arXiv subjects

J. E. Gizis

Publications and source records attributed to J. E. Gizis.

32 records · Page 2Linked to original sources

arXiv2002

Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs

Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for studying various color-color and color-absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late-M and L dwarfs. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late-L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections, and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have T_eff ~ 1360 K.

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arXiv2002

High resolution spectroscopy of ultracool M dwarfs

(abridged) We present high-resolution echelle spectroscopy of a photometricaly-selected sample if thirty-nine dwarfs with spectral types between M6.5 and L0.5. Two stars, 2MASSI 0253202+271333 and 2MASSW 0952219-192431, are double-lined spectroscopic binaries. We have used our observations to search for Li I 6708 A absorption, characteristic of sub-stellar mass; estimate the level of chromospheric activity through measurement of H-alpha emission fluxes; measure rotational velocities via line broadening; and determine radial velocities and Galactic space motions. Two dwarfs have strong lithium absorption, the previously-known brown dwarf, LP 944-20, and 2MASSI J0335020+234235, which we identify as a probable 0.06 M_sun brown dwarf, age ~1 Gyr. We have investigated the prospect of using the observed frequency of lithium absorption amongst ultracool M dwarfs (M7 to M9.5) as a probe of the initial mass function. The available observations are difficult to reconcile with Salpeter-like power-law mass functions (alpha > 2) for masses below 0.1M_Sun. A comparison between the rotational velocities and -alpha fluxes shows no evidence for significant correlation. Velocity dispersions are significantly lower than those measured for nearby M dwarfs, but show remarkable similarity to results for earlier-type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ~3 Gyrs.

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arXiv2002

The Palomar/MSU Nearby Star Spectroscopic Survey III: Chromospheric Activity, M-dwarf Ages and the Local Star Formation History

We present high-resolution echelle spectroscopy of 676 nearby M dwarfs. Our measurements include radial velocities, equivalent widths of important chromospheric emission lines, and rotational velocities for rapidly rotating stars. We identify several distinct groups by their H alpha properties, and investigate variations in chromospheric activity amongst early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limited sample together with a relationship between age and chromospheric activity, we show that the rate of star formation in the immediate Solar Neighbourhood has been relatively constant over the last 4 Gyr. In particular our results are inconsistent with recent large bursts of star formation. We use the correlation between H alpha activity and age as a function of colour to set constraints on the properties of L and T dwarf secondary components in binary systems. We also identify a number of interesting stars, including rapid rotators, radial velocity variables, and spectroscopic binaries.

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arXiv2001

Three Wide-Separation L dwarf Companions from the Two Micron All Sky Survey: Gl 337C, Gl 618.1B, and HD 89744B

We present two confirmed wide separation L-dwarf common proper motion companions to nearby stars and one candidate identified from the Two Micron All Sky Survey. Spectral types from optical spectroscopy are L0 V, L2.5 V, and L8 V. Near-infrared low resolution spectra of the companions are provided as well as a grid of known objects spanning M6 V -- T dwarfs to support spectral type assignment for these and future L-dwarfs in the z'JHK bands. Using published measurements, we estimate ages of the companions from physical properties of the primaries. These crude ages allow us to estimate companion masses using theoretical low-mass star and brown dwarf evolutionary models. The new L-dwarfs in this paper bring the number of known wide-binary (Separation >= 100 AU) L-dwarf companions of nearby stars to nine. One of the L-dwarfs is a wide separation companion to the F7 IV-V + extrasolar planet system HD89744Ab.

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arXiv2001

Three newly-discovered M-dwarf companions of Solar Neighbourhood stars

We present low-resolution spectroscopy of newly-discovered candidate companions to three stars in the Solar Neighbourhood. All three companions are M dwarfs, with spectral types ranging from M4 to M9.5. In two cases, G85-55`B' (M6) and G87-9`B' (M4), we have circumstantial evidence from spectroscopy, photometry and limited astrometry that the systems are physical binaries; in the third, G216-7B (M9.5), comparison of POSS II IIIaF plate material and the 2MASS image indicates common proper motion. The primary star in this system, G216-7A (M0), appears itself to be an unresolved, nearly equal-mass binary. All three low-mass companions are highly likely to be stellar in nature, although G216-7B lies very close to the hydrogen-burning limit.

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arXiv2000

Near-Infrared Spectral Classification of late-M and L Dwarfs

We present near-infrared (1 to 2.5 micron) low-resolution spectroscopy of eleven ultracool dwarfs with spectral types ranging from M7 to L8. Combining our observations with data published previously by Leggett et al. (2001), we have measured equivalent widths for the strongest atomic features and constructed narrowband indices to gauge the strength of the strongest molecular features. Those measurements show that the behaviourr at near-infrared wavelengths is well correlated with spectral type, where the latter is defined from observations between 6300 and 10000 Angstroms. In particular, four indices designed to measure absorption in the wings of the 1.4 and 1.85 micron steam bands exhibit a linear, monotonic correlation with spectral type on the Kirkpatrick et al. (1999) system, allowing consistent calibration from both optical and near-infrared spectroscopy.

