arXiv subjects

Todd J. Henry

Publications and source records attributed to Todd J. Henry.

At least 73 records · Page 4Linked to original sources

arXiv2013

The Solar Neighborhood XXIX: The Habitable Real Estate of our Nearest Stellar Neighbors

We use the sample of known stars and brown dwarfs within 5 pc of the Sun, supplemented with AFGK stars within 10 pc, to determine which stellar spectral types provide the most habitable real estate --- defined to be locations where liquid water could be present on Earth-like planets. Stellar temperatures and radii are determined by fitting model spectra to spatially resolved broad-band photometric energy distributions for stars in the sample. The locations of the HZ's are calculated using an empirical formula for planetary surface temperature and assuming the condition of liquid water, called here the empirical habitable zone, or EHZ. Systems that have dynamically disruptive companions, assuming a 5:1 separation ratios for primary/secondary pairs and either object and a planet, are considered not habitable. We then derive a simple formula to predict the location of the EHZ for main sequence stars based on V-K color. We consider EHZ widths as more useful measures of the habitable real estate around stars than areas because multiple planets are not expected to orbit stars at identical stellar distances. This EHZ provides a qualitative guide on where to expect the largest population of planets in the HZ of main sequence stars. M stars provide more EHZ real estate than other spectral types, possessing 36.5% of the habitable real estate en masse. K stars are second with 21.5%, while A, F, and G stars offer 18.5%, 6.9% and 16.6%, respectively. Our calculations show that three M dwarfs within 10 pc harbor planets in their EHZs. GJ 581 may have two planets (d with msini = 6.1 M_E; g with msini = 3.1 M_E), GJ 667 C has one (c with msini = 4.5 M_E), and GJ 876 has two (b with msini = 1.89 M_J and c with msini = 0.56 M_J). If Earth-like planets are as common around low mass stars as recent Kepler results suggest, M stars are the most likely place to find Earth-like planets in HZ's.

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arXiv2012

The Solar Neighborhood XXVIII: The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert Around M Dwarfs

We report on our analysis of HST/NICMOS snapshot high resolution images of 255 stars in 201 systems within ~10 parsecs of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas/pixel, NICMOS can easily resolve binaries with sub-arcsecond separations in the 19".5x19".5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0{-0.0}^{+3.5}% for L companions to M dwarfs in the separation range of 5 to 70 AU, and 2.3_{-0.7}^{+5.0}% for L and T companions to M dwarfs in the separation range of 10 to 70 AU. We also discuss trends in the color-magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called "brown dwarf desert" extends to binary systems with low mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

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arXiv2011

The Solar Neighborhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star

We present the results of a multi-technique investigation of the M4.5Ve flare star AP Col, which we discover to be the nearest pre-main-sequence star. These include astrometric data from the CTIO 0.9m, from which we derive a proper motion of 342.0+/-0.5 mas yr^-1, a trigonometric parallax of 119.21+/-0.98 mas (8.39+/-0.07 pc), and photometry and photometric variability at optical wavelengths. We also provide spectroscopic data, including radial velocity (22.4+/-0.3 km s^-1), lithium Equivalent Width (EW) (0.28+/-0.02 A), H-alpha EW (-6.0 to -35 A), {\it vsini} (11+/-1 km s^-1), and gravity indicators from the Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle spectrographs. The combined observations demonstrate that AP Col is the closer of only two known systems within 10 pc of the Sun younger than 100 Myr. Given its space motion and apparent age of 12-50 Myr, AP Col is likely a member of the recently proposed ~40 Myr old Argus/IC 2391 association.

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arXiv2011

The Solar Neighborhood XXVII: Discovery of New Proper Motion Stars with mu > 0.18 "/yr in the Southern Sky with 16.5 > R_59F > 18.0

Here we present 1584 new southern proper motion systems with mu > 0.18 "/yr and 16.5 > R_59F > 18.0. This search complements the six previous SuperCOSMOS-RECONS (SCR) proper motion searches of the southern sky for stars within the same proper motion range, but with R_59F < 16.5. As in previous papers, we present distance estimates for these systems and find that three systems are estimated to be within 25 pc, including one, SCR 1546-5534, possibly within the RECONS 10 pc horizon at 6.7 pc, making it the second nearest discovery of the searches. We find 97 white dwarf candidates with distance estimates between 10 and 120 pc, as well as 557 cool subdwarf candidates. The subdwarfs found in this paper make up nearly half of the subdwarf systems reported from our SCR searches, and are significantly redder than those discovered thus far. The SCR searches have now found 155 red dwarfs estimated to be within 25 pc, including 10 within 10 pc. In addition, 143 white dwarf candidates and 1155 cool subdwarf candidates have been discovered. The 1584 systems reported here augment the sample of 4724 systems previously discovered in our SCR searches, and imply that additional systems fainter than R_59F = 18.0 are yet to be discovered.

