WISEA J064750.85-154616.4: a new nearby L/T transition dwarf
AAstronomy & Astrophysics manuscript no. 24070 c (cid:13)
ESO 2018October 24, 2018
WISEA J064750.85-154616.4: a new nearby L/T transition dwarf (cid:63) (Research Note)
R.-D. Scholz, G. Bihain, and J. Storm
Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germanye-mail: [email protected], [email protected], [email protected]
Received 25 April 2014; accepted 4 June 2014
ABSTRACT
Aims.
Our aim is to detect and classify previously overlooked brown dwarfs in the solar neighbourhood.
Methods.
We performed a proper motion search among bright sources observed with the Wide-field Infrared Survey Explorer (WISE)that are also seen in the Two Micron All Sky Survey (2MASS). Our candidates appear according to their red J − K s colours asnearby late-L dwarf candidates. Low-resolution near-infrared (NIR) classification spectroscopy in the HK band allowed us to getspectroscopic distance and tangential velocity estimates. Results.
We have discovered a new L9.5 dwarf, WISEA J064750.85-154616.4, at a spectroscopic distance of about 14 pc and witha tangential velocity of about 11 km / s, typical of the Galactic thin disc population. We have confirmed another recently found L / Ttransition object at about 10 pc, WISEA J140533.13 + Key words.
Astrometry – Proper motions – Stars: distances – Stars: kinematics and dynamics – brown dwarfs – solar neighbourhood
1. Introduction
Proper motion searches continue to play an important role in thesearch for still missing cool neighbours of the sun. For findinghidden brown dwarfs (BDs) in the solar neighbourhood by theirhigh proper motion (HPM), it is essential to use near-infrared(NIR) and mid-infrared (MIR) multi-epoch observations. TheNIR observations of the Two Micron All Sky Survey (2MASS;Skrutskie et al. 2006) with observing epochs between 1997 and2001 have been successfully combined with recent ( ≈ d <
10 pc) new L / T and Mdwarf neighbours, which were observed but not discovered be-fore in 2MASS, were found by Luhman (2013) and Kirkpatricket al. (2014). The even cooler Y-type BDs discovered thanks toWISE observations (Cushing et al. 2011; Kirkpatrick et al. 2012,2013; Cushing et al. 2014) are not seen in 2MASS. The recentlydiscovered coolest BD (Luhman 2014b), ranging now as thethird fastest among all HPM objects and fourth nearest amongthe solar neighbours, is undetected in the J band down to 23rdmagnitude. (cid:63) based on observations with the Large Binocular Telescope (LBT) In their HPM search for previously overlooked (in 2MASS)nearby BDs, Scholz et al. (2011) and Bihain et al. (2013) con-centrated on relatively bright MIR ( w (cid:46)
13) BD candidates se-lected from the WISE preliminary and all-sky catalogues, re-spectively. They looked for their 2MASS counterparts with sig-nificant proper motion and colours typical of T dwarfs. Someof the candidates in the search of Bihain et al. (2013) exhib-ited relatively red NIR colours expected for late-L dwarfs. Herewe report the results of spectroscopic follow-up observations forthree of these candidates, one of which turned out to be a newL9.5 dwarf within about 15 pc. For all objects described in thisresearch note, we use their AllWISE (Kirkpatrick et al. 2014)designations.
2. Candidate selection and proper motion
The new object WISEA J064750.85-154616.4 (hereafterWISEA J0647 − J − K s colour (Table 1), WISEA J0647 − I -band measurements of the Super-COSMOS Sky Surveys (SSS; Hambly et al. 2001). But it canbe seen in the corresponding image (top right in Fig. 1) at theexpected position at epoch 1983, close to the south-west inner Article number, page 1 of 5 a r X i v : . [ a s t r o - ph . S R ] J un & A proofs: manuscript no. 24070
Fig. 2.
Proper motion and parallax solutions using the software of Gudehus (2001): a) proper motion including the DENIS position with large errorbars, b) proper motion from only four accurate positions (adopted solution), c) combined proper motion and parallax solution using all availablepositions.
Fig. 1.
