UKIDSS detections of cool brown dwarfs - proper motions of 14 known >T5 dwarfs and discovery of three new T5.5-T6 dwarfsbased on observations with the Large Binocular Telescope (LBT)

UKIDSS detections of cool brown dwarfs - proper motions of 14 known T5 dwarfs and discovery of three new T5.5-T6 dwarfs1

Key Words.:
Astrometry – Proper motions – Stars: distances – Stars: kinematics and dynamics – brown dwarfs – solar neighbourhood

Abstract

Context:

Aims:We contribute to improving the census of cool brown dwarfs (late-T and Y dwarfs) in the immediate solar neighbourhood.

Methods:By combining near-infrared (NIR) data of UKIDSS with mid-infrared WISE and other available NIR (2MASS) and red optical (SDSS -band) multi-epoch data we detect high proper motion (HPM) objects with colours typical of late spectral types (T5). We use NIR low-resolution spectroscopy for the classification of new candidates.

Results:We determined new proper motions for 14 known T5.5-Y0 dwarfs, many of them being significantly (2-10 times) more accurate than previous ones. We detected three new candidates, ULAS J09540623, ULAS J11520359, and ULAS J12040150, by their HPMs and colours. Using previously published and new UKIDSS positions of the known nearby T8 dwarf WISE J02540223 we improved its trigonometric parallax to 16520 mas. For the three new objects we obtained NIR spectroscopic follow-up with LBT/LUCIFER classifying them as T5.5 and T6 dwarfs. With their estimated spectroscopic distances of about 25-30 pc, their proper motions of about 430-650 mas/yr lead to tangential velocities of about 50-80 km/s typical of the Galactic thin disk population.

Conclusions:

1 Introduction

Although brown dwarfs (BDs) might be even more numerous than M-type red dwarf stars, to date we know 10 less substellar objects than stars in the immediate solar neighbourhood (within 6 pc). The BDs are difficult to detect, especially in the optical, due to their relatively small fluxes and red optical-to-NIR colours. This is reinforced by (i) their “failed star” nature, as they dim and cool down with age, evolving across the M, L, and T spectral types (Burrows et al. burrows01 ()), and (ii) the typical age of several Gyrs of solar neighbourhood objects. Most of the BD neighbours are expected to have reached temperatures as low or lower than those of T dwarfs (500-1300 K), which is confirmed by the fact that almost 75% of the objects with trigonometric parallaxes 100 mas in the compilation of L and T dwarfs of Gelino et al.  (gelino11 (), hereafter G11) are T dwarfs, and their fraction continues to grow, in particular with late-T (T5) types.

Deep large-area optical surveys like the Sloan Digital Sky Survey data releases (SDSS DR7, Abazajian et al. abazajian09 (); DR8, Aihara et al. aihara11 ()) detected only very nearby late-T dwarfs. The Canada-France BD survey is deeper, although smaller in sky area (Delorme et al. delorme08 (), hereafter D08; Albert et al. albert11 ()). New near-infrared (NIR) and mid-infrared surveys covering big sky portions like the Large Area Survey (LAS) within the UKIRT InfraRed Deep Sky Surveys (UKIDSS)6 that goes deeper than the Two Micron All Sky Survey (2MASS; Skrutskie et al. skrutskie06 ()), and the Wide-field Infrared Survey Explorer (WISE; Wright et al. wright10 ()) that extends the 2MASS to the mid-infrared over the whole sky, are very sensitive to late-T dwarfs and even cooler Y dwarfs, as recently shown by Kirkpatrick et al. (kirkpatrick11 (), hereafter K11) and Cushing et al. (cushing11 (), herafter C11).

In continuation of the study of Scholz (scholz10 (), hereafter S10), who used UKIDSS DR6 and SDSS DR7 to detect late-T dwarf candidates, and Scholz et al. (scholz11 (), hereafter S11), who combined the WISE preliminary data release (PDR) with SDSS (DR7DR8) and 2MASS discovering two very nearby ultracool BDs and classifying one of them (WISE J17412553) spectroscopically as a T9.5 dwarf, we look for further late-T dwarf candidates with HPMs. We combine recent UKIDSS data releases (DR7DR8DR9) with WISE, 2MASS, and SDSS, where available. The UKIDSS releases represent a growing database, whereas SDSS DR7 and DR8 data are independent and may differ for a given object. Sects. 3 and 5 provide proper motions and photometry of 14 previously known and three newly found objects, respectively. Sect. 4 provides the parallax measurement of a very nearby object. Sect. 6 presents spectroscopic follow-up for our new candidates, and Sect. 7 contains our brief discussion and conclusions.

