We present new Spitzer Infrared Array Camera (IRAC) photometry of 12 very late-type T dwarfs: nine have 3.6, 4.5, 5.8, and 8.0 photometry and three have 3.6 and 4.5 photometry only. Combining this ...with previously published photometry, we investigate trends with type and color that are useful for both the planning and interpretation of infrared surveys designed to discover the coldest T or Y dwarfs. The online appendix provides a collation of MKO-system YJHKL'M' and IRAC photometry for a sample of M, L, and T dwarfs. Brown dwarfs with effective temperature (T{sub eff}) below 700 K emit more than half their flux at wavelengths longer than 3 {mu}m, and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to T{sub eff} at these low temperatures. We confirm that the color H (1.6 {mu}m) - 4.5 is a good indicator of T{sub eff} with a relatively weak dependence on metallicity and gravity. Conversely, the colors H - K (2.2 {mu}m) and 4.5 - 5.8 are sensitive to metallicity and gravity. Thus, near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs, and if temperature and gravity are known, then mass and age can be reliably determined from evolutionary models. There are 12 dwarfs currently known with H- 4.5 >3.0, and 500 K {approx}< T{sub eff} {approx}<800 K, which we examine in detail. The ages of the dwarfs in the sample range from very young (0.1-1.0 Gyr) to relatively old (3-12 Gyr). The mass range is possibly as low as 5 Jupiter masses to up to 70 Jupiter masses, i.e., near the hydrogen burning limit. The metallicities also span a large range, from m/H = -0.3 to m/H = +0.3. The small number of T8-T9 dwarfs found in the UK Infrared Telescope Infrared Deep Sky Survey to date appear to be predominantly young low-mass dwarfs. Accurate mid-infrared photometry of cold brown dwarfs is essentially impossible from the ground, and extensions to the mid-infrared space missions, warm-Spitzer and Wide-Field Infrared Survey Explorer, are desirable in order to obtain the vital mid-infrared data for cold brown dwarfs, and to discover more of these rare objects.
VIRAC: the VVV Infrared Astrometric Catalogue Smith, L. C.; Lucas, P. W.; Kurtev, R. ...
Monthly Notices of the Royal Astronomical Society,
02/2018, Volume:
474, Issue:
2
Journal Article
Peer reviewed
Open access
Abstract
We present VIRAC version 1, a near-infrared proper motion and parallax catalogue of the VISTA Variables in the Via Lactea (VVV) survey for 312 587 642 unique sources averaged across all ...overlapping pawprint and tile images covering 560 deg2 of the bulge of the Milky Way and southern disc. The catalogue includes 119 million high-quality proper motion measurements, of which 47 million have statistical uncertainties below 1 mas yr−1. In the 11 < Ks < 14 magnitude range, the high-quality motions have a median uncertainty of 0.67 mas yr−1. The catalogue also includes 6935 sources with quality-controlled 5σ parallaxes with a median uncertainty of 1.1 mas. The parallaxes show reasonable agreement with the Tycho-Gaia Astrometric Solution, though caution is advised for data with modest significance. The SQL data base housing the data is made available via the web. We give example applications for studies of Galactic structure, nearby objects (low-mass stars and brown dwarfs, subdwarfs, white dwarfs) and kinematic distance measurements of young stellar objects. Nearby objects discovered include LTT 7251 B, an L7 benchmark companion to a G dwarf with over 20 published elemental abundances, a bright L subdwarf, VVV 1256−6202, with extremely blue colours and nine new members of the 25 pc sample. We also demonstrate why this catalogue remains useful in the era of Gaia. Future versions will be based on profile fitting photometry, use the Gaia absolute reference frame and incorporate the longer time baseline of the VVV extended survey.
