NEW M, L, AND T DWARF COMPANIONS TO NEARBY STARS FROM THE WIDE-FIELD INFRARED SURVEY EXPLORER^*

Through our 2MASS/WISE proper motion analysis, we identified 2MASS J03184214+0828002 (hereafter 2M 0318+0828) as a possible companion to the 6'' binary system LSPM J0318+0827S/N (Lépine & Shara 2005). The brighter component of the binary, LSPM J0318+0827S, has a spectral type of M3. LSPM J0318+0827N is unclassified, but it is probably later than the primary by ∼1 subclass based on the relative fluxes of the pair (ΔK_s = 1.0 mag). We estimated a distance of 23 pc for LSPM J0318+0827S by combining its 2MASS photometry with the typical absolute magnitudes for an M3 dwarf (Kraus & Hillenbrand 2007). At this distance, the 75'' angular separation between 2M 0318+0828 and LSPM J0318+0827S corresponds to 1700 AU. The proper motion of 2M 0318+0828 measured from 2MASS and WISE agrees with that of LSPM J0318+0827S/N (Table 1), i.e., their 1σ errors overlap. The colors and magnitudes of 2M 0318+0828 are consistent with those expected for a late-M or early-L dwarf at the distance of the primary, as shown in Figures 1 and 2. Spectroscopy is needed to confirm that it has a spectral type in this range.

Our examination of WISE sources lacking 2MASS counterparts uncovered WISE J041328.73+821854.7 (hereafter WISE 0413+8218) as a candidate companion to LHS 1643. Law et al. (2008) estimated a spectral type of M4.5 for the latter from a measurement of V − K. The 2MASS photometry of LHS 1643 would then imply a distance of 22 pc. If this distance is adopted for WISE 0413+8218, its position in M_W2 versus W1 − W2 is consistent with that of a T dwarf, as illustrated in Figure 1 (W1 − W2 = 1.5), and its angular separation of 215'' would correspond to 4700 AU. Because this separation is small enough that companionship is plausible (≲10,000 AU; Lépine & Bongiorno 2007), we pursued follow-up imaging of WISE 0413+8218 to measure its proper motion and better constrain its spectral type. Using the WIYN High-Resolution Infrared Camera (WHIRC), we obtained nine dithered 30 s exposures of WISE 0413+8218 through a J-band filter on 2011 October 11. Only a weak detection of WISE 0413+8218 appeared in the reduced mosaic of these data, but it was sufficient to demonstrate that the candidate is much redder in J − W2 than expected for a T dwarf with its value of W1 − W2 (Figure 1). In addition, no motion was detected for WISE 0413+8218 between the WISE and WHIRC images, which is marginally inconsistent with companionship (Table 1). Therefore, we conclude that WISE 0413+8218 is probably not a companion to LHS 1643.

The primary for our next candidate companion is LHS 6176. Because this star has not been previously classified, we performed spectroscopy on it with SpeX (Rayner et al. 2003) at the NASA Infrared Telescope Facility (IRTF). On the night of 2011 April 23, we observed LHS 6176 in the SXD mode with a 08 slit, which provided a wavelength coverage of 0.8–2.5 μm and a resolution of R = 750. The data were reduced with the Spextool package (Cushing et al. 2004) and corrected for telluric absorption (Vacca et al. 2003). The reduced spectrum is shown in Figure 3, where it has been smoothed to the resolution of the SpeX prism data that we have collected for some of our other candidates (R = 150). By comparing its spectrum to SpeX data for standard dwarfs (Cushing et al. 2005; Rayner et al. 2009), we measured a spectral type of M3.5V±1 for LHS 6176. We estimated a distance of 27 pc based on this classification and the photometry from 2MASS. Proper motions have been previously measured for LHS 6176 using DSS images (Bakos et al. 2002; Lépine & Shara 2005). However, it is blended with another star in the images from the Second Palomar Sky Survey, which may have introduced errors into those data. Therefore, we have performed a new measurement using the first-epoch Palomar Sky Survey and archival Spitzer 4.5 μm images from 2011 (Table 1).