2 Selection of candidates and cross-identification

Nearby cool BDs are expected to be very faint HPM objects. We identify them not only based on their characteristic colours in one deep survey but as moving objects showing up in various surveys with time baselines of several years. For our search of nearby T5 dwarfs, primarily based on latest UKIDSS, i.e. DR9 LAS and Galactic Clusters Survey (GCS), and SDSS data, we used the same criteria as described in S10:

,        ,        .

Only late-T dwarfs and possibly low-metallicity T dwarfs meet these constraints. White dwarfs, late-M and L dwarfs are excluded by the two colour cuts. To identify a HPM object, we needed at least two different epochs from UKIDSS or other available survey data, if it had no suitable SDSS counterpart. For additional survey data we looked at 2MASS and other catalogues accessible via the CDS7. We also checked 2MASS FITS images from IRSA Finder Chart8 and detected objects visually using ”pick object” within the ESO skycat tool. After the WISE PDR, we checked our UKIDSS candidates, including those with lacking/uncertain proper motions, for WISE counterparts. In the WISE PDR, we also selected suspected HPM objects (no 2MASS counterpart within 3 arcsec) with late-T dwarf mid-infrared colours9 (similar to the criteria used by S11):

,      ,      

and subsequently looked for UKIDSS counterparts. However, using WISE as a starting point, we found only already known objects. We checked the UKIDSS DR9 LAS, GCS, and DR7 Galactic Plane Survey (GPS) for possible measurements of the T5 dwarfs discovered by K11, to improve their proper motions by involving additional epochs. UKIDSS counterpart data were found in the LAS and GCS but not in the GPS. In Sects. 3 and 5, we show our proper motion results for objects with at least three distinct epochs available.

3 Proper motions & photometry of known objects

On top of Table 110 we list photometry and our proper motions of previously known late-T (T5) dwarfs, which we newly identified in UKIDSS LAS data. Individual multi-epoch positions from UKIDSS LAS (and other surveys) are listed in Tables 2, 3, and 411, where our visual 2MASS detections are marked. Imaging SDSS and UKIDSS LAS data are available in all fields/bands of the ten objects. For objects not detected in any of the bands listed in Table 1, we give lower magnitude limits corresponding to the SDSS depths (Abazajian et al. abazajian09 ()) and in specific fields of UKIDSS LAS and GCS (using SQL queries from Dye et al. dye06 ()). Most of the LAS detections are recent WISE discoveries (K11). Full designations of all known objects described in this paper can be found in K11 and G11.

We also tried to improve the proper motions for three known T5 dwarfs from Table 6 of S10 (not shown here again) by involving WISE positions if available. However, this led to larger uncertainties in . In particular, we note that one of these three (the T7 dwarf SDSS J15071027; Chiu et al. chiu06 ()) is detected by WISE as a red object (16.300.09, 14.120.05, 12.130.33) but shifted in declination by about 1.5 arcsec with respect to its expected position according to the HPM measurement of S10. According to the WISE PDR documentation, sources fainter than 14.5 may have ”declinations being offset between 0.2 and 1.0 arcsec from the true position”. This object is not included in K11. The T6.5 dwarf SDSS J13460031 (Tsvetanov et al. tsvetanov00 (); Burgasser et al. burgasser06 ()) in Table 1 is also a red WISE source (15.410.05, 13.560.04, 11.990.32) but was not listed by K11 in their Table 1 (with WISE and NIR photometry of known T dwarfs).

