# Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs

C. Reylé, R.-D. Scholz, M. Schultheis, A.C. Robin, M. Irwin

► To cite this version:

C. Reylé, R.-D. Scholz, M. Schultheis, A.C. Robin, M. Irwin. Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs. Monthly Notices of the Royal Astronomical Society, 2006, 373(2), pp.705-714. 10.1111/j.1365-2966.2006.11051.x. hal-00131477

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# Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs

C. Reylé,<fnref n="1" />* R.-D. Scholz,<fnref n="2" /> M. Schultheis,<fnref n="1" /> A. C. Robin<fnref n="1" /> and M. Irwin<fnref n="3" />

Accepted 2006 September 8. Received 2006 September 8; in original form 2006 July 27

## ABSTRACT

We present spectra of 59 nearby star candidates, M dwarfs and white dwarfs, previously identified using high proper motion catalogues and the DENIS database. We review the existing spectral classification schemes and spectroscopic parallax calibrations in the near-infrared J band and derive spectral types and distances of the nearby candidates. Forty-two stars have spectroscopic distances smaller than 25 pc, three of them being white dwarfs. Two targets lie within 10 pc, one M8 star at 10.0 pc (APMPM J0103–3738), and one M4 star at 8.3 pc (L 225–57). One star, LHS 73, is found to be among the few subdwarfs lying within 20 pc. Furthermore, together with LHS 72, it probably belongs to the closest pair of subdwarfs we know.

**Key words:** stars: late-type – subdwarfs – white dwarfs – solar neighbourhood.

## 1 INTRODUCTION

Recent discoveries of cool objects, such as M stars or brown dwarfs, closer than 5 pc show that even the immediate solar neighbourhood sample is still incomplete (Delfosse et al. 2001; Scholz et al. 2003; Teegarden et al. 2003; Hambly et al. 2004).

Henry et al. (1997) estimated that about 130 systems out of 359 (36 per cent) are missing within 10 pc. The missing fraction is even larger within 25 pc (63 per cent) with a deficit of about 3500 systems out of the 5500 expected ones (Henry et al. 2002). Statistical comparisons from the local sample in the Northern hemisphere led to a less-pessimistic result, the current 10-pc sample being ~75 per cent complete (Reid et al. 2003a).

New surveys in the near-infrared (NIR), such as the DENIS (Epchtein et al. 1997) and the 2MASS (Cutri et al. 2003), provide unprecedented data for a systematic search for low-luminosity cool dwarfs. The use of these data together with high proper motion catalogues is a powerful tool for discovering our neighbours. Hundreds of stars closer than 25 pc have been discovered this way (e.g. Phan-Bao et al. 2001, 2003; Reid & Cruz 2002; Reid, Kilkenny & Cruz 2002; Reylé et al. 2002; Reid et al. 2003b, 2004; Hambly et al. 2004; Reylé & Robin 2004; Lodieu et al. 2005; Scholz, Meusinger & Jahreiß 2005).

As spectroscopy provides much more information than photometry alone, spectroscopic observations were also carried out to identify and classify nearby stars. Recent studies dealing with sample sizes larger than 10 objects are cited in Table 1. All together, they

revealed 490 stars in the 25 pc sample and 25 within 10 pc. However, we note that the samples investigated by the different authors do have many stars in common so that the total number of newly discovered neighbours is much smaller.

In previous papers (Reylé et al. 2002; Reylé & Robin 2004), we have determined the photometric distances of high proper motion stars that we cross-identified with the DENIS survey. We have reported the discovery of 115 nearby candidates, probably lying within 25 pc, the limit of the Catalogue of Nearby Stars (CNS3 Gliese & Jahreiss 1991). We selected the closest candidates for a spectroscopic follow-up performed at the La Silla Observatory (Chile).

Our sample of nearby candidates is described in Section 2. Section 3 describes the spectroscopic observations. In Section 4, we review the spectroscopic classification schemes and compute the spectral type of the stars. Computation of distances is detailed in Section 5. In Section 6, we discuss the chromospheric activity of the stars in our sample and the conclusions are given in Section 7.

## 2 NEARBY CANDIDATE SAMPLE

Due to their small distance, most of the nearby stars have high proper motions. However, many stars in high proper motion catalogues have no distance determined yet. Note that a high proper motion does not imply a short distance but may also indicate a high space velocity with respect to the Sun.

As a contribution towards improving the completeness of the solar neighbourhood census, we made a systematic search for nearby stars among two new high proper motion catalogues. The cross-identification of those with the DENIS database allowed us to

<footnote n="1">¹CNRS UMR6091, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France</footnote>

<footnote n="2">²Astrophysikalisches Institut Potsdam, An der Sternwarte 16, Potsdam D-14482, Germany</footnote>

<footnote n="3">³Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA</footnote>

<footnote>*E-mail: celine@obs-besancon.fr</footnote>
<table><caption>**Table 1.** Nearby stars identified by spectroscopy.</caption><thead><tr><th>Reference</th><th>Sample size</th><th>Number of stars within 25 pc</th><th>Number of stars within 10 pc</th></tr></thead><tbody><tr><td>Cruz & Reid (2002)</td><td>70</td><td>28</td><td></td></tr><tr><td>Henry et al. (2002)</td><td>34</td><td>6</td><td>2</td></tr><tr><td>Reid et al. (2003b)</td><td>357</td><td>127</td><td>9</td></tr><tr><td>Lodieu et al. (2005)</td><td>71</td><td>25</td><td>3</td></tr><tr><td>Crifo et al. (2005)</td><td>39</td><td>31</td><td>1</td></tr><tr><td>Scholz et al. (2005)</td><td>322</td><td>226</td><td>8</td></tr><tr><td>Phan-Bao & Bessel (2006)</td><td>45</td><td>5</td><td></td></tr><tr><td>This study</td><td>59</td><td>42</td><td>2</td></tr></tbody></table>

determine the distances of the stars from NIR photometry, an appropriate wavelength range for M-type dwarfs.

In Reylé et al. (2002), we present preliminary distance estimates for 301 Automatic Plate Measuring (APM) high proper motion stars (Scholz et al. 2000). Fifteen stars were found to be within the 25 pc limit of the CNS3 (Gliese & Jahreiss 1991). In Reylé & Robin (2004), we used the Liverpool–Edinburgh high proper motion survey (LEHPMS; Pokorny, Jones & Hambly 2003) and found 100 stars that probably lie within 25 pc from the Sun. They are mainly M dwarf candidates, with the exception of few white dwarf candidates.

Given the large uncertainties on the photometric distances (up to 45 per cent in some cases), follow-up observations of these candidates were needed. We selected 54 stars, with photometric distances smaller than 20 pc, for spectroscopic follow-up. Using the reduced proper motion versus colour diagram, we showed that these objects are likely M dwarfs, except five white dwarf candidates. We also observed four stars that could be either disc or thick-disc M dwarfs,

for which we determined two photometric distances with two metallicity hypotheses. They are nearby candidates if they belong to the thick disc. Finally, we obtained spectra for one star that may be a nearby subdwarf and one star detected in the *J* and *K* bands, but not in the *I* band. Optical and NIR DENIS photometry, proper motion, as well as photometric distance determination are given in Tables 2 and 3 for the APM and LEHPMS nearby candidates, respectively. Fig. 1 shows their reduced proper motion versus colour diagram, where the reduced proper motion is $H = I + 5 \log \mu + 5$.

### 3 SPECTROSCOPIC OBSERVATIONS

In order to ensure the spectral types and therefore distances, we carried out spectroscopic observations on the 3.6-m New Technology Telescope (NTT) at the La Silla Observatory [European Southern Observatory (ESO), Chile] in 2003 November. Optical low-resolution spectra were obtained in the Red Imaging and Low-dispersion spectroscopy (RILD) observing mode with the EMMI instrument. The spectral dispersion of the grism we used is 0.28 nm pixel⁻¹, with a wavelength range 385–950 nm. Except for the white dwarf candidates, we used an order blocking filter to avoid the second-order overlap that occurs beyond 800 nm. Thus, the effective wavelength coverage ranges from 520 to 950 nm. The slit was 1 arcsec wide and the resulting resolution was 10.4 Å. The seeing varied from 0.5 to 1.5 arcsec. Exposure time ranged from 15 s for the brightest to 120 s for the faintest dwarf (*I* = 15.3). The reduction of the spectra was done using the context LONG of MIDAS. Fluxes were calibrated with the spectrophotometric standards LTT 2415 and Feige 110.