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arXiv2011

The Solar Neighborhood XXV: Discovery of New Proper Motion Stars with 0.40 "/yr > mu > 0.18 "/yr between Declinations -47 degrees and 00 degrees

We present 2817 new southern proper motion systems with 0.40 "/yr > mu > 0.18 "/yr and declination between -47 degrees and 00 degrees. This is a continuation of the SuperCOSMOS-RECONS (SCR) proper motion searches of the southern sky. We use the same photometric relations as previous searches to provide distance estimates based on the assumption that the objects are single main sequence stars. We find 79 new red dwarf systems predicted to be within 25 pc, including a few new components of previously known systems. Two systems - SCR 1731-2452 at 9.5 pc and SCR 1746-3214 at 9.9 pc - are anticipated to be within 10 pc. We also find 23 new white dwarf candidates with distance estimates of 15-66 pc, as well as 360 new red subdwarf candidates. With this search, we complete the SCR sweep of the southern sky for stars with mu > 0.18 "/yr and R_59F < 16.5, resulting in a total of 5042 objects in 4724 previously unreported proper motion systems. Here we provide selected comprehensive lists from our SCR proper motion search to date, including 152 red dwarf systems estimated to be within 25 pc (nine within 10 pc), 46 white dwarfs (ten within 25 pc), and 598 subdwarf candidates. The results of this search suggest that there are more nearby systems to be found at fainter magnitudes and lower proper motion limits than those probed so far.

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arXiv2011

The Solar Neighborhood XXIV. Parallax Results from the CTIOPI 0.9-m Program: Stars with $μ$ $\ge$ 1\farcs0 yr$^{-1}$ (MOTION Sample) and Subdwarfs

We present 41 trigonometric parallaxes of 37 stellar systems, most of which have proper motions greater than 1\farcs0 yr$^{-1}$. These are the first trigonometric parallaxes for 24 systems. Overall, there are 15 red dwarf systems and 22 red subdwarf systems in the sample. Five of the systems are multiples with directly detected companions, and we have discovered perturbations caused by unseen companions in two additional cases, the dwarf LHS 501 and the subdwarf LHS 440. The latter system may eventually provide important dynamical mass points on the subdwarf mass-luminosity relation. Two additional stars of note are LHS 272, the third closest M-type subdwarf at a distance of only 13.6 pc, and LHS 2734AB, a high velocity subdwarf binary with $V_{tan}>$ 700 km/sec, which likely exceeds the escape velocity of the Milky Way. We also report the first long term variability study of cool subdwarfs indicating that cool subdwarfs are less photometrically variable than their main sequence counterparts.

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arXiv2011

Astrophysical Parameters and Habitable Zone of the Exoplanet Hosting Star GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be $0.299 \pm 0.010 R_{\odot}$. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature $T_{\rm EFF}=3498 \pm 56$ K and its luminosity $L=0.01205 \pm 0.00024 L_{\odot}$. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

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arXiv2010

Nearby Motionless Stars

We present methods and preliminary results of a relatively novel search for nearby stars. The method relies on photometric distance estimates as its primary search criterion, thus distinguishing itself from proper motion-based searches that have produced the bulk of nearby star discoveries.

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arXiv2010

The Solar Neighborhood XXIII CCD Photometric Distance Estimates of SCR Targets -- 77 M Dwarf Systems within 25 Parsecs

We present CCD photometric distance estimates of 100 SCR (SuperCOSMOS RECONS) systems with $μ$ $\geq$ 0$\farcs$18/yr, 28 of which are new discoveries previously unpublished in this series of papers. These distances are estimated using a combination of new $VRI$ photometry acquired at CTIO and $JHK$ magnitudes extracted from 2MASS. The estimates are improvements over those determined using photographic plate $BRI$ magnitudes from SuperCOSMOS plus $JHK$, as presented in the original discovery papers. In total, 77 of the 100 systems investigated are predicted to be within 25 pc. If all 77 systems are confirmed to have $π$$_{trig}$ $\ge$ 40 milliarcseconds, this sample would represent a 23% increase in M dwarf systems nearer than 25 pc in the southern sky.

