absorption: Methods, Case Studies, And Release Of The TelFit Code

Ground-based astronomical spectra are contaminated by the Earth's atmosphere to varying degrees in all spectral regions. We present a Python code that can accurately fit a model to the telluric absorption spectrum present in astronomical data, with residuals of similar to 3%-5% of the continuum for moderately strong lines. We demonstrate the quality of the correction by fitting the telluric spectrum in a nearly featureless A0V star, HIP 20264, as well as to a series of dwarf M star spectra near the 819 nm sodium doublet. We directly compare the results to an empirical telluric correction of HIP 20264 and find that our model-fitting procedure is at least as good and sometimes more accurate. The telluric correction code, which we make freely available to the astronomical community, can be used as a replacement for telluric standard star observations for many purposes.UT Austin Hutchinson fellowshipUniversity of TexasAstronom

Hot Stars With Cool Companions

Young intermediate-mass stars have become high-priority targets for direct-imaging planet searches following the recent discoveries of planets orbiting e.g. HR 8799 and Beta Pictoris. Close stellar companions to these stars can affect the formation and orbital evolution of any planets, and so a census of the multiplicity properties of nearby intermediate mass stars is needed. Additionally, the multiplicity can help constrain the important binary star formation physics. We report initial results from a spectroscopic survey of 400 nearby A- and B-type stars. We search for companions by cross-correlating high resolution and high signal-to-noise ratio echelle spectra of the targets stars against model spectra for F- to M-type stars. We have so far found 18 new candidate companions, and have detected the spectral lines of the secondary in 4 known spectroscopic binary systems. We present the distribution of mass-ratios for close companions, and find that it differs from the distribution for wide ($a < 100$ AU) intermediate-mass binaries, which may indicate a different formation mechanism for the two populations.Comment: Submitted as part of the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun Proceedings of Lowell Observatory (9-13 June 2014

Spectroscopic detection and characterization of extreme flux-ratio binary systems

Binary stars and higher-order multiple systems are a ubiquitous outcome of star formation, especially as the system mass increases. The companion mass-ratio distribution is a unique probe into the conditions of the collapsing cloud core and circumstellar disk(s) of the binary fragments. Inside a ~1000 AU the disks from the two forming stars can interact, and additionally companions can form directly through disk fragmentation. We might therefore expect the mass-ratio distribution of close companions to differ from that of wide companions. This prediction is difficult to test with intermediate-mass primary stars using traditional methods because the contrast ratios that would be required to detect low-mass companions at narrow working angles are not yet achievable. In this thesis, we present a spectroscopic method to detect and characterize close companions to a variety of stars. We demonstrate applications of the method to detection of stars and even planets around sun-like stars, and present the results of a survey searching for companions to A- and B-type stars. As part of the survey, we estimate the temperatures and surface gravity of most of the 341 sample stars, and derive their masses and ages. We additionally estimate the temperatures and masses of the 64 companions we find, 23 of which are new detections. We find that the mass-ratio distribution for our sample has a turnover near q ~0.3, in contrast to the scale-free power law that describes the widely separated binary systems. We take this characteristic scale as evidence that companions are accreting a significant of material through disk interactions as they form, and that the scale is largely set by the disk lifetime and the time at which the fragments form.Astronom

Revisiting rho 1 Cancri e: A New Mass Determination Of The Transiting super-Earth

