3. Source selection

Out of the 1084 RASS sources in our studied region, only 179 (about ) can be identified with known stellar and extragalactic counterparts in the Simbad database. Clearly, for efficient ground-based optical follow-up observations a pre-selection among the remaining sample of more than 900 X-ray sources is needed. A general statistical discrimination method was introduced by Sterzik et al.\ (1995) in order to find promising TTS candidates in the ROSAT database and was applied to study the spatial distribution of TTS candidates in the Orion SFR. In the present case, we apply the same method to our sample of RASS sources. A detailed description of the procedure can be found in Sterzik et al. (1995). Here we only summarize the relevant steps:

1. We establish a "training set'', consisting of members of previously classified groups, in our case TTS and non-TTS. As our observations are carried out close to the Taurus SFR, we conveniently draw the sources of the training set from follow-up observations of ROSAT sources reported by Wichmann et al. (1996). We use the properties of 63 TTS and 87 non-TTS of their survey (all detected in the RASS). We added also 54 RASS-detected TTS from the Herbig & Bell (1988) catalog. Our training set presents a uniform spatial distribution.
2. As discrimination parameters we choose: and determined for each X-ray source, V of the closest optical counterpart in the GSC within around the X-ray source, and the X-ray to optical flux ratio . If no GSC counterpart is present within we assign V=20, indicating a faint optical X-ray source not detected in the GSC.
3. A k-nearest neighbourhood analysis is applied to all sources in the sample. A discrimination probability P_i for each source i is defined as
   
   where is the number of TTS in the training set "in the neighbourhood'' of source i in the four-dimensional discrimination parameter space. The neighbourhood is defined as the sphere containing exactly
   
   members of the training set. For the present study we have chosen k=16, because this value proved to give the best results in telling TTS from non-TTS within the training set. High values of P_i indicate that the source properties are similar to those in the sample of TTS of the training set.

Source selection on this basis implies a bias towards (X-ray and optical) properties of typical TTS in the training set, as desired. However, the selection cannot guarantee a "pure'' TTS sample, because other classes of stars such as active binaries, RS CVn binaries, emission line dwarfs, etc., share similar values within the discrimination parameter set, and are expected to contaminate any sub-sample.

Figure 2 (click here) summarizes the definition of the X-ray sample according to the discrimination probability P. We note that, above a discrimination threshold of 0.6 (0.5), we have observed 97 (110) sources out of a total of 180 (246) candidates. It can be seen that many X-ray sources with high discrimination probability, i.e. many TTS candidates, indeed are new PMS stars (see Sects. 4, 5), while there are no new PMS stars among X-ray sources with low discrimination probability.

  
Figure 2: Source selection: The histogram gives the distribution of discrimination probability P for all 1084 X-ray sources in our studied area. Plotted is the number of stars per P bin (with bin size 0.05). The hatched histogram indicates the observed sub-sample with dKe/dMe stars and new PMS stars (including certain and possible new PMS stars). There are a total of 47 X-ray counterparts classified as either PMS or PMS? and 22 other X-ray counterparts classified as dKe or dMe stars. There are no RASS sources with P > 0.9

Whereas previously this classification scheme was primarily applied to demonstrate the discrimination ability within the training set itself (as the only a priori known classified sample), the results of this work indeed prove the prediction quality of the suggested procedure and allow a reliable extrapolation of the number of different types of active stars in the sample.

In Table 1 (click here) we list all the RASS sources selected in the way described above. We list ROSAT source name, X-ray counts (background subtracted and vignetting corrected) detected in the broad ROSAT band, exposure time, hardness ratios, the maximum likelihood for existence of the source, and spectral fit results. Spectral fits are performed - as described in Neuhäuser et al. (1995b) - using the hardness ratios and assuming a one-temperature Raymond-Smith model (Raymond & Smith 1977). Both the emission energy and the absorbing foreground column density are taken as free fit parameters. The procedure searches for the (, ) pair that best fits the observed (, ) pair. From the ROSAT PSPC response matrix and the fit results, we can then compute the individually spectral-corrected X-ray flux (also given in Table 1 (click here)). The X-ray source positions can be obtained from the positions of the optical counterparts and the offsets between X-ray and optical positions (given in Table 2 (click here)).

 

(s) (keV) (cm^-2) (erg s^-1 cm^-2)