Aims. We present the results from high-resolution spectroscopic measurements to determine metallicities and activities of bright stars in the southern hemisphere. Methods. We measured the iron ...abundances (Fe/H's) and chromospheric emission indices (log $h{R}'_{{HK}}$) of 353 solar-type stars with V = 7.5-9.5. Fe/H abundances are determined using a custom χ2 fitting procedure within a large grid of Kurucz model atmospheres. The chromospheric activities were determined by measuring the amount of emission in the cores of the strong Caii HK lines. Results. Our comparison of the metallicity sample to other Fe/H determinations was found to agree at the ±0.05 dex level for spectroscopic values and at the ±0.1 dex level for photometric values. The distribution of chromospheric activities is described by a bimodal distribution, agreeing with the conclusions from other works. Also an analysis of Maunder minimum status was attempted, and it was found that 6 ± 4 stars in the sample could be in a Maunder minimum phase of their evolution and hence the Sun should only spend a few per cent of its main sequence lifetime in Maunder minimum.
We describe the procedure to determine effective temperatures, rotational velocities, microturbulent velocities and chemical abundances in the atmospheres of Sun-like stars. We use independent ...determinations of iron abundances using the fits to the observed Fe i and Fe ii atomic absorption lines. We choose the best solution from the fits to these spectral features for the model atmosphere that provides the best confidence in the determined log N(Fe), V
t and v sin i. Computations were performed in the framework of local thermodynamic equilibrium. Blending effects were accounted for explicitly. First, we compute the abundance of iron for a set of adopted microturbulent velocities. In some cases, a few points of log N(Fe i) = log N(Fe ii) can be found. To determine the most self-consistent effective temperature and microturbulent velocity in any star's atmosphere, we used an additional constraint where we minimize the dependence of the derived abundances of Fe i and Fe ii on the excitation potential of the corresponding lines. Using this procedure we analyse the spectra of the Sun and two well-known solar-type stars, HD 1835 and HD 10700, to determine their abundances, microturbulent velocity and rotational velocity. Our approach allows us to determine self-consistent values for the effective temperatures, abundances, V
t and v sin i. For the Sun, we obtain the best agreement for a model atmosphere of T
eff/log g/Fe/H = 5777/4.44/0.0, iron abundances and microturbulent velocities of log N(Fe) = 4.44, V
t= 0.75 km s−1, for the Fe i lines, and log N(Fe) =−4.47 and V
t= 1.5 km s−1 for the Fe ii lines. Furthermore, abundances of other elements obtained from the fits of their absorption features agree well enough (±0.1 dex) with the known values for the Sun. We determined a rotational velocity of v sin i= 1.6 ± 0.3 km s−1 for the spectrum of the Sun as a star. For HD 1835, the self-consistent solution for Fe i and Fe ii lines log N(Fe) =+0.2 was obtained with a model atmosphere of 5807/4.47/+0.2 and microturbulent velocity of V
t= 0.75 km s−1, and leads to v sin i= 7.2 ± 0.5 km s−1. For HD 10700, the self-consistent solution log N(Fe) =−4.93 was obtained using a model atmosphere of 5383/4.59/−0.6 and microturbulent velocity of V
t= 0.5 km s−1. The Fe i and Fe ii lines give rise to a v sin i= 2.4 ± 0.4 km s−1. Using T
eff found from the ionization equilibrium parameters for all three stars, we found abundances of a number of other elements: Ti, Ni, Ca, Si and Cr. We show that uncertainties in the adopted values of T
eff of 100 K and V
t of 0.5 km s−1 change the abundances of elements up to 0.1 and 0.2 dex, respectively. Galactic abundance variations can generally be larger than this measurement precision and therefore we can study abundance variations throughout the Galaxy.
Rotational Velocities for M Dwarfs Jenkins, J. S; Ramsey, L. W; Jones, H. R. A ...