We obtained spectra for 59 nearby candidates. They are shown in Figs 2 and 3. In addition, we observed two radial velocity standard

<table><caption>**Table 2.** Nearby APM star candidates observed at the ESO/NTT. $B_J$ and $R$ are from the APM high proper motion catalogue, obtained on digitized plates. *I* and *I* − *J* are from the DENIS.</caption><thead><tr><th>Name</th><th>α<sub>J2000</sub></th><th>δ<sub>J2000</sub></th><th>ESO epoch</th><th>μ<sub>x</sub> ("yr<sup>-1</sup>)</th><th>μ<sub>y</sub></th><th>B<sub>J</sub></th><th>R</th><th>I</th><th>I - J</th><th>d<sub>phot</sub></th></tr></thead><tbody><tr><td>L 291-115</td><td>00 33 13.26</td><td>-47 33 17.8</td><td>1993.7</td><td>0.26</td><td>0.15</td><td>16.45</td><td>14.20</td><td>12.26</td><td>1.79</td><td>15.7</td></tr><tr><td>APMPM J0103-3738</td><td>01 02 49.77</td><td>-37 37 46.8</td><td>1990.7</td><td>1.44</td><td>0.30</td><td>19.73</td><td>17.30</td><td>13.83</td><td>2.67</td><td>12.2</td></tr><tr><td>APMPM J0145-3205</td><td>01 45 11.27</td><td>-32 05 09.6</td><td>1991.9</td><td>0.58</td><td>0.18</td><td>13.83</td><td>12.14</td><td>11.36</td><td>1.52</td><td>18.4</td></tr><tr><td>LP 940-20</td><td>01 49 42.37</td><td>-33 19 21.5</td><td>1991.9</td><td>0.38</td><td>0.11</td><td>16.60</td><td>14.36</td><td>12.65</td><td>1.76</td><td>19.6</td></tr><tr><td>LHS 5045</td><td>01 52 52.07</td><td>-48 05 39.2</td><td>1988.9</td><td>-0.55</td><td>-0.20</td><td>14.52</td><td>12.40</td><td>10.77</td><td>1.60</td><td>11.0</td></tr><tr><td>L 225-57</td><td>02 34 20.95</td><td>-53 05 35.4</td><td>1994.0</td><td>0.25</td><td>-0.34</td><td>12.88</td><td>10.78</td><td>9.79</td><td>1.49</td><td>9.6</td></tr><tr><td>L 127-33</td><td>02 40 36.99</td><td>-60 44 48.5</td><td>1991.9</td><td>0.19</td><td>0.29</td><td>15.35</td><td>13.85</td><td>12.34</td><td>1.13</td><td>116.7/30.4<sup>a</sup></td></tr><tr><td>APMPM J0255-5140</td><td>02 55 13.92</td><td>-51 40 23.1</td><td>1992.8</td><td>0.59</td><td>0.23</td><td>15.03</td><td>12.68</td><td>11.56</td><td>1.67</td><td>13.7</td></tr><tr><td>LP 831-45</td><td>03 14 17.96</td><td>-23 09 31.2</td><td>1993.0</td><td>0.35</td><td>0.19</td><td>13.29</td><td>11.46</td><td>9.90</td><td>1.44</td><td>11.7</td></tr><tr><td>LHS 5090</td><td>04 04 31.58</td><td>-62 59 10.4</td><td>1990.0</td><td>0.25</td><td>-0.46</td><td>16.34</td><td>14.49</td><td>12.85</td><td>1.25</td><td>103.6/25.5<sup>a</sup></td></tr><tr><td>LHS 1656</td><td>04 18 50.80</td><td>-57 14 06.0</td><td>1995.0</td><td>0.27</td><td>0.75</td><td>12.97</td><td>11.02</td><td>10.75</td><td>1.22</td><td>42.5/10.7<sup>a</sup></td></tr><tr><td>APMPM J0541-5349</td><td>05 41 27.18</td><td>-53 49 17.9</td><td>1991.1</td><td>0.10</td><td>0.37</td><td>13.61</td><td>11.82</td><td>11.77</td><td>1.13</td><td>90.3/23.6<sup>a</sup></td></tr><tr><td>LP 904-51</td><td>10 41 43.77</td><td>-31 11 52.7</td><td>1993.1</td><td>0.37</td><td>-0.25</td><td>16.80</td><td>15.10</td><td>12.84</td><td>1.83</td><td>19.3</td></tr><tr><td>APMPM J1107-2202</td><td>11 06 45.50</td><td>-22 02 16.2</td><td>1992.2</td><td>0.34</td><td>-0.27</td><td>13.92</td><td>11.87</td><td>11.48</td><td>1.56</td><td>16.8</td></tr><tr><td>APMPM J1932-4834</td><td>19 31 53.09</td><td>-48 33 50.5</td><td>1991.6</td><td>0.01</td><td>-0.37</td><td>16.21</td><td>14.15</td><td>12.39</td><td>1.85</td><td>15.4</td></tr><tr><td>APMPM J2101-4115</td><td>21 01 03.44</td><td>-41 14 31.8</td><td>1991.7</td><td>0.36</td><td>-0.25</td><td>15.40</td><td>13.46</td><td>11.50</td><td>1.55</td><td>17.5</td></tr><tr><td>APMPM J2101-4907</td><td>21 01 07.50</td><td>-49 07 23.7</td><td>1992.6</td><td>-0.31</td><td>-0.15</td><td>12.01</td><td>11.25</td><td>10.52</td><td>1.43</td><td>16.5</td></tr><tr><td>LTT 8708 A/B</td><td>21 49 11.00</td><td>-41 33 28.6</td><td>1990.7</td><td>0.30</td><td>-0.17</td><td>12.65</td><td>10.06</td><td>9.29</td><td>1.48</td><td>7.8</td></tr><tr><td>LHS 3800</td><td>22 23 08.90</td><td>-43 27 35.3</td><td>1990.8</td><td>0.79</td><td>-0.37</td><td>15.39</td><td>13.85</td><td>12.23</td><td>1.33</td><td>15.7</td></tr><tr><td>LHS 3842</td><td>22 40 57.59</td><td>-45 43 23.1</td><td>1992.6</td><td>0.33</td><td>-0.30</td><td>14.39</td><td>12.35</td><td>11.30</td><td>1.55</td><td>16.0</td></tr><tr><td>APMPM J2330-4737</td><td>23 30 16.48</td><td>-47 36 38.2</td><td>1992.9</td><td>-0.59</td><td>-0.95</td><td>19.13</td><td>17.19</td><td>13.70</td><td>2.38</td><td>15.0</td></tr><tr><td>APMPM J2352-3609<sup>b</sup></td><td>23 52 27.35</td><td>-36 09 12.2</td><td>1996.6</td><td>0.31</td><td>-0.37</td><td>18.55</td><td>16.60</td><td>J = 13.28</td><td>J - K<sub>s</sub> = 1.10</td><td>23.3</td></tr></tbody></table>

<footnote><sup>a</sup>Photometric distance estimate assuming that the star belongs to the disc/to the thick disc, obtained using the (M<sub>J</sub>, I − J) theoretical relation from Baraffe et al. (1998) with a metallicity of 0 dex/−0.8 dex (see Reylé et al. 2002).</footnote>