DSS R and I , 2MASS J and K s , and WISE w w × , north is up, east to the left) forWISEA J0647 − J band. border of the circle. On the other hand, its proper motion leadsto a shifted position in north-east direction with respect to thecircle centre at the later WISE epoch (bottom row).The target also appears as a very faint object in the DEepNear-Infrared Survey (DENIS; Epchtein et al. 1997). However,the DENIS position deviates by about 0.8 arcsec from the ex-pected position at the DENIS observing epoch. The latter is inaddition not known exactly (see footnote in Table 1). We cor-rected the DENIS position of the target for the mean o ff set ofthree reference stars close to the target (marked in J -band imageof Fig. 1) with respect to their 2MASS positions, as we also didfor the positions measured in the LBT acquisition image. Thesethree stars have no significant proper motions ( <
10 mas / yr) inthe PPMXL (Röser, Demleitner & Schilbach 2010).The corrected DENIS position (Table 1) is still o ff by about0.5 arcsec in right ascension and 0.3 arcsec in declination fromthe expected position (see left and right panels in Fig. 2). There-fore we tried to measure the target position in the DENIS J and K s FITS images, but failed because the target image isalmost completely buried in noise. We assigned large errorbars ( ±
300 mas) to the (corrected) DENIS catalogue position,whereas the errors of the 2MASS, LBT, and two WISE posi-tions range between 60 and 100 mas. The linear proper motionfit with and without the DENIS position does not change sig-nificantly, but the proper motion error in the solution withoutDENIS is much smaller so that we adopted this as our final so-lution (Table 1, Fig. 2, central panel). The four precise positionsare consistent with a purely linear motion. They are also consis-tent with a combined proper motion and parallax solution, wherethe parallax and its error are of the order of 50 mas (Fig. 2 rightpanel). However, we consider this parallax as unreliable not onlydue to the large formal error but also because of the small num-ber of positions, their partly similar seasons, and the di ff erentNIR and MIR observations used.The multi-epoch positions, the determined proper mo-tion, and the available NIR and MIR photometry ofWISEA J0647 − µ α cos δ =+ ± µ δ =+ ±
66 mas / yr. Because of the very small time interval be-tween the WISE observations of about six months, it is much lessprecise and probably more a ff ected by parallactic motion thanour proper motion. Article number, page 2 of 5.-D. Scholz et al.: New L / T transition dwarf (RN)
3. Near-infrared spectroscopic classification
Our target WISEA J0647 − + + ± + + J − K s > K s <
14, we used the NIR spectrographLUCI 1 (Mandel et al. 2008; Seifert et al. 2010; Ageorgeset al. 2010) in long-slit spectroscopic mode only with the HK (200 lines / mm + order separation filter) grating. Thethree objects, WISEA J0647 − + + K s filter with exposure times of 15, 5, and 15 sec-onds, respectively. The seeing was ≈ / LUCI spectra of the new ob-ject WISEA J0647 − + − H -band spectrum fits slightly better with the T0 than withthe L9 standard spectrum, whereas in the K band it lies rightin between the L9 and T0 standard spectra. All three measuredspectral indices (Table 1) are consistent with a spectral type ofT0 as shown by Burgasser et al. (2006, his Table 5). The spec-tral index / spectral type relation for the H O- H index provided byBurgasser (2007, their Table 3) leads to a spectral type of L9.3,the relation for the CH - H index is only applicable in the T1-T8range, and that for the CH - K index yields an L9.5 type. Due tothe good agreement between the results from direct comparisonwith standard spectra and from the spectral indices, we classi-fied WISEA J0647 − JHK s magnitudes of L9 and T0 dwarfs andthe absolute w w − w
1) to 14.8 pc (from H ) with a mean distance of13.9 + . − . pc, where we conservatively assumed a large uncertaintyof 0.5 mag in the absolute magnitude. The resulting tangentialvelocity is about 11 km / s.The spectrum of WISEA J1405 + H band and well-fittedby L8 in K band), whereas Castro et al. (2013) classified it asL8 in the optical but L9 in the NIR. From three spectral indices,which we were able to measure for WISEA J1405 + O- H = - H = - K = + + Table 1.