We determined the new proper motions in Table 1 from weighted linear fitting over all available multi-epoch positions, including 2MASS and SDSS (assuming 100 mas errors), WISE PDR (with their given errors), and UKIDSS positions (assuming 70 mas errors) listed in Tables 2, 3 and 4, as well as all WISE and Spitzer measurements (with their errors) given in K11 if available. In Sect. 4, we compare our error assumptions with others. For faint UKIDSS (18 mag) objects we assumed larger errors (140 mas), and for faint SDSS (20) objects and our visual 2MASS detections, we increased the assumed errors to 150 mas and 200 mas, respectively. For CFBDS J00590114 we included the CFBDS position (assuming 150 mas errors) from D08 in our proper motion fitting. In case of WISE J02540223, we combined 2MASS and SDSS DR8 positions already used by S11 with Pan-STARRS1 measurements from Liu et al. (liu11 (), hereafter L11), multiple WISE (instead of WISE PDR) and Spitzer epochs provided by K11, and UKIDSS DR9 data. For the latter object, we improved its trigonometric parallax (see Sect. 4).

Our proper motions of the first six WISE discoveries in Table 1 are at least 2-5 times more accurate than previous values in S11 and K11, whereas those of CFBDS J00590114, ULAS J08270204 and SDSS J13460031 agree with more accurate other data. Pinfield et al. (pinfield12 ()) measured the proper motion of WISE J07502725 using only two UKIDSS epochs. By involving the WISE and Spitzer epochs, we obtained a smaller , whereas our , despite a poor fit, is in good agreement with the former value. For SDSS J11100116 our 2 more accurate result agrees well with that of Jameson et al. (jameson08 ()), who did not use the SDSS.

As WISE J07502725 was not detected in SDSS -, UKIDSS -bands, and 2MASS, the maximum epoch difference between the first epoch UKIDSS -band and the last epoch Spitzer data was less than four years. The resulting proper motion errors are relatively large, but the total proper motion is highly significant. Even larger errors for CFBDS J00590114, WISE J22260440 and WISE J23441034 are due to maximum epoch differences of only about 1.5 to 2 years. The worst case is WISE J13110122, where the UKIDSS data do not significantly extend the time baseline of only 1 year, resulting in the largest proper motion errors. However, all large proper motion components, already indicated in K11, were confirmed.

For four of the T5 discoveries of K11, we found UKIDSS GCS -band data (Table 1). However, one of the Y0 dwarfs was not detected therein. For the others we used the positions of their apparent counterparts (Table 3) together with 2010/2011 WISE and Spitzer epochs given in K11 for new proper motion solutions. For the T6 dwarf WISE J04101411, we included available SDSS DR8 data as well. We also found the known late-T dwarf 2MASS J05160445 in our search combining WISE and UKIDSS GCS data. We achieved 3 to 10 more accurate proper motions for the T dwarfs with GCS detection (Table 1) but not for the Y0 dwarf WISE J15412250, which lies in a relatively crowded region close to Upper Sco (Lodieu et al. lodieu08 ()) and has highly discrepant preliminary trigonometric (2-4 pc) and photometric (8 pc) distances in K11. If the latter is correct, we would not expect to see a counterpart in the GCS -band, similar to what we found for the other Y0 dwarf, WISE J04101502, which has according to K11 a photometric distance of 9 pc. Therefore, we think the GCS counterpart of WISE J15412250 and its revised proper motion are doubtful.