The Astrophysical journal,
10/2009, Volume:
704, Issue:
2
Journal Article
Peer reviewed
We present spectroscopic rotation velocities (v sin i) for 56 M dwarf stars using high-resolution Hobby-Eberly Telescope High Resolution Spectrograph red spectroscopy. In addition, we have also ...determined photometric effective temperatures, masses, and metallicities (Fe/H) for some stars observed here and in the literature where we could acquire accurate parallax measurements and relevant photometry. We have increased the number of known v sin i values for mid M stars by around 80% and can confirm a weakly increasing rotation velocity with decreasing effective temperature. Our sample of v sin is peak at low velocities (~3 km s-1). We find a change in the rotational velocity distribution between early M and late M stars, which is likely due to the changing field topology between partially and fully convective stars. There is also a possible further change in the rotational distribution toward the late M dwarfs where dust begins to play a role in the stellar atmospheres. We also link v sin i to age and show how it can be used to provide mid-M star age limits. When all literature velocities for M dwarfs are added to our sample, there are 198 with v sin i <= 10 km s-1 and 124 in the mid-to-late M star regime (M3.0-M9.5) where measuring precision optical radial velocities is difficult. In addition, we also search the spectra for any significant H Delta *a emission or absorption. Forty three percent were found to exhibit such emission and could represent young, active objects with high levels of radial-velocity noise. We acquired two epochs of spectra for the star GJ1253 spread by almost one month and the H Delta *a profile changed from showing no clear signs of emission, to exhibiting a clear emission peak. Four stars in our sample appear to be low-mass binaries (GJ1080, GJ3129, Gl802, and LHS3080), with both GJ3129 and Gl802 exhibiting double H Delta *a emission features. The tables presented here will aid any future M star planet search target selection to extract stars with low v sin i.
We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ...ULAS J133553.45+113005.2 (ULAS1335). We detail optical and near-infrared (NIR) photometry for all three sources, and mid-IR photometry for ULAS1335. We use NIR spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1238) and T9 (ULAS1335) to these objects. ULAS1017 is classed as a peculiar T8 (T8p) due to appearing as a T8 dwarf in the J band, whilst exhibiting H- and K-band flux ratios consistent with a T6 classification. Through comparison to BT-Settl model spectra we estimate that ULAS1017 has 750 K ≲Teff≲ 850 K, and 5.0 ≲ log g(cm s−2) ≲ 5.5, assuming solar metallicity. This estimate for gravity is degenerate with varying metallicity. We estimate that ULAS1017 has an age of 1.6–15 Gyr, a mass of 33–70MJ and lies at a distance of 31–54 pc. We do not estimate atmospheric parameters for ULAS1238 due to a lack of K-band photometry. We extend the unified scheme of Burgasser et al. to the type T9 and suggest the inclusion of the WJ index to replace the now saturated J-band indices. We propose ULAS1335 as the T9 spectral type standard. ULAS1335 is the same spectral type as ULAS J003402.77−005206.7 and CFBDS J005910.90−011401.3. We argue that given the similarity of the currently known >T8 dwarfs to the rest of the T dwarf sequence, the suggestion of the Y0 spectral class for these objects is premature. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600 K, possibly combined with low gravity and/or high metallicity. We find ULAS1335 to be extremely red in NIR to mid-IR colours, with H−4.49= 4.34 ± 0.04. This is the reddest NIR to mid-IR colour yet observed for a T dwarf. The NIR to mid-IR spectral energy distribution of ULAS1335 further supports Teff < 600 K, and we estimate Teff∼ 550–600 K for ULAS1335. We estimate that ULAS1335 has an age of 0.6–5.3 Gyr, a mass of 15–31MJ and lies at a distance of 8–12 pc.
The properties of the T8.5p dwarf Ross 458C Burningham, Ben; Leggett, S. K.; Homeier, D. ...
Monthly Notices of the Royal Astronomical Society,
07/2011, Volume:
414, Issue:
4
Journal Article
Peer reviewed
Open access
We present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints ...(≤1 Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has T
eff= 695 ± 60 K, log g= 4.0-4.7 and an inferred mass of 5-20M
J. We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5-μm region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of the dust opacity in the spectra of Ross 458C that was raised by Burgasser et al. We have found that the BT Settl models which also incorporate a condensate cloud model provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with f
sed= 2 and we briefly discuss the influence of condensate clouds on T dwarf spectra.