<footnote><sup>b</sup>I drop-out star. The DENIS magnitude is in the *J* band, the colour is *J* − *K*<sub>s</sub>. The photometric distance has been computed using the (M<sub>J</sub>, J − K<sub>s</sub>) theoretical relation from Baraffe et al. (1998).</footnote>
<table><caption>**Table 3.** Nearby stars candidates found in the LEHPMS observed at the ESO/NTT. $B_J$ and $R$ are from the LEHPMS high proper motion catalogue, obtained on digitized plates. $I$ and $I-J$ are from the DENIS.</caption><thead><tr><th>Name</th><th>α<sub>J2000</sub></th><th>δ<sub>J2000</sub></th><th>ESO epoch</th><th>μ<sub>α</sub><br>("yr<sup>-1</sup>)</th><th>μ<sub>δ</sub></th><th>B<sub>J</sub></th><th>R</th><th>I<br>DENIS</th><th>(I - J)<br>(pc)</th><th>d<sub>phot</sub></th></tr></thead><tbody><tr><td>LEHPMS J0029-2733</td><td>00 28 54.55</td><td>-27 33 34.3</td><td>1984.8</td><td>0.23</td><td>0.03</td><td>16.65</td><td>14.63</td><td>12.47</td><td>1.67</td><td>19.9</td></tr><tr><td>LEHPMS J0030-5442</td><td>00 29 50.33</td><td>-54 41 32.5</td><td>1988.7</td><td>-0.27</td><td>-0.22</td><td>14.51</td><td>12.89</td><td>10.99</td><td>1.64</td><td>14.6</td></tr><tr><td>LEHPMS J0102-5524</td><td>01 01 57.31</td><td>-55 23 47.1</td><td>1981.8</td><td>0.28</td><td>0.07</td><td>15.34</td><td>13.20</td><td>11.24</td><td>1.54</td><td>18.5</td></tr><tr><td>LHS 1201</td><td>01 08 46.93</td><td>-37 10 16.8</td><td>1989.7</td><td>-0.56</td><td>-0.45</td><td>17.29</td><td>14.92</td><td>12.72</td><td>1.92</td><td>17.9</td></tr><tr><td>LHS 1249<sup>a</sup></td><td>01 24 03.54</td><td>-42 40 30.0</td><td>1983.8</td><td>0.31</td><td>-0.54</td><td>15.62</td><td>14.81</td><td>14.46</td><td>0.25</td><td>16.9</td></tr><tr><td>LEHPMS J0141-5544<sup>a</sup></td><td>01 40 31.11</td><td>-55 43 39.4</td><td>1984.9</td><td>0.21</td><td>-0.01</td><td>15.12</td><td>15.28</td><td>14.97</td><td>-0.63</td><td>-</td></tr><tr><td>LHS 1293</td><td>01 45 11.07</td><td>-32 05 09.7</td><td>1985.9</td><td>0.67</td><td>0.22</td><td>15.37</td><td>13.25</td><td>11.20</td><td>1.51</td><td>18.0</td></tr><tr><td>LEHPMS J0146-5230</td><td>01 45 30.37</td><td>-52 30 20.1</td><td>1984.9</td><td>0.18</td><td>0.03</td><td>15.26</td><td>13.86</td><td>12.17</td><td>1.71</td><td>17.5</td></tr><tr><td>LEHPMS J0146-5340</td><td>01 46 29.07</td><td>-53 39 30.9</td><td>1984.9</td><td>0.18</td><td>-0.11</td><td>15.52</td><td>13.18</td><td>10.90</td><td>1.59</td><td>12.7</td></tr><tr><td>LHS 1367</td><td>02 15 07.38</td><td>-30 39 57.2</td><td>1988.8</td><td>0.77</td><td>-0.34</td><td>20.23</td><td>17.34</td><td>14.03</td><td>2.46</td><td>17.4</td></tr><tr><td>LP 829-41</td><td>02 16 21.55</td><td>-22 00 52.0</td><td>1984.8</td><td>-0.12</td><td>0.22</td><td>18.14</td><td>15.55</td><td>12.74</td><td>1.96</td><td>19.6</td></tr><tr><td>LP 941-57</td><td>02 22 18.07</td><td>-36 51 52.8</td><td>1989.8</td><td>0.29</td><td>0.10</td><td>17.45</td><td>15.16</td><td>12.57</td><td>1.93</td><td>18.2</td></tr><tr><td>LHS 1421 8435<sup>a</sup></td><td>02 35 21.91</td><td>-24 00 38.3</td><td>1985.8</td><td>-0.11</td><td>-0.63</td><td>16.58</td><td>15.36</td><td>14.92</td><td>0.56</td><td>14.3</td></tr><tr><td>LP 942-107</td><td>03 05 10.79</td><td>-34 05 23.1</td><td>1983.8</td><td>0.37</td><td>-0.05</td><td>14.41</td><td>11.97</td><td>9.72</td><td>1.55</td><td>14.8</td></tr><tr><td>LEHPMS J0309-4925</td><td>03 08 59.98</td><td>-49 24 53.5</td><td>1981.8</td><td>0.16</td><td>0.10</td><td>16.19</td><td>13.67</td><td>11.45</td><td>1.69</td><td>17.5</td></tr><tr><td>LP 994-114</td><td>03 09 21.59</td><td>-39 11 02.8</td><td>1981.9</td><td>0.38</td><td>-0.01</td><td>13.56</td><td>11.25</td><td>9.99</td><td>1.42</td><td>15.9</td></tr><tr><td>LP 772-8</td><td>03 09 50.86</td><td>-19 06 46.9</td><td>1986.9</td><td>0.41</td><td>-0.09</td><td>15.41</td><td>12.85</td><td>10.94</td><td>1.56</td><td>17.4</td></tr><tr><td>LHS 1524</td><td>03 17 17.57</td><td>-19 40 14.7</td><td>1986.9</td><td>0.53</td><td>-0.25</td><td>16.57</td><td>14.23</td><td>11.86</td><td>1.78</td><td>19.3</td></tr><tr><td>LEHPMS J0321-6352</td><td>03 20 51.84</td><td>-63 51 47.9</td><td>1984.7</td><td>0.04</td><td>-0.31</td><td>14.29</td><td>11.77</td><td>9.49</td><td>1.48</td><td>14.0</td></tr><tr><td>LTT 1732</td><td>03 38 55.65</td><td>-52 34 15.0</td><td>1981.8</td><td>0.15</td><td>0.21</td><td>15.74</td><td>13.18</td><td>11.15</td><td>1.58</td><td>14.6</td></tr><tr><td>LEHPMS J0428-6209</td><td>04 28 05.44</td><td>-62 09 28.6</td><td>1989.8</td><td>0.21</td><td>0.32</td><td>14.15</td><td>11.76</td><td>9.96</td><td>1.50</td><td>11.9</td></tr><tr><td>LEHPMS J0503-5353</td><td>05 03 24.90</td><td>-53 53 20.9</td><td>1982.0</td><td>0.48</td><td>0.10</td><td>15.61</td><td>13.14</td><td>11.02</td><td>1.64</td><td>16.6</td></tr><tr><td>L 83-11</td><td>22 10 10.48</td><td>-70 10 06.8</td><td>1984.6</td><td>0.08</td><td>-0.27</td><td>14.03</td><td>11.78</td><td>10.53</td><td>1.46</td><td>17.9</td></tr><tr><td>LEHPMS J2210-7010</td><td>22 10 19.97</td><td>-70 10 04.4</td><td>1984.6</td><td>0.08</td><td>-0.26</td><td>14.88</td><td>12.66</td><td>11.17</td><td>1.67</td><td>14.2</td></tr><tr><td>LEHPMS J2230-5345</td><td>22 30 09.59</td><td>-53 44 44.8</td><td>1985.7</td><td>-0.03</td><td>-0.76</td><td>15.81</td><td>13.39</td><td>11.19</td><td>1.68</td><td>12.0</td></tr><tr><td>LEHPMS J2234-6108</td><td>22 34 04.12</td><td>-61 07 40.4</td><td>1985.5</td><td>0.26</td><td>-0.05</td><td>16.36</td><td>13.84</td><td>10.88</td><td>1.65</td><td>15.9</td></tr><tr><td>LP 1033-31</td><td>22 35 04.52</td><td>-42 17 45.3</td><td>1981.8</td><td>0.27</td><td>-0.16</td><td>14.40</td><td>11.80</td><td>10.34</td><td>1.40</td><td>18.3</td></tr><tr><td>L 166-3</td><td>22 41 59.52</td><td>-59 15 12.2</td><td>1986.8</td><td>0.34</td><td>0.00</td><td>15.34</td><td>13.07</td><td>11.25</td><td>1.71</td><td>12.5</td></tr><tr><td>LEHPMS J2248-4715<sup>a</sup></td><td>22 48 08.68</td><td>-47 14 45.3</td><td>1983.8</td><td>0.05</td><td>-0.29</td><td>17.77</td><td>15.84</td><td>15.26</td><td>0.50</td><td>18</td></tr><tr><td>LP 877-72</td><td>23 09 59.91</td><td>-21 11 43.1</td><td>1984.8</td><td>0.34</td><td>0.01</td><td>13.67</td><td>10.89</td><td>9.82</td><td>1.36</td><td>19.1</td></tr><tr><td>LEHPMS J2325-6740</td><td>23 25 24.54</td><td>-67 40 05.6</td><td>1984.8</td><td>0.24</td><td>-0.15</td><td>15.60</td><td>13.22</td><td>11.09</td><td>1.61</td><td>16.5</td></tr><tr><td>LP 878-73</td><td>23 40 47.99</td><td>-22 25 27.9</td><td>1986.6</td><td>-0.37</td><td>-0.25</td><td>16.36</td><td>14.10</td><td>12.07</td><td>1.77</td><td>19.2</td></tr><tr><td>LHS 73<sup>b</sup></td><td>23 43 15.49</td><td>-24 10 47.2</td><td>1986.8</td><td>1.37</td><td>-2.20</td><td>14.03</td><td>11.72</td><td>11.17</td><td>1.09</td><td>12.3</td></tr><tr><td>LTT 9783</td><td>23 53 08.12</td><td>-42 32 04.5</td><td>1989.8</td><td>0.18</td><td>0.10</td><td>13.93</td><td>11.24</td><td>9.86</td><td>1.44</td><td>15.9</td></tr><tr><td>LP 987-47</td><td>23 53 40.74</td><td>-35 59 07.6</td><td>1988.8</td><td>0.30</td><td>0.16</td><td>14.77</td><td>12.46</td><td>10.68</td><td>1.43</td><td>19.1</td></tr><tr><td>LEHPMS J2354-3634<sup>a</sup></td><td>23 54 18.80</td><td>-36 33 47.3</td><td>1988.8</td><td>0.05</td><td>-0.68</td><td>15.60</td><td>15.71</td><td>15.34</td><td>-0.50</td><td>-</td></tr><tr><td>LEHPMS J2359-5400</td><td>23 58 49.37</td><td>-54 00 13.2</td><td>1989.6</td><td>-0.15</td><td>-0.16</td><td>15.64</td><td>13.05</td><td>11.57</td><td>1.56</td><td>19.6</td></tr></tbody></table>