\onllongtabL

1

previously measured proper motion new proper motion (this work)
object SpT (Ref) (Ref)
known (LAS):
CFBDS J00590114 T8.5 (1,3) 20.5 18.770.06 18.030.05 18.860.25 18.400.26 879008 51005 (11) 904069 22128
WISE J02540223 T8.0 (2,3) 19.860.07 17.000.01 15.920.01 16.290.03 16.730.05 2496046 276047 (2) 2537022 236008
WISE J07502725 T8.5 (3,3) 20.5 19.750.09 18.720.04 18.7 18.1 732017 194017 (12) 666037 202076
ULAS J08270204 T5.5 (4,4) 20.410.17 18.360.05 17.160.02 17.430.05 17.520.11 27003 109002 (11) 24012 98006
WISE J09290409 T6.5 (3,3) 20.750.12 18.000.02 16.870.01 17.370.07 17.400.09 105374 773367 (3) 525022 460021
SDSS J11100116 T5.5 (5,7) 19.680.11 17.340.01 16.160.01 16.200.02 16.050.03 243021 238018 (13) 237008 266003
WISE J13110122 T9: (3,3) 20.5 19.890.10 18.970.08 18.8 18.3 68324 827325 (3) 150229 847047
SDSS J13460031 T6.5 (6,7) 19.330.04 16.800.01 15.640.01 15.970.01 15.960.02 503003 114001 (14) 480019 131014
WISE J22260440 T8/T8.5 (3,3) 20.5 18.040.03 16.900.02 17.450.07 17.240.09 56584 529596 (3) 150082 516128
WISE J23441034 T9 (3,3) 20.5 19.880.12 18.840.09 19.240.29 18.2 574667 211668 (3) 859114 10128
known (GCS):
WISE J03072904 T6.5 (3,3) n/a - n/a n/a 18.080.12 91307 79312 (3) 154025 23016
WISE J04101502 Y0 (3,8) 20.5 - n/a n/a 18.2
WISE J04101411 T6 (3,3) 20.520.12 - n/a n/a 17.820.20 19302 198326 (3) 81037 65047
2MASS J05160445 T5.5 (9,7) 19.310.08 - n/a 15.770.01 15.790.02 270030 210030 (15) 225007 193009
WISE J15412250 Y0 (3,8) n/a - n/a n/a 17.930.19 780234 218249 (3) 984293 516187
new (LAS):
ULAS J09540623 T5.5 (10,10) 19.580.11 17.870.02 16.590.01 16.890.02 17.170.07 495014 422008
ULAS J11520359 T6 (10,10) 20.690.19 18.540.03 17.280.02 17.700.05 17.770.12 388017 209033
ULAS J12040150 T5.5 (10,10) 20.490.20 17.990.03 16.810.02 17.070.04 17.290.09 399016 166011
Table 1: SDSS DR7 (DR8) and UKIDSS DR9 photometry and old and newly determined proper motions (in mas/yr) of known and new T5 dwarfs

Notes: not in WISE PDR, SDSS DR8, mean values from multiple measurements, doubtful GCS counterpart/new proper motion, n/a - not available, magnitudes in AB system, = aperMag3 for point sources (Dye et al. dye06 ()) in Vega system using MKO photometric system. References (discovery, SpT, proper motions) are: (1) - D08, (2) - S11, (3) - K11, (4) - Lodieu et al. (lodieu07 ()), (5) - Geballe et al. (geballe02 ()), (6) - Tsvetanov et al. (tsvetanov00 ()), (7) - Burgasser et al. (burgasser06 ()), (8) - C11, (9) - Burgasser et al. (burgasser03 ()), (10) - New (this paper, SpT accurate to 0.5 sub-types), (11) - Marocco et al. (marocco10 ()), (12) - Pinfield et al. (pinfield12 ()), (13) - Jameson et al. (jameson08 ()), (14) - Tinney, Burgasser & Kirkpatrick (tinney03 ()), (15) - Faherty et al. (faherty09 ()).