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arXiv2010

The Solar Neighborhood. XXII. Parallax Results from the CTIOPI 0.9m Program: Trigonometric Parallaxes of 64 Nearby Systems with 0\farcs5 $\leq μ\leq$ 1\farcs0 yr$^{-1}$ (SLOWMO sample)

We present trigonometric parallaxes of 64 stellar systems with proper motions between 0\farcs5 yr$^{-1}$ and 1\farcs0 yr$^{-1}$ from the ongoing RECONS (Research Consortium On Nearby Stars) parallax program at CTIO (the Cerro Tololo Interamerican Observatory). All of the systems are south of DEC $= +30$, and 58 had no previous trigonometric parallaxes. In addition to parallaxes for the systems, we present proper motions, Johnson-Kron-Cousins $VRI$ photometry, variability measurements, and spectral types. Nine of the systems are multiple; we present results for their components, three of which are new astrometric detections. Of the 64 systems, 56 are within 25 parsecs of the Sun and 52 of those are in the southern hemisphere, comprising 5.7\% of the total number of known southern 25 parsec systems.

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arXiv2010

UCAC3 Proper Motion Survey. I. Discovery of New Proper Motion Stars in UCAC3 with 0.40 "/yr > mu >= 0.18 "/yr between Declinations -90 deg and -47 deg

Presented here are 442 new proper motion stellar systems in the southern sky between declinations -$90\degr$ and -47$\degr$ with 0$\farcs$40 yr$^{-1}$ $>$ $μ$ $\ge$ 0$\farcs$18 yr$^{-1}$. These systems constitute a 25.3% increase in new systems for the same region of the sky covered by previous SuperCOSMOS RECONS (SCR) searches that used Schmidt plates as the primary source of discovery. Among the new systems are 25 multiples, plus an additional seven new common proper motion companions found to previously known primaries. All stars have been discovered using the third U.S. Naval Observatory (USNO) CCD Astrograph Catalog (UCAC3). A comparison of the UCAC3 proper motions to those from the Hipparcos, Tycho-2, Southern Proper Motion (SPM4), and SuperCOSMOS efforts is presented, and shows that UCAC3 provides similar values and precision to the first three surveys. The comparison between UCAC3 and SuperCOSMOS indicates that proper motions in RA are systematically shifted in the SuperCOSMOS data but are consistent in DEC data, while overall showing a significantly higher scatter. Distance estimates are derived for stars having SuperCOSMOS Sky Survey (SSS) $B_J$, $R_{59F}$, and $I_{IVN}$ plate magnitudes and Two-Micron All Sky Survey (2MASS) infrared photometry. We find 15 systems estimated to be within 25 pc, including UPM 1710-5300 our closest new discovery estimated at 13.5 pc. Such new discoveries suggest that more nearby stars are yet to be found in these slower proper motion regimes, indicating that more work is needed to develop a complete map of the solar neighborhood.

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arXiv2010

A Survey of Stellar Families: Multiplicity of Solar-Type Stars

We present the results of a comprehensive assessment of companions to solar-type stars. A sample of 454 stars, including the Sun, was selected from the Hipparcos catalog with π > 40 mas, σ_π/π < 0.05, 0.5 < B - V < 1.0 (~ F6-K3), and constrained by absolute magnitude and color to exclude evolved stars. New observational aspects of this work include surveys for (1) very close companions with long-baseline interferometry at the CHARA Array, (2) close companions with speckle interferometry, and (3) wide proper motion companions identified by blinking multi-epoch archival images. In addition, we include the results from extensive radial-velocity monitoring programs and evaluate companion information from various catalogs. The overall observed fractions of single, double, triple, and higher order systems are 56% \pm 2%, 33% \pm 2%, 8% \pm 1%, and 3% \pm 1%, respectively, counting all confirmed stellar and brown dwarf companions. Our completeness analysis indicates that only a few undiscovered companions remain in this well-studied sample, implying that the majority (54% \pm 2%) of solar-type stars are single, in contrast to the results of prior multiplicity studies. The orbital-period distribution of companions is unimodal and roughly log-normal with a peak of about 300 years. The period-eccentricity relation shows a roughly flat distribution beyond the expected circularization for periods below 12 days. The mass-ratio distribution shows a preference for like-mass pairs, which occur more frequently in relatively close pairs. The fraction of planet hosts among single, binary, and multiple systems are statistically indistinguishable, suggesting that planets are as likely to form around single stars as they are around components of binary or multiple systems with sufficiently wide separations.

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arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell

Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ~5 m/s are obtainable over long timescales, and precisions of better than 3 m/s can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.

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arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. II. The Proposed Giant Planet Orbiting VB10 Does Not Exist

We present high-precision relative radial velocities of the very low-mass star VB10 that were obtained over a time span of 0.61 yr as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the VLT with an ammonia gas cell. The typical internal precision of the measurements is 10 m/s. These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo and Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the ~1 km/s radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M_p > 3 M_Jup and the orbital period and inclination suggested by Pravdo and Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo and Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low-mass stars.