<sup>a</sup>White dwarf candidates. The photometric distance has been computed with (M<sub>J</sub>, I - J) theoretical relation from Bergeron et al. (1995) for 0.6 M<sub>☉</sub> white dwarfs. No distances were determined for the bluest candidates as they are beyond the colour limit of the theoretical relation.

<sup>b</sup>Subdwarf candidate. The photometric distance has been obtained using the (M<sub>J</sub>, I - J) theoretical relation from Baraffe et al. (1998) with a metallicity of -1.8 dex.

M stars (Nidever et al. 2002) with spectral types sdM0 and M1, 12 comparison stars with known spectral types from M0 to M6, found in the NASA NStars,<sup>1</sup> the ARICNS<sup>2</sup> and the SIMBAD<sup>3</sup> databases, and one M9 star (Delfosse et al. 2001). Discrepancies are found in the spectral types given by the different databases (see Table 4), probably due to the fact that different spectral classification systems are used. These templates, and our finally adopted types are in currently accepted system of Kirkpatrick, Henry & McCarthy (1991). Deviations between our adopted sequence and other published spectral types are usually 0.5-1 subtype and discrepancies mostly occur at earlier types (M0-M3).

# 4 SPECTRAL CLASSIFICATION

## 4.1 M dwarfs

In the past years, several classification schemes have been proposed for cool dwarfs. First, the least-squares minimization technique by Kirkpatrick et al. (1991) has the advantage of assigning types based on both the molecular or atomic features and the slope of the spectrum. Recently, Henry et al. (2002) developed a similar but more accurate method that matches a target spectrum to one from a database of standard spectra (software program called ALLSTAR). Scholz et al. (2005) used the minimization of the absolute differences between target and template spectra in their low-resolution classification spectroscopy.

A different approach was taken by Reid, Hawley & Gizis (1995), who measured the strengths of the prominent TiO and CaH features of M dwarfs. They defined several indices being the ratio between

<footnote n="1"><sup>1</sup>http://nstars.arc.nasa.gov.</footnote>

<footnote n="2"><sup>2</sup>http://www.ari.uni-heidelberg.de/aricns/.</footnote>

<footnote n="3"><sup>3</sup>http://cdsweb.u-strasbg.fr/Simbad.</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1. (H, I − J) diagram for the high proper motion stars cross-identified with the DENIS. Dots: M dwarfs. Squares: white dwarf candidates. Triangle: subdwarf candidate.</figcaption></figure>

the flux within a given spectral feature and the flux in a nearby pseudo-continuum region.

For the latest M dwarfs, Kirkpatrick, Henry & Simons (1995) defined an index based on the VO bandstrength, whereas Kirkpatrick et al. (1999) and Martín et al. (1999) proposed spectral indices for classification down to the L type where TiO and VO bands fade due to the condensation of Ti and V to dust. The pseudo-continuum spectral ratios (namely PC3) defined by Martín et al. (1999) cover a narrower wavelength range than for the computation of indices, thus it is less sensitive to possible wavelength shifts.

Lépine, Rich & Shara (2003a) pointed out that a unified scheme for the classification of M dwarfs is needed and they defined an enhanced classification based on a set of existing (Reid et al. 1995; Hawley et al. 2002) and new spectral indices that measure the most important features in the optical spectral range.

Furthermore, the classification procedure of Gizis (1997) allows us to distinguish potential subdwarfs (sdM) or extreme subdwarfs (esdM) from the M dwarfs. Accordingly, Lépine, Shara & Rich (2003b) used a CaH2+CaH3 versus TiO5 diagram to discriminate between the three different object classes. This diagram was also used by Lépine, Shara & Rich (2004) and Scholz et al. (2004a) when reporting the discovery of an esdM6.5 and an sdM9.5 star, respectively. Fig. 4 shows the locus of our candidates (filled circles) in the CaH2+CaH3 versus TiO5 diagram superimposed with the data used in the same diagram by Lépine et al. (2003b) and newly discovered objects by Lépine et al. (2004), Scholz et al. (2004a), Scholz, Lodieu & McCaughrean (2004b) and Farihi, Wood & Stalder (2005).

Most of our candidates lie in the M-dwarf region, including the four stars for which alternative distances were computed assuming that they belong to the thick disc (see the last column in Table 2). Thus, these stars are more distant disc stars instead of nearby low-metallicity dwarfs.

Spectral types are obtained by a visual comparison with our spectral templates of comparison stars, observed together with our target stars (Table 4). For comparison, we also derive spectral types, using the classification scheme based on the TiO and CaH bandstrengths. We also measured the pseudo-continuum spectral ratio PC3, useful for late dwarfs, in particular for M7 and M8 stars that are missing in our comparison sequence. The results are given in Table 5. The uncertainty of our spectral type determination is 0.5 subclass.

Fig. 5 shows the spectral type determined from the TiO5 and PC3 indices versus our adopted spectral type obtained from the

comparison with templates. As already pointed out by the different authors who defined the indices, the TiO5 index starts to deviate from our spectral sequence for spectral types later than M6, whereas the PC3 index is not reliable for stars earlier than M3. Both indices are well in agreement in the overlapping region M3–M6.