Parameters of WISEA J0647 − Parameter WISEA J0647 − α (J2000) 06 47 50.7632MASS δ (J2000) −
15 46 18.002MASS epoch 1999.041DENIS α (J2000) a
06 47 50.812DENIS δ (J2000) a −
15 46 18.06DENIS epoch b α (J2000) 06 47 50.848WISE all-sky δ (J2000) −
15 46 16.55WISE all-sky epoch 2010.244WISE post-cryo α (J2000) c
06 47 50.858WISE post-cryo δ (J2000) c −
15 46 16.40WISE post-cryo epoch 2010.770LBT α (J2000) 06 47 50.880LBT δ (J2000) −
15 46 16.06LBT epoch 2014.036 µ α cos δ (mas / yr) + ± µ δ (mas / yr) + ± J (mag) 15.31 ± H (mag) 14.29 ± K s (mag) 13.74 ± J (mag) 16.04 ± K s (mag) 13.90 ± w ± w ± w ± O- H (SpT) d - H (SpT) d - K (SpT) d ± d spec (pc) 13.9 + . − . v tan (km / s) 11 + − Notes. ( a ) The original DENIS coordinates of the target were correctedfor the mean o ff sets of three reference stars around the target with re-spect to their 2MASS coordinates. The DENIS data were not used forthe final proper motion determination. ( b ) The DENIS catalogue givesan epoch of 2000.47, whereas according to the DENIS FITS images it is2001.47. ( c ) Mean position from 15 single exposures. ( d ) Spectral index(and corresponding spectral type) as defined in Burgasser et al. (2006).
WISEA J0421 + µ α cos δ , µ δ ) = ( + ± − ±
11) mas / yr. This translates to a tangential velocity of morethan about 100 km / s, if we assume WISEA J0421 +
4. Conclusions and discussion
Best et al. (2013) have shown that previous surveys for L / Ttransition dwarfs have been incomplete. We were able to dis-cover a new L / T transition dwarf within about 15 pc fromthe sun. With its spectral type of L9.5 that we found fromLBT / LUCI HK -band spectroscopy, WISEA J0647 − + = WISEA J032842.65 + ± Article number, page 3 of 5 & A proofs: manuscript no. 24070
Fig. 3.
LBT / LUCI spectra (black) ofWISEA J0647 − + + − + + ff ected by strongresiduals of telluric absorption correction inthe region of 2.00..2.03 micrometers (dottedline), as the A0V standard star used for thecorrection was observed at higher airmass thanthe target. L9.5 dwarf WISEA J0647 − K s ≈ + = WISEA J092055.41 + − / T transition objects, the new L9.5dwarf WISEA J0647 − − H band and very red J − K s (cid:38) − / T transition objectslisted in Dupuy & Liu (2012).For WISEA J1405 + J -band spectroscopy. In their Fig. 5, they alsoshowed an HK -band spectrum, which they described as beingconsistent with the L9 type. They used the same L8 and L9standards for comparison as we did in Fig. 3. We find their HK -band spectrum fits in between the L8 and L9 standards,whereas ours fits better with the L8 standard as do our mea-sured spectral indices. Nevertheless, our bad-weather spectrumof WISEA J1405 + HK -band spectrum so that we think their more accurate classification in the J bandis more reliable.For the reddened star WISEA J0421 + + < M0.
Acknowledgements.
The authors thank Jochen Heidt, Barry Rothberg, and theobservers at the LBT, D. Rosario and E. Wuyts, for assistance during the prepa-ration and execution of LUCI observations, Adam Burgasser for providing tem-plate spectra at http: // pono.ucsd.edu / ∼ adam / browndwarfs / spexprism and the ref-eree, Étienne Artigau, for helpful hints, mainly concerning the discussion of ourresults. This research has made use of the NASA / IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Institute of Tech-nology, under contract with the National Aeronautics and Space Administration,of data products from WISE, which is a joint project of the University of Califor-nia, Los Angeles, and the Jet Propulsion Laboratory / California Institute of Tech-nology, funded by the National Aeronautics and Space Administration, and from2MASS. We have also extensively used the M, L, T, and Y dwarf compendiumhoused at DwarfArchives.org, the SIMBAD database and the VizieR catalogueaccess tool operated at the CDS / Strasbourg, including the CDS xMatch service.
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