ABSTRACT
We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky Survey, Visible and Infrared Survey Telescope for Astronomy , and Wide-field Infrared Survey Explorer surveys, ...and confirmed with optical to near-infrared spectra obtained with the Very Large Telescope and the Gran Telescopio Canarias. One of these new T dwarfs is mildly metal-poor with slightly suppressed K-band flux. We presented a new X-shooter spectrum of a known benchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational properties of brown dwarfs, we discussed transition zones (mass ranges) with low-rate hydrogen, lithium, and deuterium burning in brown dwarf population. The hydrogen burning transition zone is also the substellar transition zone that separates very low-mass stars, transitional, and degenerate brown dwarfs. Transitional brown dwarfs have been discussed in previous works of the Primeval series. Degenerate brown dwarfs without hydrogen fusion are the majority of brown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and halo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the literature by their suppressed K-band flux. We studied the spectral-type–colour correlations, spectral-type–absolute magnitude correlations, colour–colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these of L–T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery capability of deep sky surveys in the 2020s.
We present near-infrared long-slit and multislit spectra of low-mass brown dwarf candidates in the Orion nebula cluster. The long-slit data were observed in the H and K bands using NIRI on the ...Gemini-North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo-Australian Telescope at H band. We develop a spectral typing scheme based on optically calibrated, near-infrared spectra of young sources in the Taurus and IC 348 star-forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on Hertzsprung–Russell diagrams overlaid with theoretical pre-main-sequence isochrones. The majority of our objects lie close to or above the 1-Myr isochrone, leading to an average cluster age that is <1 Myr. We find 38 sources lie at or below the hydrogen-burning limit (0.075 M⊙). 10 sources potentially have masses below the deuterium-burning limit (0.012 M⊙). We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions. The lowest χ2 values are produced by an age distribution centred at 1 Myr, with a mass function that declines at substellar masses according to an Mα power law in the range α= 0.3–0.6. We find that truncating the mass function at 0.012 M⊙ produces luminosity functions that are starved of the faintest magnitudes, even when using bimodal age populations that contain 10-Myr-old sources. The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC.
47 new T dwarfs from the UKIDSS Large Area Survey Burningham, Ben; Pinfield, D. J.; Lucas, P. W. ...
Monthly Notices of the Royal Astronomical Society,
08/2010, Volume:
406, Issue:
3
Journal Article
Peer reviewed
Open access
We report the discovery of 47 new T dwarfs in the Fourth Data Release (DR4) from the Large Area Survey (LAS) of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey with spectral ...types ranging from T0 to T8.5. These bring the total sample of LAS T dwarfs to 80 as of DR4. In assigning spectral types to our objects we have identified eight new spectrally peculiar objects, and divide seven of them into two classes. H2O-H-early have a H2O-H index that differs with the H2O-J index by at least two subtypes. CH4-J-early have a CH4-J index that disagrees with the H20-J index by at least two subtypes. We have ruled out binarity as a sole explanation for both types of peculiarity, and suggest that they may represent hitherto unrecognized tracers of composition and/or gravity. Clear trends in z′(AB) −J and Y−J are apparent for our sample, consistent with weakening absorption in the red wing of the K i line at 0.77 μm with decreasing effective temperature. We have used our sample to estimate space densities for T6–T9 dwarfs. By comparing our sample to Monte Carlo simulations of field T dwarfs for various mass functions of the form ψ(M) ∝ M−α pc−3 M−1⊙, we have placed weak constraints on the form of the field mass function. Our analysis suggests that the substellar mass function is declining at lower masses, with negative values of α preferred. This is at odds with results for young clusters that have been generally found to have α > 0.
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