RXJ 0209.1-1536 16.3 5.5 444.2 0.95 0.49 -0.09 0.35 21.8 0.72 20.85 5.17

RXJ 0210.4-1308 21.5 7.3 548.2 0.35 0.31 0.20 0.39 17.7 1.07 18.10 3.53

RXJ 0212.3-1330 17.9 6.8 556.2 0.20 0.38 0.23 0.45 10.9 1.22 18.00 2.98

RXJ 0215.0-1402 9.9 4.5 529.3 0.67 0.38 0.43 0.53 12.4 1.67 20.35 3.06

RXJ 0218.6-1004 17.9 5.6 486.2 1.00 0.12 -0.04 0.31 21.7 0.58 21.55 11.53

RXJ 0219.4-1321 11.6 4.7 523.8 0.92 0.63 -0.03 0.40 12.3 0.81 20.60 2.76

RXJ 0219.7-1026 46.2 9.4 530.5 0.49 0.19 0.46 0.19 53.1 1.70 20.10 12.16

RXJ 0220.4-1305 15.8 6.5 506.5 0.07 0.45 -0.23 0.25 10.5 0.39 18.00 2.31

RXJ 0223.3-1615 8.6 4.1 532.1 1.00 0.22 0.18 0.46 9.6 0.57 21.85 21.09

RXJ 0229.5-1224 12.3 5.0 516.4 0.60 0.35 0.41 0.37 12.8 1.68 20.25 3.70

RXJ 0237.3-0527 9.2 4.5 279.4 0.22 0.48 0.44 0.68 7.7 1.63 19.45 3.60

RXJ 0239.1-1028 6.5 3.3 326.0 0.33 0.51 0.26 0.24 12.7 1.15 18.90 1.91

RXJ 0243.9-0850 8.3 4.5 341.7 0.44 0.44 0.55 0.74 7.5 1.67 20.00 3.22

RXJ 0248.3-1117 8.0 4.3 418.7 0.16 0.28 0.01 0.38 7.7 1.15 18.00 1.76

RXJ 0251.8-0203 8.5 3.9 325.7 0.43 0.43 0.09 0.46 12.6 0.96 18.00 2.28

RXJ 0254.8-0709 10.4 4.4 329.7 0.70 0.32 0.76 0.26 15.7 1.94 21.80 20.03

RXJ 0255.8-0750 8.7 4.3 330.5 0.89 0.80 0.30 0.42 10.0 1.17 20.60 4.15

RXJ 0309.1+0324 28.5 7.2 614.3 0.81 0.19 0.39 0.24 38.1 1.39 20.50 7.75

RXJ 0312.8-0414 34.0 7.9 424.7 0.64 0.20 0.07 0.23 43.4 0.93 19.70 8.31

RXJ 0314.8-0406 6.7 3.9 465.8 1.00 0.29 0.46 0.48 8.5 0.76 22.00 24.47

RXJ 0317.9+0231 14.3 5.6 541.9 1.00 0.16 -0.01 0.39 12.2 0.58 21.60 10.67

RXJ 0319.3+0003 8.7 4.4 603.2 1.00 0.26 -0.10 0.51 8.8 0.55 21.45 4.66

RXJ 0324.4+0231 21.7 7.1 461.1 0.36 0.34 0.15 0.38 16.6 1.03 18.00 4.23

RXJ 0329.1+0118 11.9 4.9 637.8 1.00 0.14 0.50 0.35 12.5 0.83 22.00 28.73

RXJ 0330.7+0306 54.4 9.8 737.7 1.00 0.04 0.05 0.18 54.9 0.59 21.70 40.64

RXJ 0333.0+0354 9.9 4.5 634.2 1.00 0.22 0.57 0.46 10.5 0.94 22.00 21.98

RXJ 0333.1+1036 20.3 6.2 564.3 0.36 0.25 0.48 0.26 30.8 1.66 19.85 4.55

RXJ 0336.0+0846 8.7 4.0 543.0 0.78 0.85 -0.26 0.44 10.7 0.51 20.30 1.72

RXJ 0338.1+1224 8.3 4.2 555.1 1.00 0.23 0.25 0.50 10.0 0.60 21.90 18.40

RXJ 0338.3+1020 13.2 4.8 529.8 1.00 0.16 0.17 0.35 15.1 0.64 21.80 19.04

RXJ 0339.6+0624 8.6 4.1 569.3 1.00 0.20 0.47 0.40 9.7 0.78 22.00 24.42

RXJ 0340.3+1220 26.9 6.6 517.6 0.85 0.17 -0.18 0.25 49.3 0.64 20.35 5.77

RXJ 0340.5+0639 5.3 2.9 522.8 1.00 0.26 0.26 0.25 9.8 0.61 21.90 12.48

RXJ 0341.2+0453 37.6 8.2 538.1 1.00 0.04 0.26 0.21 56.6 0.63 21.90 81.67

RXJ 0341.2+0759 8.1 3.9 567.9 1.00 0.23 0.19 0.47 8.3 0.59 21.85 17.56

RXJ 0343.6+1039 17.5 6.0 555.3 0.89 0.30 0.60 0.24 21.0 2.02 21.45 11.78

RXJ 0344.8+0359 25.5 6.9 529.7 0.88 0.13 -0.15 0.29 29.0 0.67 20.45 5.55

RXJ 0347.2+0933 16.1 5.4 527.6 0.58 0.28 0.85 0.16 19.9 2.02 22.00 32.54