We present a mass determination for the transiting super-Earth rho 1 Cancri e based on nearly 700 precise radial velocity (RV) measurements. This extensive RV data set consists of data collected by the McDonald Observatory planet search and published data from Lick and Keck observatories (Fischer et al. 2008). We obtained 212 RV measurements with the Tull Coude Spectrograph at the Harlan J. Smith 2.7 m Telescope and combined them with a new Doppler reduction of the 131 spectra that we have taken in 2003-2004 with the High-Resolution-Spectrograph (HRS) at the Hobby-Eberly Telescope (HET) for the original discovery of rho 1 Cancri e. Using this large data set we obtain a 5-planet Keplerian orbital solution for the system and measure an RV semi-amplitude of K = 6.29 +/- 0.21 m/s for rho 1 Cnc e and determine a mass of 8.37 +/- 0.38 M_Earth. The uncertainty in mass is thus less than 5%. This planet was previously found to transit its parent star (Winn et al. 2011, Demory et al. 2011), which allowed them to estimate its radius. Combined with the latest radius estimate from Gillon et al. (2012), we obtain a mean density of rho = 4.50 +/- 0.20 g/cm^3. The location of rho 1 Cnc e in the mass-radius diagram suggests that the planet contains a significant amount of volitales, possibly a water-rich envelope surrounding a rocky core.Comment: 16 pages, 5 figures, accepted for publication in the Astrophysical Journal (the 300+ RV measurements will be published as online tables or can be obtained from the author

Assessing the Readability of Medical Documents: A Ranking Approach

BACKGROUND: The use of electronic health record (EHR) systems with patient engagement capabilities, including viewing, downloading, and transmitting health information, has recently grown tremendously. However, using these resources to engage patients in managing their own health remains challenging due to the complex and technical nature of the EHR narratives. OBJECTIVE: Our objective was to develop a machine learning-based system to assess readability levels of complex documents such as EHR notes. METHODS: We collected difficulty ratings of EHR notes and Wikipedia articles using crowdsourcing from 90 readers. We built a supervised model to assess readability based on relative orders of text difficulty using both surface text features and word embeddings. We evaluated system performance using the Kendall coefficient of concordance against human ratings. RESULTS: Our system achieved significantly higher concordance (.734) with human annotators than did a baseline using the Flesch-Kincaid Grade Level, a widely adopted readability formula (.531). The improvement was also consistent across different disease topics. This method\u27s concordance with an individual human user\u27s ratings was also higher than the concordance between different human annotators (.658). CONCLUSIONS: We explored methods to automatically assess the readability levels of clinical narratives. Our ranking-based system using simple textual features and easy-to-learn word embeddings outperformed a widely used readability formula. Our ranking-based method can predict relative difficulties of medical documents. It is not constrained to a predefined set of readability levels, a common design in many machine learning-based systems. Furthermore, the feature set does not rely on complex processing of the documents. One potential application of our readability ranking is personalization, allowing patients to better accommodate their own background knowledge

Direct Spectral Detection: An Efficient Method to Detect and Characterize Binary Systems

Young, intermediate-mass stars are experiencing renewed interest as targets for direct-imaging planet searches. However, these types of stars are part of multiple systems more often than not. Close stellar companions affect the formation and orbital evolution of any planets, and the properties of the companions can help constrain the binary formation mechanism. Unfortunately, close com- panions are difficult and expensive to detect with imaging techniques. In this paper, we describe the direct spectral detection method wherein a high-resolution spectrum of the primary is cross-correlated against a template for a companion star. Variants of this method have previously been used to search for stellar, brown dwarf, and even planetary companions. We show that the direct spectral detection method can detect companions as late as M-type orbiting A0 or earlier primary stars in a single epoch on small-aperture telescopes. In addition to estimating the detection limits, we determine the sources of uncertainty in characterizing the companion temperature, and find that large systematic biases can exist. After calibrating the systematic biases with synthetic binary star observations, we apply the method to a sample of 34 known binary systems with an A- or B-type primary star. We detect nine total companions, including four of the five known companions with literature temperatures between 4000 K < T < 6000 K, the temperature range for which our method is optimized. We additionally characterize the companion for the first time in two previously single-lined binary systems and one binary identified with speckle interferometry. This method provides an inexpensive way to use small- aperture telescopes to detect binary companions with moderate mass-ratios, and is competitive with high-resolution imaging techniques inside ∼ 100 − 200 mas