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arXiv2000

CORMASS: A Compact and Efficient NIR Spectrograph for Studying Low-Mass Objects

CorMASS (Cornell Massachusetts Slit Spectrograph) is a compact, low-resolution (R=300), double-pass prism cross-dispersed near-infrared (NIR) spectrograph in operation on the Palomar Observatory 60-inch telescope. Its 2-dimensional spectral format provides simultaneous coverage from lambda ~ 0.75 microns to lambda ~ 2.5 microns (z'JHK bands). A remotely operated cold flip mirror permits its NICMOS3 detector to function as a K_s slit viewer to assist object placement into the 2 arcsec x 15 arcsec slit. CorMASS was primarily designed for the rapid spectral classification of low-mass stellar and sub-stellar objects identified by the Two-Micron All Sky Survey (2MASS). CorMASS' efficiency and resolution also make it a versatile instrument for the spectral observation and classification of many other types of bright objects (K<14) including quasars, novae, and emission line objects.

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arXiv2000

Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf Significantly Cooler than Gliese 229B

We report the discovery of a widely separated (258$\farcs3\pm0\farcs$4) T dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was initially identified from the Two Micron All Sky Survey (2MASS). Its near-infrared spectrum shows the 1.6 and 2.2 $\micron$ CH$_4$ absorption bands characteristic of T dwarfs, while its common proper motion with the Gl 570ABC system confirms companionship. Gl 570D (M$_J$ = 16.47$\pm$0.07) is nearly a full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and estimates of L = (2.8$\pm$0.3)x10$^{-6}$ L$_{\sun}$ and T$_{eff}$ = 750$\pm$50 K make it significantly cooler and less luminous than any other known brown dwarf companion. Using evolutionary models by Burrows et al. and an adopted age of 2-10 Gyr, we derive a mass estimate of 50$\pm$20 M$_{Jup}$ for this object.

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arXiv1999

Four nearby L dwarfs

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000Åin the L0 dwarf 2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I 4227Åand Na I 5890/5896Åare extremely strong, with the latter having an equivalent width of 240Åin the L4 dwarf. By spectral type L5, the D lines extend over $\sim1000$Åand absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband colour. The KI resonance doublet at 7665/7699Åincreases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.

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arXiv1999

Two nearby M dwarf binaries from 2MASS

We report the discovery of two binary M dwarf systems in the immediate solar neighborhood using 2MASS. The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsecs away. The M dwarf is highly active with both H alpha and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age, and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcseconds separation. We estimate a distance of 16 parsecs for this very low mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen burning limit.

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arXiv1999

Discovery of Four Field Methane (T-type) Dwarfs with 2MASS

We report the discovery of four field methane (T-type) brown dwarfs using 2MASS survey data. One additional methane dwarf, previously discovered by SDSS, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 um CH4 absorption bands characteristic of objects with T_eff < 1300 K, as well as broadened H2O bands at 1.4 and 1.9 um. Comparing the spectra of these objects with Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, J1225-27, J1047+21 and J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs/sq. deg., or ~ 90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs/pc^3, is comparable to that of the L dwarf sample from Kirkpatrick et al. (1999).

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arXiv1999

L Dwarfs and the Substellar Mass Function

Analysis of initial observations from near-infrared sky surveys has shown that the resulting photometric catalogues, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp) indicates that these L dwarfs have effective temperatures between ~2000\pm100 K and 1500\pm100 K, while the available trigonometric parallax data place their luminosities at between 10^{-3.5} and 10^{-4.3} L_solar. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the Solar Neighbourhood population and constrain Psi(M) by comparing the predicted L-dwarf surface densities and temperature distributions against observations from the DENIS and 2MASS surveys. The data, although sparse, can be represented by a power-law mass function, Psi(M) ~ M^{-alpha}, with 1 < alpha < 2. Current results favour a value nearer the lower limit. If alpha = 1.3, then the local space density of 0.075 > M/M_solar > 0.01 brown dwarfs is 0.10 systems pc^{-3}. In that case brown dwarfs are twice as common as main-sequence stars, but contribute no more than ~15% of the total mass of the disk.

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arXiv1998

M Subdwarfs: The Population II Luminosity Function

We present results of a study of very low mass halo stars. Using a sample of proper motion stars identified from plate material taken as part of the first and second Palomar Sky Surveys, we measure the space density, metallicity distribution, and kinematics of the Population II M subdwarfs. Our overall luminosity function is in good agreement with previous analyses of the space density of nearby very-low-mass halo subdwarfs, and confirms the discrepancy between local analyses and the space densities inferred from deep HST starcounts. We show for the first time that both the metallicity distribution and kinematics of late-type halo subdwarfs are consistent with those of their higher mass metal-poor counterparts. Dividing our sample by abundance, we find no evidence that the mass function of field halo stars is dependent upon metallicity. We provide data for three nearby subdwarfs that may merit additional observations.

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arXiv1997

Faint M-dwarfs and the structure of the Galactic disk

We use broadband photometry and low-resolution spectra of a complete sample of late-K and M dwarfs brighter than I=22 in three fields at high galactic latitude to study issues relating to galactic structure and large scale abundance gradients in the Galaxy. The observed starcounts in each field are a good match to the predictions of models based on deep starcount data in other intermediate-latitude fields, and these models identify the late-type stars as members of the Galactic disk. Abundances for these late type stars are estimated via narrowband indices that measure the strength of the TiO and CaH bands in their spectra. Our results show that the average abundance in the Galactic disk remains close to solar even at heights of more than 2 kpc above the Plane.

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