UKIDSS detections of cool brown dwarfs - proper motions of 14 known T5 dwarfs and discovery of three new
Key Words.:Astrometry – Proper motions – Stars: distances – Stars: kinematics and dynamics – brown dwarfs – solar neighbourhood
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.
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.
optical surveys like the Sloan Digital Sky Survey
data releases (SDSS DR7, Abazajian et al. abazajian09 ();
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
like the Large Area Survey (LAS) within the UKIRT InfraRed Deep Sky Surveys
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
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
For additional survey data we looked at 2MASS and other catalogues accessible
via the CDS
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 1
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.
|previously measured proper motion||new proper motion (this work)|
|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|
|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|
|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|
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 ()).
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).
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.
|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)|
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
using standard routines in IRAF
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 SIMBAD
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.
- thanks: based on observations with the Large Binocular Telescope (LBT)
- email: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
- email: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
- email: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
- email: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
- 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 ()).
- , , and bands are centered at 3.4 m, 4.6 m, and 12 m
- Table 1 is available electronically only
- Tables 2, 3, and 4 are available electronically only
- - 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.
- - additional WISE and Spitzer epochs in K11, - doubtful counterpart. ID are run and multiframe numbers for SDSS DR8 and UKIDSS GCS (UGCS), respectively.
- ID are run and multiframe numbers for SDSS and UKIDSS LAS (ULAS), respectively. indicate our visual detections.
- 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.
- Table 5 is available electronically only
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