\onltab

2

Object
[deg] [deg] Epoch ID Source
CFBDS J00590114:
14.7949709 01.2336947 2005.732 292702 ULAS
14.7949181 01.2337060 2005.732 293033 ULAS
14.7952080 01.2337464 2006.885 1581752 ULAS
14.7952146 01.2337521 2006.885 1584033 ULAS
WISE J02540223:
43.5399358 02.3996412 2010.734 3859643 ULAS
43.5399337 02.3996466 2010.734 3859715 ULAS
43.5399295 02.3996464 2010.718 3840324 ULAS
43.5399246 02.3996529 2010.718 3840484 ULAS
WISE J07502725:
117.5164404 27.4293472 2007.126 1231545 ULAS
117.5159819 27.4292434 2009.129 2367125 ULAS
117.5159827 27.4292398 2009.129 2367307 ULAS
ULAS J08270204:
126.7819292 02.0687722 1998.953 2M
126.781892 02.068816 2001.293 2259 SDSS
126.7819523 02.0689431 2005.940 465119 ULAS
126.7819511 02.0689491 2005.940 473125 ULAS
126.7819424 02.0689509 2005.940 478440 ULAS
126.7819506 02.0689568 2005.940 477757 ULAS
WISE J09290409:
142.276914 04.167209 2001.140 2125 SDSS
142.2776989 04.1663975 2007.107 1219425 ULAS
142.277670 04.166407 2007.197 6749 SDSS
142.2777383 04.1663741 2007.288 1399129 ULAS
142.2781424 04.1660432 2009.923 3028681 ULAS
142.2781455 04.1660397 2009.923 3029035 ULAS
SDSS J11100116:
167.541711 01.270302 2000.126 2M
167.541674 01.270298 2000.343 1462 SDSS
167.5411353 01.2697012 2008.373 2325845 ULAS
167.5411422 01.2697045 2008.373 2325865 ULAS
167.5410610 01.2695783 2009.978 3085143 ULAS
167.5410582 01.2695767 2009.978 3085325 ULAS
WISE J13110122:
197.7760412 01.3817322 2010.156 3351901 ULAS
197.7760299 01.3817386 2010.156 3352083 ULAS
SDSS J13460031:
206.693495 00.530639 1999.221 756 SDSS
206.693104 00.530592 2001.093 2M
206.692471 00.530844 2006.395 6166 SDSS
206.6921079 00.5309516 2009.129 2369313 ULAS
206.6921118 00.5309485 2009.129 2369436 ULAS
206.6921010 00.5309524 2009.134 2350848 ULAS
206.6921028 00.5309471 2009.134 2351038 ULAS
206.6920776 0.5309553 2010.182 WISE
WISE J22260440:
336.5960939 04.6678106 2009.620 2784734 ULAS
336.5961029 04.6678081 2009.620 2783676 ULAS
336.5960903 04.6678034 2009.641 2797400 ULAS
336.5960866 04.6677827 2009.641 2797524 ULAS
WISE J23441034:
356.1923904 10.5710685 2009.668 2823367 ULAS
356.1924237 10.5710277 2009.668 2823532 ULAS
356.1924059 10.5710051 2009.827 2944838 ULAS
12
Table 2: (J2000.0) of known T5 dwarfs in UKIDSS LAS, etc.
\onltab

3

Object
[deg] [deg] Epoch ID Source
WISE J03072904:
46.8525439 29.0798528 2008.786 2334900 UGCS
WISE J04101411:
62.7270953 14.1920928 2005.781 322532 UGCS
62.7269979 14.1921757 2006.367 6003 SDSS
2MASS J05160445:
79.039410 04.763872 1998.721 2M
79.0392237 04.7640431 2000.934 1927 SDSS
79.0386926 04.7644903 2009.893 2998751 UGCS
79.0386834 04.7644923 2009.893 2998937 UGCS
79.0386734 04.7645655 2010.182 WISE
WISE J15412250:
235.4642206 22.8407768 2008.301 2323120 UGCS
13
Table 3: (J2000.0) of known T5 dwarfs in UKIDSS GCS, etc.
\onltab

4

Object
[deg] [deg] Epoch ID Source
ULAS J09540623:
148.625883 06.387122 2000.153 2M
148.625654 06.386935 2002.174 3015 SDSS
148.6249784 06.3863407 2007.214 1287023 ULAS
148.6249374 06.3863301 2007.277 1390233 ULAS
148.6246477 06.3860939 2009.332 2442500 ULAS
148.6245730 06.3860068 2009.942 3052277 ULAS
ULAS J11520359:
178.124725 03.991431 2000.181 2M
178.124063 03.991187 2006.023 5973 SDSS
178.1238153 03.9910013 2008.375 2326168 ULAS
178.1237922 03.9910487 2008.375 2326188 ULAS
178.1236578 03.9909038 2009.978 3084746 ULAS
178.1236518 03.9908986 2009.978 3084839 ULAS
ULAS J12040150:
181.187079 01.843475 1999.077 2M
181.187242 01.843383 1999.077 2M
181.187080 01.843431 2000.116 1140 SDSS
181.1861442 01.8430247 2008.395 2327790 ULAS
181.1861488 01.8430134 2008.395 2327810 ULAS
181.1861335 01.8430218 2008.403 2329084 ULAS
181.1861363 01.8430238 2008.403 2329105 ULAS
14
Table 4: Multi-epoch (J2000.0) of new late-T dwarf candidates
Figure 1: Parallactic ellipse for WISE J02540223 after combined proper motion and parallax solution and subtraction of the proper motion. Coloured (blue - SDSS, green - Pan-STARRS1, yellow - 2MASS, orange - UKIDSS, red - WISE, pink - Spitzer) dots with error bars show the individual positional measurements, dashed lines show their displacements relative to the fit.