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arXiv2009

Astrometry - Challenging our Understanding of Stellar Structure and Evolution

Stellar mass plays a central role in our understanding of star formation and aging. Stellar astronomy is largely based on two maps, both dependent on mass, either indirectly or directly: the Hertzprung-Russell Diagram (HRD) and the Mass-Luminosity Relation (MLR). The extremes of both maps, while not terra incognita, are characterized by large uncertainties. A precise HRD requires precise distance obtained by direct measurement of parallax. A precise MLR requires precise measurement of binary orbital parameters, with the ultimate goal the critical test of theoretical stellar models. Such tests require mass accuracies of ~1%. Substantial improvement in both maps requires astrometry with microsecond of arc measurement precision. Why? First, the 'tops' of both stellar maps contain relatively rare objects, for which large populations are not found until the observing horizon reaches hundreds or thousands of parsecs. Second, the 'bottoms' and 'sides' of both maps contain stars, either intrinsically faint, or whose rarity guarantees great distance, hence apparent faintness. With an extensive collection of high accuracy masses that can only be provided by astrometry with microsecond of arc measurement precision, astronomers will be able to stress test theoretical models of stars at any mass and at every stage in their aging processes.

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arXiv2009

Ground-Based Astrometry 2010-2020

We discuss the opportunities for astronomical discovery via ground-based astrometry carried out in the United States during the 2010-2020 decade. We describe imminent scientific breakthroughs that can be achieved at both classic astrometric scales -- narrow angle astrometry done by individual groups and large A*Omega astrometry carried out by consortia. The two most compelling questions to be addressed are (1) What is the composition of the stellar and substellar population near the Sun? and (2) What are the shape, size, and mass of the Milky Way? We provide a short list of five recommendations that we believe will allow us to take best advantage of the intellectual and financial investments made for what some have called "The Decade of Astrometry." The most important recommendation is to provide the educational foundation required so that a new generation of astrometrists can make best use of the rich datasets that will arrive in the coming decade.

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arXiv2009

Cool Subdwarf Investigations II: Multiplicity

Cool subdwarfs of types K and M are the fainter counterparts of cool main sequence dwarfs that dominate the Galactic population. In this paper we present the results of an optical speckle survey of 62 confirmed cool subdwarf systems within 60 pc. We have resolved two new companions and confirmed two previously known companions with separations 0\farcs13 to 3\farcs29. After including previously known wide companions and all known spectroscopic binaries, we determine the multiplicity rate of cool subdwarfs to be 26$\pm$6%, which is somewhat lower than comparable main sequence stars, which have a multiplicity rate of 37$\pm$5%. We find that only 3% of the cool subdwarfs surveyed have companions within 10 AU, 3% have companions between 10 and 100 AU, and 14% have companions beyond 100 AU. The other 6% of cool subdwarfs are spectroscopic binaries. This is very different from K/M dwarfs that have most companions (13%) at separations closer than 10 AU. However, because a search for close binaries among a large sample of nearby cool subdwarfs remains elusive, it is not yet settled whether or not the multiplicity rates are significantly different. Nonetheless, several different observational results and theories pointing to a possible dearth of subdwarf multiples are discussed.

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arXiv2009

The Solar Neighborhood. XXI. Parallax Results from the CTIOPI 0.9m Program: 20 New Members of the 25 Parsec White Dwarf Sample

We present accurate trigonometric parallaxes for 20 new members of the 25 pc white dwarf sample as part of the DENSE project (Discovery and Evalution of Nearby Stellar Embers, http://www.DenseProject.com). Previously, there were a total of 112 white dwarf systems with trigonometric parallaxes placing them within 25 pc and of these, 99 have trigonometric parallaxes known to better than 10%. Thus, the 20 new members presented in this work represent a 20% increase in the number of white dwarfs accurately known to be within 25 pc. In addition, we present updated parallaxes for seven known white dwarfs within 10 pc that have been observed as part of the ASPENS initiative (Astrometric Search for Planets Encircling Nearby Stars) to monitor nearby southern red and white dwarfs for astrometric perturbations from unseen companions. Including a few white dwarf companions and white dwarfs beyond 25 pc, we present a total of 33 trigonometric parallaxes. We perform atmospheric modeling for white dwarfs to determine physical parameters (i.e., effective temperature, log g, mass, and white dwarf age). Finally, a new ZZ Ceti pulsating white dwarf was identified and revised constraints are placed on two mixed H/He atmosphere cool white dwarfs that display continuum absorption in the near-infrared.

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