Zoom In Zoom Out Reset image size

The candidate companion to LHS 6176 is ULAS J095047.28+011734.3 (hereafter ULAS 0950+0117). B. Burningham (2012, in preparation) discovered this object using UKIDSS data and classified it as a T8 dwarf. We identified it as a possible companion through two separate surveys. In the first one, we searched for common proper motion companions using multi-epoch images from Spitzer (Luhman et al. 2011). Because ULAS 0950+0117 was observed by Spitzer in 2010 (program 60093, S. Leggett), we selected it for a second epoch of imaging with the satellite in 2011 (program 70021, K. Luhman). By comparing the astrometry for all objects detected in both epochs, we found the proper motion of LHS 6176 was similar to that of ULAS 0950+0117. We later recovered ULAS 0950+0117 as a candidate companion during our analysis of WISE sources lacking 2MASS counterparts that are in close proximity to nearby stars like LHS 6176 (Section 2).

ULAS 0950+0117 and LHS 6176 have an angular separation of 52'', corresponding to 1400 AU at the distance of the latter. UKIDSS provides the earliest astrometry of ULAS 0950+0117. It was detected in H and K in 2006 and 2007, but the signal-to-noise ratios were low. Therefore, we use the J-band data from 2008 February 23 for the first epoch in our measurement of the proper motion of ULAS 0950+0117. For the second epoch, we used the most recent publicly available images that were taken near the same time of year as the first epoch to minimize parallactic motion, which consist of J-band data acquired on the night of 2012 March 3 with SOFI on the European Southern Observatory New Technology Telescope (NTT, program 186.C-0756, R. Smart). The resulting proper motion measurement for ULAS 0950+0117 is similar to that of LHS 6176 such that their 1σ error bars overlap (Table 1). The diagram of M_W2 versus spectral type in Figure 2 shows that ULAS 0950+0117 is brighter than expected for a T8 dwarf at the spectrophotometric distance of LHS 6176, which may indicate that it is an unresolved binary. ULAS 0950+0117 has been imaged several times over the last few years with both the NTT and the Canada–France–Hawaii Telescope (CFHT) in an effort to measure its parallax (Marocco et al. 2010; Dupuy & Liu 2012), which will eventually provide an additional test of its companionship and overluminous nature.

2MASS J10313234−5338010 (hereafter 2M 1031−5338) was found as a possible companion to HD 91324 (F9V, 21.8 pc) through our 2MASS/WISE proper motion survey. The angular separation of the two stars is 309'' (6700 AU). Our proper motion measurement for 2M 1031−5338 from 2MASS and WISE is within 1σ of that of HD 91324 (Table 1). Using Figures 1 and 2, we find that the photometry of 2M 1031−5338 is indicative of an M5–M6 dwarf near the distance of HD 91324. As with 2M 0318+0828, spectroscopy is needed to verify this spectral type.

We identified WISE J142320.84+011638.0 (hereafter WISE 1423+0116) as a candidate companion to LHS 2907 (BD+01°2920) from among WISE sources lacking 2MASS counterparts. LHS 2907 is a moderately metal-poor solar-type star ([Fe/H] ∼−0.35; Valenti & Fischer 2005; Casagrande et al. 2011). The pair has an angular separation of 156'' or 2680 AU at the distance of the primary (17.2 pc). After checking data archives from various surveys and observatories, we found detections of WISE 1423+0116 in Y- and J-band images from UKIDSS and J-band images from WIRCam at the CFHT (program 09BD95, L. Albert). The values of W2, W1 − W2, J − W2 are consistent with a late T dwarf at the distance of LHS 2907 (Figure 1). Using the images from UKIDSS (2008 May 16) and CFHT (2009 December 29), we measured the proper motion of WISE 1423+0116, which was found to be similar to that of LHS 2907 (Table 1). Because of this promising evidence that WISE 1423+0116 was a T dwarf companion, we obtained a spectrum of it with SpeX at the IRTF. These data were collected in the prism mode with a 08 slit, providing a wavelength coverage of 0.8–2.5 μm and a resolution of R = 150. The spectrum exhibits the strong absorption bands that characterize late T dwarfs, as illustrated in Figure 3 where we compare it to data for a standard T8 dwarf (2MASS J04151954−0935066; Burgasser et al. 2004). During their independent discovery of WISE 1423+0116, Pinfield et al. (2012) obtained a spectrum with higher signal-to-noise ratio, producing a more accurate classification than is possible with our data. Using their spectral type of T8 and the distance of the primary, WISE 1423+0116 falls within the sequence of T dwarfs in the diagram of M_W2 versus spectral type in Figure 2, which supports its companionship.