4 Trigonometric parallax for WISE J02540223

L11 and K11 published preliminary trigonometric parallaxes for WISE J02540223, which has the largest proper motion in Table 1. By combining all available measurements used by them (less than 10 epochs in both cases) with new UKIDSS DR9 data (Table 2), we collected 17 epochs and expected an improved fit. We applied the software of Gudehus (gudehus01 ()) for combined proper motion and parallax solutions with and without weights corresponding to the positional errors of the different surveys. In our final weighted solution, which yielded 10% more accurate parallax and proper motion results compared to the unweighted one, we used the following errors: 70 mas for UKIDSS (as we can assume for an object at intermediate galactic latitude according to Lawrence et al. lawrence07 ()), 100 mas for 2MASS and SDSS, 120 mas for Pan-STARRS1 (corresponding to the typical size of the error bars shown in Fig. 4 of L11, and the individual errors of the three WISE (between 76 mas and 89 mas) and two Spitzer epochs (from 138 mas to 275 mas) given in K11. Our assumed 2MASS errors are smaller than the values given in K11 and correspond to those of L11. For the SDSS counterpart we assumed slightly larger errors than L11 as we took into account that the astrometric accuracy at the faint end of the SDSS is limited by photon statistics (Pier et al. pier03 ()). Our proper motion and parallax of WISE J02540223 are:

, (25447, 2377) mas/yr,   16520 mas.

The proper motion is more accurate than the one in Table 1 from linear fitting not taking into account the parallactic motion. In Fig. 1 we show the parallactic ellipse and the individual positions with their error bars and with respect to their expected location (indicated by dashed lines) according to the fit after subtracting the proper motion. Only in four cases the shift between expected location and measured position exceeds the error bars. This concerns three out of six Pan-STARRS1 and one out of three WISE positions, whose errors may be underestimated.

The distance estimate of 6.10.7 pc from the new parallax agrees well with the photometric distances of 5.5 pc (S11), 7.20.7 pc (L11), and 6.9 pc (K11). Our new parallax is also very similar to previous trigonometric measurements of 17145 mas (from 9 positions) and 16546 mas (from 6 positions), respectively by L11 and K11, but has a 50% smaller error. However, our new parallax is still a preliminary result, not obtained within a dedicated parallax programme and making use of different optical, NIR, and mid-infrared surveys. As we neglected colour-dependent systematic errors in the positions (differential refraction), our parallax errors are lower limits. We also derived significant parallaxes for ULAS J08270204 (9721 mas), SDSS J11100116 (565 mas), and the new T5.5 dwarf (Sects. 5 and 6) ULAS J09540623 (13628 mas), but these results are uncertain (from only six epochs in each case).

Figure 2: LBT/LUCIFER1 - (left) and -band (right) spectra of ULAS J09540623 (top), ULAS J12040150 (centre), and ULAS J11520359 (bottom). Target spectra (black) were smoothed for the comparison with the lower-resolution (120) template spectra (red) of T5-T6.5 dwarfs from Adam Burgasser’s SpeX Prism Spectral Libraries. Original references for the template spectra are: T5, T6 - Burgasser et al. (burgasser04 ()); T5.5, T6.5 - Burgasser et al. (burgasser08 ()).

5 New late-T dwarf candidates

Using the UKIDSS LAS as a starting point, we discovered three new late-T dwarf candidates: ULAS J095429.91062309.9, ULAS J115229.67035927.2, ULAS J120444.67015034.8 (hereafter names are abbreviated). They have -only SDSS counterparts shifted by several arcsec with respect to their two LAS epochs. For our proper motion fits we measured their positions at a fourth (earliest) epoch by visually detecting 2MASS counterparts (Table 4). The resulting well-measured HPMs, and the magnitudes of the objects are shown in Table 1. WISE PDR data are not available for these objects.

All three new candidates can be preliminarily classified as T72 dwarfs based on their negative NIR colours 0.410.26 and 0.580.48. As shown by S10, this colour-based classification cannot be made more accurately for this range of negative NIR colour indices. This is due to the colour reversal of T8-T9 dwarfs, whereas the sequence of T dwarfs with earlier types shows a clear trend with both colours, which allows a more accurate colour-based classification for positive and moderately negative indices (see Fig. 3 in S10). The more accurate spectral types (SpT) given in Table 1 are the result of our spectroscopic follow-up described in the next section.