## 4.2 Subdwarfs

The sdM0 stars, LHS 29 and LHS 73, are also marked in Fig. 4. The latter could also be a subdwarf, given its location in the reduced proper motion versus colour diagram. Its position in the CaH2+CaH3 versus TiO5 diagram is compatible with the position of other known subdwarfs, although dwarfs, subdwarfs and extreme subdwarfs are not well separated in this part of the diagram.

For this star, LHS 73, we determined the radial velocity. We compared the position of the potassium line at 769.9 nm, the iron line at 838.8 nm, and the calcium triplet at 849.8, 854.2 and 866.2 nm with the position of the same lines in the spectrum of the standard star LHS 1827 with known radial velocity (47.2 km s⁻¹). We obtained a radial velocity of 100.6 km s⁻¹, with an accuracy of 15 km s⁻¹. We then computed the galactocentric velocities and found (U, V, W) = (−0.4 ± 3.8, −207.0 ± 23.3, −141.4 ± 15.2) km s⁻¹, compatible with the space motion of a star belonging to an old population, thick disc or halo. Thus, following the procedure of Gizis (1997), we derived an sdK7 spectral type.

## 4.3 White dwarfs

The spectra of the five white dwarf candidates are shown in Fig. 3. The two bluest candidates shown on the top panel are hot white dwarfs. LEHPMS J2354−3634 is an early-type DA white dwarf with broad Balmer lines and LEHPMS J0141−5544 is a helium-rich DB white dwarf. For such blue objects, the atmosphere models of Bergeron, Wesemael & Beauchamp (1995) for DA and DB white dwarfs do not give an absolute magnitude. To compute their distance, we used instead the relation from Oppenheimer et al. (2001):

$$M_{BJ} = 12.73 + 2.58(B_J - R).$$

We obtained $d_{BJM} = 44.9$ pc for LEHPMS J2354−3634. Distances were also derived by Pauli et al. (2003) and Salim et al. (2004) from spectroscopic observations. They found a distance of 62.6 and 59 ± 14 pc, respectively. From spectral fitting, Koester et al. (2001) found an effective temperature around 14 500 K. For LEHPMS J0141−5544, we computed $d_{Bj} = 36.3$ pc.

In the bottom panel, LHS 1249 and LHS 1421 are featureless DC white dwarfs, although they could show weak H α absorption, in particular LHS 1249. Spectra with better signal-to-noise ratio would be required to assign a DC or DA class. We derived $d_{Bj} = 14.4$ pc for LHS 1249 and $d_{Bj} = 13.8$ pc for LHS 1421. For those objects, we also estimated the distance using the M$_J$ versus I − J theoretical relation for white dwarfs of 0.6 M☉ from Bergeron et al. (1995) and obtained 16.9 pc for LHS 1249 and 14.3 pc for LHS 1421. Following the conventional number 50 400/T$_{eff}$ to assign a spectral index, we performed blackbody fits to the spectra and obtained 6200 K for LHS 1249 and 4800 K for LHS 1421, with an accuracy of ±100 K. The derived spectral types are DC8 and DC10, respectively. Kawka, Vennes & Thorstensen (2004) obtained spectroscopy of LHS 1421 and found it to be a DC white dwarf at 16 pc and with T$_{eff}$ = 5300 ± 200 K.

The lower spectrum shows clear indications of K dwarfs (MgH, Na features). LEHPMS J2248−4715 is not a nearby white dwarf but a distant K subdwarf. Following the spectral classification scheme
<figure><img src="image_3.png" /><figcaption>**Figure 2.** NTT spectra of K and M dwarfs, from spectral types K7 to M4 in the left-hand panel, and from M4 to M8 in the right-hand panel.</figcaption></figure>

of Gizis (1997), we measured the Ti05, CaH1, CaH2 and CaH3 indices and estimated a spectral type sdK5.

# 5 SPECTROSCOPIC DISTANCES

## 5.1 M dwarfs

Spectral type versus absolute magnitude relations can be used to derive the distances of our red dwarf targets. Reid et al. (1995), Henry, Kirkpatrick & Simons (1994) and Henry et al. (2002) computed relations between spectral type and absolute magnitude in the V band, and Lépine et al. (2003a) computed in the R, B and Ks bands.

Several calibrations have also been computed in the J-band.

(i) Dahn et al. (2002) determined trigonometric parallaxes for 28 late-type dwarfs and derived a *M*<sub>J</sub> (spectral type) relation valid for types later than M6.5.

(ii) Hawley et al. (2002) used a well observed sample of nearby dwarfs with measured trigonometric parallaxes, 2MASS J magnitude and spectral type to calibrate a spectroscopic parallax relation *M*<sub>J</sub> (spectral type). They adopted a fit with several line segments in the spectral type range from K7 to T8.

(iii) Cruz & Reid (2002) used a sample of 68 late-type dwarfs with well determined trigonometric parallaxes surveyed by the Palomar/Michigan State University (PMSU) survey to derive a
<figure><img src="image_4.png" /><figcaption>**Figure 3.** NTT spectra of the white dwarf candidates. Two hot white dwarfs are shown in the top panel, whereas two featureless cool white dwarfs and a K subdwarf (bottom) are shown in the lower panel.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Relative strengths of the CaH and TiO molecular bands for spectroscopically identified M dwarfs, subdwarfs (sdM) and extreme subdwarfs (esdM) according to the classification scheme of Gizis (1997). Our nearby candidates are shown with the filled circles.</figcaption></figure>

$M_J$ (TiO5) relation. Cruz et al. (2003) supplemented these data to fit a fourth-order polynomial $M_J$ (spectral type) relation, valid for types M6 to L8.

(iv) Scholz et al. (2005) provided $M_J$ for spectral types over the whole range from K0 to L8.

(v) Crifo et al. (2005) identified in the literature 27 stars with good trigonometric parallaxes and photometry and with known PC3 indices to derive a calibration of absolute magnitudes in the NIR as a function of the PC3 index.

Since the more accurate available photometry of our candidates is in the NIR bands, we prefer to use spectral-type versus NIR absolute magnitude relations. The available $M_J$ against spectral-type calibration relations are shown in Fig. 6, in the spectral-type range where they are valid. We note that the absolute magnitudes, $M_J$, given by most of calibration relations do not differ by more than the differences due to the 0.5 subclass uncertainty in the spectral type.

<table><caption>**Table 4.** Comparison stars for spectroscopic classification. The spectral types found in the literature are given, as well as the final adopted spectral types.</caption><thead><tr><th>Name</th><th>NStars</th><th>ARICNS</th><th>SIMBAD</th><th>Other</th><th>Adopted</th></tr></thead><tbody><tr><td>LHS 29</td><td>—</td><td>M1</td><td>M1</td><td>sdM1.0<sup>a</sup>/sdM0<sup>b</sup></td><td>sdM0</td></tr><tr><td>GI 143.1</td><td>M0</td><td>M0</td><td>K7.0</td><td>—</td><td>M0</td></tr><tr><td>LHS 141</td><td>M0.5</td><td>M1.0</td><td>K7</td><td>—</td><td>M0</td></tr><tr><td>LHS 3833</td><td>—</td><td>M0</td><td>M0</td><td>—</td><td>M0.5</td></tr><tr><td>LHS 1827</td><td>M1</td><td>M1</td><td>M1/M2</td><td>—</td><td>M1</td></tr><tr><td>LHS 65</td><td>M3</td><td>M2</td><td>M1</td><td>—</td><td>M1.5</td></tr><tr><td>LHS 14</td><td>M2.5</td><td>M1.5</td><td>M1.5</td><td>—</td><td>M1.5</td></tr><tr><td>LHS 1208</td><td>M2</td><td>—</td><td>M2</td><td>—</td><td>M2.5</td></tr><tr><td>LHS 502</td><td>M4</td><td>M3</td><td>M2.5</td><td>—</td><td>M3</td></tr><tr><td>LHS 183</td><td>M3.5</td><td>M4</td><td>M3.5</td><td>—</td><td>M3.5</td></tr><tr><td>LHS 138</td><td>M4.5</td><td>M4.5</td><td>M4.5</td><td>—</td><td>M4.5</td></tr><tr><td>LHS 168</td><td>M5.0</td><td>M5.0</td><td>M5.0</td><td>—</td><td>M5</td></tr><tr><td>LHS 546</td><td>M5.5</td><td>M5.5</td><td>M5.5</td><td>—</td><td>M5.5</td></tr><tr><td>LHS 1326</td><td>M6</td><td>M6</td><td>M6</td><td>—</td><td>M6</td></tr><tr><td>DENIS J1048</td><td>—</td><td>—</td><td>M8</td><td>M9<sup>c</sup></td><td>M9</td></tr></tbody></table>