Our 2MASS/WISE proper motion search uncovered 2MASS J14351087−2333025 (hereafter 2M 1435−2333) as a possible companion to 2MASS J14345819−2335572 (hereafter 2M 1434−2335). The latter has a spectral type of M7V and a corresponding distance of 27 pc. The pair had an angular separation of 247'' in the 2MASS images (6700 AU). The proper motion that we measure for 2M 1435−2333 using the first-epoch Palomar Sky Survey, 2MASS, and WISE (Table 1) agrees with the value of (μ_α, μ_δ) = (−306 ± 30, +51 ± 5) mas yr⁻¹ for 2M 1434−2335 from Faherty et al. (2009). To further characterize 2M 1435−2333, we obtained a spectrum of it using the prism mode of SpeX on the night of 2012 May 6. The spectrum matches our SpeX data for the M8V standard 2MASS J03205965+1854233 (Kirkpatrick et al. 1995), as shown in Figure 3. Our classification of M8V ± 0.5 is roughly consistent with companionship given its photometry. However, when we measure the proper motion of 2M 1434−2335 in the same manner as for 2M 1435−2333, we arrive at (μ_α, μ_δ)=(−314, −19) ± 15 mas yr⁻¹, which differs from the motion from Faherty et al. (2009). Thus, the motions that we measure for the components of this pair indicate that they are not companions.

2MASS J16220644+0101156 (hereafter 2M 1622+0101) is a candidate companion to HD 147449 (F0V, 27.3 pc) based on our 2MASS/WISE proper motion analysis. 2M 1622+0101 is relatively close to HD 147449 with an angular separation of only 43'' (1200 AU). The proper motion of 2M 1622+0101 that we measured from 2MASS and WISE agrees with that of HD 147449 (Table 1). To measure the spectral type of 2M 1622+0101, we performed spectroscopy on it with SpeX in its SXD mode (R = 750) on the night of 2012 April 24. Through comparison to SpeX data for standard stars (Cushing et al. 2005; Rayner et al. 2009), we classified 2M 1622+0101 as M2V±1. The spectrum of 2M 1622+0101 is plotted with data for the M2V star Gl 411 in Figure 3, both of which have been smoothed to the resolution of the prism spectra (R = 150). Our classification is consistent with the expected spectral type based on its photometry if it has the same distance as HD 147449. Because of its relatively early spectral type, 2M 1622+0101 does not appear within the boundaries of Figures 1 and 2.

Using 2MASS and WISE, we identified 2MASS J17430860+8526594 (hereafter 2M 1743+8526) as a candidate companion to G259-20. Neither a spectral type nor a parallax has been measured previously for G259-20, but its optical and IR photometry are indicative of an early-M dwarf at 20–30 pc. The proper motion of 2M 1743+8526 based on 2MASS and WISE agrees with that of G259-20 (Table 1) and the pair has a relatively small separation (30'', 650 AU). In addition, the photometry of 2M 1743+8526 is consistent with that of a late-type object near the distance of G259-20 (Figure 1). Because of the promising nature of this candidate, we pursued spectroscopy of it and G259-20 with the near-infrared spectrometer (NIRSPEC; McLean et al. 1998, 2000) on the 10 m telescope at W. M. Keck Observatory on the night of 2012 June 7. The instrument was operated in its low-resolution mode with the N3 filter and 057 slit, which provides spectra across the J band with a resolution of R = 1700. The data were reduced with the REDSPEC package (McLean et al. 2003) and the resulting spectra are presented in Figure 4. We measured spectral types of M2.5V ± 1 and L5 ± 1 for G259-20 and 2M 1743+8526, respectively, using spectra of standards obtained with SpeX (Cushing et al. 2005; Rayner et al. 2009) and NIRSPEC (McLean et al. 2003). Based on the metallicities of the early-M SpeX standards (e.g., Rojas-Ayala et al. 2012), we estimate that G259-20 has [Fe/H] ≈−0.2 to 0, which is relevant to the interpretation of unusual colors for 2M 1743+8526 (Section 4). In Figure 4, we show the standards that produced the best matches, consisting of Gl 581 (M2.5V) for the primary and DENIS-P J1228.2−1547 (L5) for the secondary. When combined with its 2MASS photometry, a spectral type of M2.5V for G259-20 implies a distance of 22 pc. 2M 1743+8526 is plotted on the diagram of M_W2 versus spectral type in Figure 2 assuming this distance, which places it within the sequence of known L dwarfs, as expected for a companion.