\onltab

5

Index ULAS J09540623 ULAS J11520359 ULAS J12040150
HO-H 0.3190.012 (T6/T5) 0.2990.021 (T6) 0.3150.015 (T6/T5)
CH-H 0.3470.012 (T6/T5) 0.3040.016 (T6) 0.3690.012 (T5/T6)
CH-K 0.2330.022 (T5/T4) 0.1990.030 (T5) 0.2250.024 (T5)
SpT(J) T5.5 T6.5 T5.5
SpT(HK) T5.5 T6 T5.5
SpT T5.5 T6 T5.5
Table 5: Spectral indices and corresponding types and classification from comparison with templates for three new late-T dwarfs

6 Spectroscopic observations with LBT/LUCIFER

For follow-up we used the Large Binocular Telescope (LBT) NIR spectrograph LUCIFER1 (Mandel et al. mandel08 (); Seifert et al. seifert10 (); Ageorges et al. ageorges10 ()) in long-slit spectroscopic mode with the (200 lines/mm + order separation filter) and gratings (210 lines/mm + filter). ULAS J12040150 and ULAS J09540623 were observed with total integrations of 30 min in and 20 min in on 11 April 2011 and 12 April 2011, respectively. The slighly fainter ULAS J11520359 was observed on 11 May 2011 with 36 min in and 24 min in . As in S11, central wavelengths were chosen at 1.835 m () and 1.25 m () yielding a coverage of 1.38–2.26 and 1.17–1.32 m, respectively. The slit width was 1 arcsec for ULAS J11520359, corresponding to 4 pixels, with a spectral resolving power =/4230, 940, and 1290 at 1.24, 1.65, and 2.2 m, respectively. For the other objects the slit width was 2 arcsec. Observations consisted of individual exposures of 75 s in and 200 s in (90 and 240 s, respectively, for ULAS J1152+0359) with shifting the target along the slit using an ABCCBA pattern until the total integration time was reached. A0V standards were observed just before/after the targets with similar airmasses.

The raw spectroscopic data were reduced as described in S11 using standard routines in IRAF15. The calibrated and spectra of our new objects and the corresponding spectra of the templates shown in Fig. 2 were normalised by the average flux in the range 1.20–1.30 and 1.52–1.61 m, respectively. Each spectrum was classified by visual comparison with template spectra, separately for the - and -band, and by three measured spectral indices in the -band (Table 516) as defined by Burgasser et al. (burgasser06 ()). No other useful indices could be determined in the -band, since the LUCIFER grating provides a very narrow wavelength interval. Fig. 2 shows the three closest matches of template (in red) with smoothed target spectra (overplotted in black). Both methods, comparison with templates and classification with indices, provided consistent results (with 0.5 sub-types accuracy, as in S11): ULAS J09540623 and ULAS J12040150 were classified as T5.5, whereas ULAS J11520359 turned out to be a T6 dwarf.

Using the relationship between spectral types and absolute magnitudes determined by Marocco et al. (marocco10 ()) from L0-T9 dwarfs with known trigonometric parallaxes (excluding known and possible binaries), we get 14.70, 14.92, 14.99 for T5.5 and 14.92, 15.22, 15.32 for T6 dwarfs. Assuming conservatively absolute magnitude uncertainties of about 0.4 mag, involving our 0.5 sub-types classification, we get spectroscopic distances of 255 pc, 316 pc, and 276 pc, respectively for ULAS J09540623, ULAS J11520359, and ULAS J12040150.

7 Discussion and conclusions

Searching HPM T5 dwarfs in the latest UKIDSS LAS/GCS data we detected some previously known objects, including nine recent WISE discoveries (K11), and revised their proper motions. For the very nearby T8 dwarf WISE J02540223 (S11, L11, K11) we derived an improved trigonometric parallax. The coolest known objects in our sample, two recently classified Y0 dwarfs (C11), were either too faint to be detected or have a doubtful counterpart in the UKIDSS GCS.