<footnote><sup>a</sup>Gizis (1997). <sup>b</sup>Nidever et al. (2002). <sup>c</sup>Delfosse et al. (2001).</footnote>
<table><caption>**Table 5.** Spectroscopic indices, spectral type, absolute magnitude *M*<sub>J</sub> and spectroscopic distance of the red dwarf candidates observed at the NTT. The additional letter 'e' in the spectral type indicates a star showing H α emission.</caption><thead><tr><th>Name</th><th>TiO5</th><th>CaH1</th><th>CaH2</th><th>CaH3</th><th>PC3</th><th>SpT</th><th>J</th><th>M<sub>J</sub></th><th>d<sub>spectro</sub><br>(pc)</th><th>Comments</th></tr></thead><tbody><tr><td>L 291-115</td><td>0.281</td><td>0.784</td><td>0.334</td><td>0.646</td><td>1.268</td><td>M5.0</td><td>10.47</td><td>9.72</td><td>14.2 ± 2.0</td><td></td></tr><tr><td>APMPM J0103-3738</td><td>0.212</td><td>0.891</td><td>0.265</td><td>0.590</td><td>1.801</td><td>M8.0e</td><td>11.16</td><td>11.15</td><td>10.0 ± 0.8</td><td></td></tr><tr><td>APMPM J0145-3205</td><td>0.504</td><td>0.803</td><td>0.497</td><td>0.754</td><td>1.055</td><td>M2.5</td><td>9.84</td><td>7.14</td><td>34.7 ± 2.8</td><td></td></tr><tr><td>LP 940-20</td><td>0.352</td><td>0.766</td><td>0.418</td><td>0.696</td><td>1.188</td><td>M4.5e</td><td>10.89</td><td>8.84</td><td>25.7 ± 8.4</td><td>M4.5 at d = 31.4 ± 5.3 pc<sup>b</sup></td></tr><tr><td>LHS 5045</td><td>0.365</td><td>0.840</td><td>0.417</td><td>0.718</td><td>1.171</td><td>M4.0</td><td>9.17</td><td>8.63</td><td>12.9 ± 4.4</td><td></td></tr><tr><td>L 225-57</td><td>0.401</td><td>0.854</td><td>0.452</td><td>0.736</td><td>1.123</td><td>M4.0</td><td>8.23</td><td>8.63</td><td>8.3 ± 2.8</td><td></td></tr><tr><td>L 127-33</td><td>0.570</td><td>0.801</td><td>0.558</td><td>0.778</td><td>1.031</td><td>M2.0</td><td>11.21</td><td>6.97</td><td>70.6 ± 5.3</td><td></td></tr><tr><td>APMPM J0255-5140</td><td>0.394</td><td>0.858</td><td>0.441</td><td>0.745</td><td>1.132</td><td>M4.0</td><td>9.89</td><td>8.63</td><td>17.9 ± 6.1</td><td></td></tr><tr><td>LP 831-45</td><td>0.444</td><td>0.808</td><td>0.464</td><td>0.727</td><td>1.102</td><td>M3.5</td><td>7.62</td><td>8.41</td><td>10.4 ± 3.5</td><td></td></tr><tr><td>LHS 5090</td><td>0.450</td><td>0.782</td><td>0.434</td><td>0.713</td><td>1.067</td><td>M3.0</td><td>11.60</td><td>7.32</td><td>71.8 ± 6.2</td><td></td></tr><tr><td>LHS 1656</td><td>0.500</td><td>0.811</td><td>0.501</td><td>0.749</td><td>1.073</td><td>M2.5</td><td>9.53</td><td>7.15</td><td>30.0 ± 2.4</td><td></td></tr><tr><td>APMPM J0541-5349A</td><td>0.576</td><td>0.794</td><td>0.575</td><td>0.798</td><td>1.048</td><td>M2.0</td><td>10.58</td><td>6.97</td><td>52.7 ± 4.0</td><td></td></tr><tr><td>APMPM J0541-5349B</td><td>0.505</td><td>0.780</td><td>0.526</td><td>0.762</td><td>1.058</td><td>M2.0</td><td>10.64</td><td>6.97</td><td>54.2 ± 4.1</td><td></td></tr><tr><td>LP 904-51</td><td>0.280</td><td>0.743</td><td>0.333</td><td>0.606</td><td>1.292</td><td>M5.0e</td><td>11.01</td><td>9.72</td><td>18.1 ± 2.6</td><td></td></tr><tr><td>APMPM J1107-2202</td><td>0.532</td><td>0.779</td><td>0.505</td><td>0.749</td><td>1.080</td><td>M2.5</td><td>9.92</td><td>7.14</td><td>36.0 ± 2.9</td><td></td></tr><tr><td>APMPM J1932-4834</td><td>0.335</td><td>0.846</td><td>0.397</td><td>0.707</td><td>1.162</td><td>M4.5</td><td>10.54</td><td>8.84</td><td>21.9 ± 7.2</td><td></td></tr><tr><td>APMPM J2101-4115</td><td>0.381</td><td>0.755</td><td>0.396</td><td>0.652</td><td>1.161</td><td>M4.0e</td><td>9.95</td><td>8.63</td><td>18.4 ± 6.3</td><td></td></tr><tr><td>APMPM J2101-4907</td><td>0.432</td><td>0.767</td><td>0.443</td><td>0.701</td><td>1.135</td><td>M3.5</td><td>9.09</td><td>8.41</td><td>13.7 ± 4.6</td><td></td></tr><tr><td>LTT 8708 A</td><td>0.487</td><td>0.766</td><td>0.537</td><td>0.749</td><td>0.886</td><td>M0.0</td><td>7.75<sup>a</sup></td><td>6.33</td><td>27.2 ± 1.8</td><td></td></tr><tr><td>LTT 8708 B</td><td>0.494</td><td>0.780</td><td>0.542</td><td>0.755</td><td>0.879</td><td>M0.0</td><td>7.75<sup>a</sup></td><td>6.33</td><td>27.2 ± 1.8</td><td></td></tr><tr><td>LHS 3800</td><td>0.442</td><td>0.785</td><td>0.484</td><td>0.737</td><td>1.069</td><td>M3.0</td><td>10.90</td><td>7.32</td><td>52.0 ± 4.6</td><td></td></tr><tr><td>LHS 3842</td><td>0.459</td><td>0.795</td><td>0.487</td><td>0.744</td><td>1.079</td><td>M3.0</td><td>9.75</td><td>7.32</td><td>30.6 ± 2.6</td><td></td></tr><tr><td>APMPM J2330-4737</td><td>0.164</td><td>0.769</td><td>0.233</td><td>0.510</td><td>1.596</td><td>M7.0e</td><td>11.32</td><td>10.73</td><td>13.1 ± 1.5</td><td>M6 at d = 15.8 ± 1.9 pc<sup>c</sup></td></tr><tr><td>APMPM J2352-3609</td><td>0.314</td><td>0.856</td><td>0.379</td><td>0.688</td><td>1.203</td><td>M4.5</td><td>12.18</td><td>8.84</td><td>46.6 ± 15.3</td><td></td></tr><tr><td>LEHPMS J0029-2733</td><td>0.368</td><td>0.820</td><td>0.421</td><td>0.703</td><td>1.162</td><td>M4.5e</td><td>10.80</td><td>8.84</td><td>24.7 ± 8.1</td><td></td></tr><tr><td>LEHPMS J0030-5442</td><td>0.347</td><td>0.869</td><td>0.409</td><td>0.709</td><td>1.188</td><td>M4.5e</td><td>9.35</td><td>8.84</td><td>12.7 ± 4.1</td><td></td></tr><tr><td>LEHPMS