Zoom In Zoom Out Reset image size

We identified WISE J180901.07+383805.4 (hereafter WISE 1809+3838) as a candidate companion to HD 166620 (K2V, 11.0 pc) from among WISE sources that do not appear in 2MASS. The angular separation is 769'', or 8460 AU at the distance of HD 166620. Given its WISE photometry (W1 − W2 > 2.8), WISE 1809+3838 could be a companion to HD 166620 with a spectral type of >T9, as shown in Figure 1. We found a detection of WISE 1809+3838 in archival J-band images obtained by WIRCam at the CFHT (program 08BD95, L. Albert) on the night of 2008 October 12. The photometry that we measured from these data indicates a color of J − W2 = 2.2, which is bluer than expected for a >T9 companion. Nevertheless, to investigate the nature of WISE 1809+3838 further, we observed it with SpeX in the prism mode on the night of 2011 June 28. The resulting spectrum is presented in Figure 3, where we find a good match to the T7 standard 2MASS J07271824+1710012 (Burgasser et al. 2006). The uncertainty in this classification is ±1 subclass. If WISE 1809+3838 has an absolute magnitude in W2 that is similar to that of typical T7 dwarfs, then it should have a distance of roughly 28 pc, indicating that it is probably not a companion to HD 166620. In addition to the spectroscopic data, we obtained J-band images of WISE 1809+3838 with the slit-viewing camera on SpeX. The proper motion of WISE 1809+3838 measured from the SpeX and WIRCam data is inconsistent with that of HD 166620 (Table 1), confirming that the pair is not a binary system.

2MASS J18525777−5708141 (hereafter 2M 1852−5708) was found as a candidate companion to LHS 3421 (M2.5V, 25.9 pc) through our 2MASS/WISE proper motion search. The pair has an angular separation of 58'' (1500 AU). The proper motion of 2M 1031−5338 based on 2MASS and WISE is within 1σ of that of LHS 3421 (Table 1). The colors and magnitudes of 2M 1852−5708 are consistent with those of a dwarf between M5 and M6 at the distance of the primary (Figures 1 and 2).

We uncovered 2MASS J20103539+0634367 (hereafter 2M 2010+0634) as a candidate companion to the M-type spectroscopic binary LSPM J2010+0632 with data from 2MASS and WISE. The latter has a distance of 15.3 pc based on its spectral type and photometry (Shkolnik et al. 2010). The angular separation of the pair is 143'' (2100 AU). The proper motion of 2M 2010+0634 derived from 2MASS and WISE agrees with that of LSPM J2010+0632 (Table 1). The photometry of 2M 2010+0634 is also consistent with a spectral type of late M or early L at the distance of the primary (Figure 1). To verify its spectral type, we obtained a spectrum of 2M 2010+0634 with the Marcario Low-Resolution Spectrograph (LRS) on the Hobby–Eberly Telescope (HET) on the night of 2011 August 26. The instrument was operated with the G3 grism and the 2'' slit, which provided a wavelength coverage of 6200–9100 Å and a resolution of R = 1100. As illustrated in Figure 5, the spectrum agrees well with a combined spectrum of VB 10 (M8V), LHS 2243 (M8V), and LHS 2243 (M9V) in which the latter was given twice the weighting of each of the other two stars. Therefore, we classified 2M 2010+0634 as M8.5V ± 0.5. Using this spectral type and the distance of the primary, 2M 2010+0634 appears within the sequence of M dwarfs in Figure 2, which supports its companionship.

Zoom In Zoom Out Reset image size