We found new late-T candidates in the UKIDSS LAS and confirmed their T5 types by LBT/LUCIFER1 NIR spectroscopy. ULAS J09540623, ULAS J11520359, and ULAS J12040150 are classified as T5.5, T6, and T5.5 dwarfs, respectively, all residing within 30 pc from the sun. From their total proper motions of 650, 441, and 432 mas/yr, we compute tangential velocities of 7715, 6513, and 5512 km/s, respectively. These values are smaller than the 878 km/s of WISE J02540223 for which L11 found thin disk membership. We conclude that the new nearby cool BDs also belong kinematically to the thin disk population of the Galaxy.

More accurate proper motions allow a more reliable determination of companionship to nearby stars. We checked the previously known objects (with our new proper motions) and our newly discovered ones for possible wide companions with common proper motions in SIMBAD17 or in the database on L and T dwarfs provided by G11. Using a search radius of one degree around the target positions, we find no bright or faint wide companion candidate for which the proper motion agrees within the error bars and the projected separation is smaller than about 10000 AU, if our estimated (or known) distances are taken into acount.

With our HPM- and colour-based survey, we have added three relatively nearby objects to the pool of 40 previous UKIDSS discoveries of cool (T5) BDs (Warren et al. (warren07 (); Lodieu et al. lodieu07 (); Chiu et al. chiu08 (); Pinfield et al. pinfield08 (); Lodieu et al. lodieu09 (); Lucas et al. lucas10 (); Burningham et al. burningham10 ()). Most of them were identified based on their colours without proper motion measurements. Our three new objects are among the five brightest (in ) of all T5 UKIDSS discoveries.

Acknowledgements.
The authors thank Roland Gredel, Jochen Heidt, Jaron Kurk, Ric Davies, and all observers at the LBT for assistance during the preparation and execution of LUCIFER observations, and Adam Burgasser for providing template spectra at http://pono.ucsd.edu/adam/browndwarfs/spexprism. This research has made use of the WFCAM Science Archive providing UKIDSS, the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and of data products from WISE, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration, from 2MASS, and from SDSS DR7 and DR8. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy. The SDSS-III web site is http://www.sdss3.org/. We used SIMBAD and VizieR at the CDS/Strasbourg.

Footnotes

  1. thanks: based on observations with the Large Binocular Telescope (LBT)
  2. email: rdscholz@aip.de, gbihain@aip.de, oschnurr@aip.de, jstorm@aip.de
  3. email: rdscholz@aip.de, gbihain@aip.de, oschnurr@aip.de, jstorm@aip.de
  4. email: rdscholz@aip.de, gbihain@aip.de, oschnurr@aip.de, jstorm@aip.de
  5. email: rdscholz@aip.de, gbihain@aip.de, oschnurr@aip.de, jstorm@aip.de
  6. The UKIDSS project is defined in Lawrence et al. (lawrence07 ()). UKIDSS uses the UKIRT Wide Field Camera (WFCAM; Casali et al. casali07 ()) and a photometric system described in Hewett et al. (hewett06 ()) which is situated in the Mauna Kea Observatories (MKO) system (Tokunaga et al. tokunaga02 ()). The pipeline processing and science archive are described in Hambly et al. (hambly08 ()) and Irwin et al. (irwin12 ()).
  7. http://vizier.u-strasbg.fr/cgi-bin/VizieR
  8. http://irsa.ipac.caltech.edu/applications/FinderChart/
  9. , , and bands are centered at 3.4 m, 4.6 m, and 12 m
  10. Table 1 is available electronically only
  11. Tables 2, 3, and 4 are available electronically only
  12. - additional epoch in D08, - more data in S11 (SDSS DR8, 2MASS), L11 (Pan-STARRS1), - WISE and Spitzer epochs in K11, - seen in WISE image but no catalogue entry, - our visual detection. IDs are run and multiframe numbers for SDSS and UKIDSS LAS (ULAS), respectively.
  13. - additional WISE and Spitzer epochs in K11, - doubtful counterpart. ID are run and multiframe numbers for SDSS DR8 and UKIDSS GCS (UGCS), respectively.
  14. ID are run and multiframe numbers for SDSS and UKIDSS LAS (ULAS), respectively. indicate our visual detections.
  15. IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
  16. Table 5 is available electronically only
  17. http://simbad.u-strasbg.fr

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