J0102-5524</td><td>0.423</td><td>0.809</td><td>0.449</td><td>0.711</td><td>1.114</td><td>M3.5e</td><td>9.70</td><td>8.41</td><td>18.2 ± 6.2</td><td></td></tr><tr><td>LHS 1201</td><td>0.318</td><td>0.780</td><td>0.370</td><td>0.663</td><td>1.215</td><td>M4.5</td><td>10.80</td><td>8.84</td><td>24.7 ± 8.1</td><td></td></tr><tr><td>LHS 1293</td><td>0.490</td><td>0.804</td><td>0.497</td><td>0.740</td><td>1.084</td><td>M2.5</td><td>9.69</td><td>7.14</td><td>32.3 ± 2.6</td><td></td></tr><tr><td>LEHPMS J0146-5230</td><td>0.436</td><td>0.803</td><td>0.441</td><td>0.692</td><td>1.097</td><td>M3.5e</td><td>10.46</td><td>8.41</td><td>25.8 ± 8.7</td><td></td></tr><tr><td>LEHPMS J0146-5340</td><td>0.371</td><td>0.761</td><td>0.380</td><td>0.637</td><td>1.204</td><td>M4.5e</td><td>9.31</td><td>8.84</td><td>12.4 ± 4.1</td><td></td></tr><tr><td>LHS 1367</td><td>0.204</td><td>0.849</td><td>0.262</td><td>0.561</td><td>1.846</td><td>M8.0e</td><td>11.57</td><td>11.15</td><td>12.1 ± 1.0</td><td></td></tr><tr><td>LP 829-41</td><td>0.233</td><td>0.757</td><td>0.310</td><td>0.631</td><td>1.293</td><td>M5.5</td><td>10.78</td><td>10.05</td><td>14.0 ± 1.7</td><td>M5.5 at d = 19.3 ± 1.6 pc<sup>b</sup></td></tr><tr><td>LP 941-57</td><td>0.294</td><td>0.776</td><td>0.335</td><td>0.603</td><td>1.273</td><td>M5.5e</td><td>10.64</td><td>10.05</td><td>13.1 ± 1.6</td><td></td></tr><tr><td>LP 942-107</td><td>0.405</td><td>0.839</td><td>0.450</td><td>0.741</td><td>1.128</td><td>M4.0</td><td>9.63</td><td>8.63</td><td>15.9 ± 5.4</td><td></td></tr><tr><td>LEHPMS J0309-4925</td><td>0.359</td><td>0.770</td><td>0.393</td><td>0.641</td><td>1.188</td><td>M4.5e</td><td>9.76</td><td>8.84</td><td>15.3 ± 5.0</td><td></td></tr><tr><td>LP -114</td><td>0.486</td><td>0.797</td><td>0.485</td><td>0.739</td><td>1.090</td><td>M2.5</td><td>9.00</td><td>7.14</td><td>23.5 ± 1.9</td><td></td></tr><tr><td>LP -8</td><td>0.410</td><td>0.774</td><td>0.433</td><td>0.696</td><td>1.167</td><td>M4.0</td><td>9.38</td><td>8.63</td><td>14.2 ± 4.8</td><td></td></tr><tr><td>LHS 1524</td><td>0.337</td><td>0.793</td><td>0.390</td><td>0.687</td><td>1.185</td><td>M4.5</td><td>10.08</td><td>8.84</td><td>17.7 ± 5.8</td><td></td></tr><tr><td>LEHPMS J0321-6352</td><td>0.414</td><td>0.825</td><td>0.451</td><td>0.732</td><td>1.132</td><td>M4.0</td><td>9.13</td><td>8.63</td><td>12.6 ± 4.3</td><td></td></tr><tr><td>LTT 1732</td><td>0.356</td><td>0.743</td><td>0.385</td><td>0.659</td><td>1.204</td><td>M4.5</td><td>9.57</td><td>8.84</td><td>14.0 ± 4.6</td><td></td></tr><tr><td>LEHPMS J0428-6209</td><td>0.449</td><td>0.816</td><td>0.460</td><td>0.725</td><td>1.111</td><td>M3.5</td><td>8.94</td><td>8.41</td><td>12.8 ± 4.3</td><td></td></tr><tr><td>LEHPMS J0503-5353</td><td>0.378</td><td>0.814</td><td>0.440</td><td>0.746</td><td>1.162</td><td>M4.0</td><td>9.38</td><td>8.63</td><td>14.2 ± 4.8</td><td></td></tr><tr><td>L 83-11</td><td>0.607</td><td>0.856</td><td>0.609</td><td>0.818</td><td>1.011</td><td>M2.0</td><td>9.07</td><td>6.97</td><td>26.3 ± 2.0</td><td></td></tr><tr><td>LEHPMS J2210-7010</td><td>0.461</td><td>0.802</td><td>0.526</td><td>0.764</td><td>1.046</td><td>M3.5</td><td>9.50</td><td>8.41</td><td>16.6 ± 5.6</td><td></td></tr><tr><td>LEHPMS J2230-5345</td><td>0.333</td><td>0.794</td><td>0.371</td><td>0.650</td><td>1.200</td><td>M4.5</td><td>9.51</td><td>8.84</td><td>13.6 ± 4.5</td><td></td></tr><tr><td>LEHPMS J2234-6108</td><td>0.366</td><td>0.745</td><td>0.393</td><td>0.649</td><td>1.183</td><td>M4.0e</td><td>9.23</td><td>8.63</td><td>13.2 ± 4.5</td><td></td></tr><tr><td>LP 1033-31</td><td>0.422</td><td>0.836</td><td>0.461</td><td>0.733</td><td>1.111</td><td>M3.5</td><td>9.10</td><td>8.41</td><td>13.8 ± 4.7</td><td></td></tr><tr><td>L 166-3</td><td>0.396</td><td>0.827</td><td>0.443</td><td>0.726</td><td>1.096</td><td>M3.5</td><td>9.54</td><td>8.41</td><td>16.9 ± 5.7</td><td></td></tr><tr><td>LP 877-72</td><td>0.486</td><td>0.796</td><td>0.486</td><td>0.742</td><td>1.109</td><td>M3.5</td><td>8.86</td><td>8.41</td><td>12.4 ± 4.2</td><td></td></tr><tr><td>LEHPMS J2325-6740</td><td>0.397</td><td>0.791</td><td>0.416</td><td>0.677</td><td>1.142</td><td>M4.0e</td><td>9.48</td><td>8.63</td><td>14.8 ± 5.0</td><td></td></tr><tr><td>LP 878-73</td><td>0.418</td><td>0.783</td><td>0.430</td><td>0.698</td><td>1.141</td><td>M3.5</td><td>10.30</td><td>8.41</td><td>23.9 ± 8.1</td><td></td></tr><tr><td>LHS 73</td><td>0.960</td><td>0.889</td><td>0.847</td><td>0.901</td><td>0.997</td><td>sdK7</td><td>10.08</td><td>8.72</td><td>18.7</td><td></td></tr><tr><td>LTT 9783</td><td>0.524</td><td>0.808</td><td>0.514</td><td>0.757</td><td>1.068</td><td>M2.5</td><td>9.17</td><td>7.14</td><td>25.5 ± 2.0</td><td></td></tr><tr><td>LP 987-47</td><td>0.430</td><td>0.839</td><td>0.459</td><td>0.734</td><td>1.100</td><td>M3.5</td><td>9.25</td><td>8.41</td><td>14.8 ± 5.0</td><td></td></tr><tr><td>LEHPMS J2359-5400</td><td>0.431</td><td>0.836</td><td>0.466</td><td>0.735</td><td>1.105</td><td>M3.5</td><td>10.01</td><td>8.41</td><td>21.0 ± 7.1</td><td></td></tr></tbody></table>

<footnote><sup>a</sup>Total magnitude of the system. <sup>b</sup>Reid et al. (2003b). <sup>c</sup>Lodieu et al. (2005). <sup>d</sup>Scholz et al. (2005).</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Spectral type obtained from the TiO5 (filled circles) and PC3 (open circles) indices versus the adopted spectral type checked against the spectra of comparison stars.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** *M*<sub>*J*</sub> versus spectral-type calibration relations from Crifo et al. (2005) (long-dashed line), Cruz et al. (2003) (thin line), Hawley et al. (2002) (thick-dashed line), Cruz & Reid (2002) (thick line), and Dahn et al. (2002) (long-dashed thick line). For the Crifo et al. (2005) and Cruz & Reid (2002) relations, the PC3 and TiO5 indices had first to be transformed to spectral types, respectively. The calibration we adopted is a combination of these relations, and is shown by the filled circles. For comparison, the pluses show the values given by Scholz et al. (2005). Spectral types are from M0 to M9.</figcaption></figure>

For types earlier than M3.5, we computed the mean of the *M*<sub>*J*</sub> obtained using Hawley et al. (2002) and Cruz & Reid (2002) relations. For the stars falling in the region M3.5 to M4.5, where there is a clear discontinuity in the calibrating relations (see Reid & Cruz 2002, for a discussion on this jump), we averaged the absolute magnitudes obtained from the upper (Cruz & Reid 2002) and lower relations (Cruz & Reid 2002; Crifo et al. 2005), as well as the fixed value of *M*<sub>*J*</sub> = 8.34 (Hawley et al. 2002). For types M5 to M6, we used the relations by Cruz & Reid (2002) and Crifo et al. (2005). For later types, the relations are nearly equal. For practical reasons, we only used the relation by Cruz et al. (2003) which is directly given as a function of spectral type. The filled circles in Fig. 6 show the result

of this combination of existing calibrating relations. The values provided by Scholz et al. (2005) are shown for comparison (pluses).

The resulting values of *M*<sub>*J*</sub> and spectroscopic distances of our candidates are given in Table 5. The uncertainties are also given, taking into account the 0.5 subclass uncertainty on the spectral type. They are less than 15 per cent. For the spectral types M3.5 to M4.5, the uncertainties are computed, given the upper and lower calibrations. The are larger, ~34 per cent. For the brightest stars with *J* < 9, we used 2MASS *J* magnitudes instead of the DENIS due to saturation problems that may occur in the DENIS bands.

Other determinations of spectral types and spectroscopic distances are given in Table 5. They are in agreement with our values.

## 5.2 Subdwarf

We computed the distance of the subdwarf LHS 73 assuming an absolute magnitude *M*<sub>*J*</sub> = 8.72, following Reid & Gizis (2005). The resulting distance is 18.7 pc. LHS 73 has a common proper motion companion, LHS 72, with a separation of 94 arcsec. LHS 72 has been spectroscopically classified as a subdwarf by Rodgers & Eggen (1974). A quite uncertain trigonometric parallax, 37.6 ± 8.9 mas (21.5 < *d* < 34.8 pc) can be found for this star in the Yale Trigonometric Parallaxes (van Altena, Lee & Hoffleit 1995).

Most K/M subdwarfs with trigonometric parallaxes are listed in Gizis (1997). Only a handful of these (LHS 20, LHS 29, LHS 35, LHS 44, LHS 55, LHS 103, LHS 3409) are within 20 pc. Another pair of subdwarfs, LHS 52/LHS 53, is slightly more distant (*d* = 29.8 ± 1.5 pc). To our knowledge, LHS 72/LHS 73 is the closest pair of subdwarfs.

## 5.3 Binaries

Besides LHS 73 which belongs to a common proper motion pair (Section 5.2), two objects in our sample are binaries.

LTT 8708A/B is an already known system. We used the acquisition image, obtained in the ESO *R* band, to derive a separation of 8.1 pixel. With a scale of 0.33 arcsec pixel<sup>-1</sup>, this corresponds to an angular separation of 2.7 arcsec. The spectra of the two components show that they are both M0 stars, with absolute magnitudes *M*<sub>*J*</sub> = 6.33. With an observed total magnitude *J* = 7.75, we derived a distance of 27.2 ± 1.8 pc. This result is in agreement with the trigonometric parallax derived by *Hipparcos*, although the uncertainty was very large (74.42 mas ± 41.23 or a distance between 8.6 and 30.1 pc). Hawley, Gizis & Reid (1997) found a smaller distance, *d* = 17.5 pc, and a spectral type of M2.5.

APMPM J0541−5349 is clearly seen as a binary on the acquisition image (Fig. 7). The separation between the components is 10.1 pixel, corresponding to 3.33 arcsec. The examination of earlier images, such as the 2MASS or the DSS2, shows that they are a common proper motion pair. Two objects are given in the 2MASS point-source catalogue, with magnitudes *J* = 10.58 and 10.64. Both components have an M2 spectral type, and an absolute magnitude *M*<sub>*J*</sub> = 6.97. Thus, the system is at a distance of 53.4 ± 4.1 pc.

# 6 CHROMOSPHERIC ACTIVITY

*H* α is a chromospheric activity indicator for M dwarfs. Stars showing an *H* α emission line are indicated by the additional letter ‘e’ in the spectral-type column of Table 5. Although the size of our sample is too small to have a strong statistical sense, we find a correlation between the activity and the spectral type. The number of active stars is greater among late-type stars than among early-type
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Part of the R-band NTT image of the binary APMPM J0541−5349. The size of the image is 30 × 30 arcsec². North is up, and east is to the left-hand side.</figcaption></figure>

stars and is increasing across the spectral-type sequence. None of the stars with spectral type earlier than M3 (over 14 objects) shows an H α emission line, whereas all of our three stars later than M8 show evidence of activity. Twenty-four per cent of the M3.5–M4 stars (over 21 objects), and 47 per cent of the M4.5–M6 stars (over 16 objects) show activity. This is consistent with the fraction of objects with H α in emission versus spectral type reported by Gizis et al. (2000) from a larger sample of M dwarfs.

## 7 CONCLUSION

We obtained spectral types and spectroscopic distances for 59 high proper motion stars. These stars are nearby candidates, as seen from their photometric distances. Forty-two stars have indeed spectroscopic distances below the 25 pc limit of the CNS3. These new neighbours are few in regards to the large number of missing stellar systems within 25 pc (~2000), according to Henry et al. (2002). However, this study, joined to similar ones, allows to increase, step by step, the completeness of the solar neighbourhood sample and to cross-check the new nearby candidates in different samples (see e.g. Table 1). Nevertheless, it is clear that further efforts are needed in order to reach a much higher completeness level.

The neighbours recovered from our study are mainly M dwarfs; two are cool white dwarfs. One star, APMPM J0541−5349, appeared to be a binary consisting of two M2 stars with an angular separation of 3.33 arcsec, at a distance of 53.4 ± 4.1 pc from the Sun. We computed the radial velocity of the subdwarf candidate LHS 73 and found a space motion compatible with that of an object belonging to an old population. With our determined distance of 18.7 pc, LHS 73 is among the few subdwarfs within 20 pc. Furthermore, LHS 73 has a companion, LHS 72. LHS 72/LHS 73 is probably the closest known pair of subdwarfs. Two stars lie within 10 pc: APMPM J0103−3738 is an M8 star at a distance $d = 10.0 ± 0.8$ pc, and L 225−57 is an M4 star at $d = 8.3 ± 2.8$ pc.

The new sources that we identified are worthy of more detailed observations, including high-resolution imaging to search for low-mass companions, and trigonometric parallax measurements. We plan more follow-up for the nearest objects and hope to place the most promising targets on new or existing parallax programmes such

as that of the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI Jao et al. 2005).

## ACKNOWLEDGMENTS

CR acknowledges help during the observations by Olivier Hainaut and the NTT team at the ESO. This research has made use of the VizieR database, operated at the CDS, Strasbourg, France.

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