RXJ 0347.9+0616 8.6 4.0 466.0 1.00 0.24 0.43 0.19 11.3 0.70 22.00 33.14

RXJ 0348.2+1109 23.8 6.5 471.2 0.30 0.28 -0.07 0.32 29.7 0.93 18.00 4.41

RXJ 0348.5+0832 23.3 6.4 486.9 0.92 0.23 -0.28 0.29 30.6 0.44 20.80 6.81

RXJ 0349.4+1255 19.7 5.7 479.3 0.81 0.35 -0.25 0.29 42.5 0.53 20.35 4.56

RXJ 0350.2+0849 6.6 3.3 470.2 1.00 0.24 0.07 0.51 11.8 0.61 21.70 7.74

RXJ 0350.6+0454 7.4 3.7 488.1 1.00 0.23 0.09 0.50 11.6 0.58 21.75 12.09

RXJ 0350.6+1033 12.5 5.3 569.5 1.00 0.17 0.51 0.33 9.4 0.85 22.00 33.80

RXJ 0351.4+0953 11.8 4.9 491.4 0.90 0.24 -0.25 0.42 8.8 0.51 20.65 2.95

RXJ 0351.8+0413 6.5 3.3 441.1 1.00 0.25 -0.10 0.52 9.2 0.55 21.45 4.77

RXJ 0352.4+1223 11.8 4.8 522.4 1.00 0.18 -0.00 0.42 13.3 0.55 21.65 9.48

RXJ 0354.1+0528 7.4 4.1 535.0 1.00 0.26 0.55 0.19 7.6 0.91 22.00 20.34

 

 

(s) (keV) (cm^-2) (erg s^-1 cm^-2)

RXJ 0354.4+1204 8.2 3.9 474.2 0.58 0.39 1.00 1.00 13.2 1.62 20.20 2.53

RXJ 0354.8+1232 16.6 5.9 505.8 0.81 0.33 -0.11 0.36 18.3 0.73 20.15 3.42

RXJ 0356.7+0943 10.3 5.1 507.5 0.60 0.39 -0.40 0.42 9.1 0.32 20.35 2.80

RXJ 0357.3+1258 17.9 6.1 486.0 0.65 0.24 -0.39 0.30 15.9 0.33 20.40 4.63

RXJ 0358.1+0932 27.6 6.9 467.6 0.86 0.22 0.57 0.20 42.7 2.02 21.35 19.43

RXJ 0400.0+0730 10.0 4.1 459.9 1.00 0.15 0.61 0.33 14.9 1.00 22.00 29.38

RXJ 0400.1+0818 169.6 16.6 450.7 0.27 0.10 0.17 0.12 298.5 1.11 18.00 34.22

RXJ 0400.8+1116 15.7 5.7 494.8 1.00 0.15 -0.05 0.38 13.4 0.56 21.55 10.26

RXJ 0402.5+0552 18.2 5.8 414.1 0.68 0.27 0.24 0.37 19.5 1.11 20.05 5.37

RXJ 0403.5+0837 20.2 6.2 480.6 1.00 0.09 0.56 0.25 22.3 0.93 22.00 59.17

RXJ 0404.4+0519 20.3 5.6 413.6 1.00 0.06 -0.06 0.28 41.8 0.57 21.55 15.87

RXJ 0405.5+0324 34.2 7.8 445.4 1.00 0.04 0.03 0.23 54.4 0.57 21.70 44.18

RXJ 0407.2+0113 10.1 4.1 421.0 1.00 0.16 0.35 0.39 13.0 0.52 22.00 56.03

RXJ 0407.6+0638 19.2 6.4 474.5 1.00 0.08 -0.25 0.34 16.3 0.46 21.30 9.69

RXJ 0408.6+1017 5.9 3.4 375.6 1.00 0.25 0.53 0.43 8.0 0.88 22.00 23.10

RXJ 0408.8+1028 106.0 13.6 387.9 0.52 0.11 0.24 0.14 159.7 1.11 19.65 29.51

RXJ 0409.8+1209 10.5 5.1 357.3 0.76 0.65 0.30 0.23 7.7 1.19 20.30 4.21

RXJ 0410.6+0608 8.7 3.9 395.1 1.00 0.19 0.08 0.46 10.1 0.56 21.75 18.47

RXJ 0413.2+1028 13.0 4.6 374.5 1.00 0.01 0.56 0.11 20.0 0.97 22.00 48.87

RXJ 0418.6+0143 9.2 4.5 411.3 0.52 0.40 0.46 0.39 8.2 1.61 20.10 3.11

RXJ 0419.8+0214 17.3 6.1 404.5 1.00 0.12 0.11 0.35 13.6 0.62 21.75 34.11

RXJ 0419.9+0231 47.7 9.7 385.4 1.00 0.04 0.06 0.18 12.5 0.61 21.70 7.69

RXJ 0422.9+0141 15.4 5.9 444.8 0.90 0.25 0.50 0.33 10.1 1.78 20.85 7.51

RXJ 0423.5+0955 33.9 8.8 577.2 0.01 0.26 -0.00 0.35 22.9 2.02 18.00 5.54

RXJ 0425.5+1210 21.3 6.2 641.0 1.00 0.08 -0.09 0.29 23.5 0.55 21.50 10.75

RXJ 0426.4+0957 16.3 5.6 657.7 1.00 0.12 -0.09 0.34 18.6 0.54 21.50 8.01

RXJ 0427.4+1039 20.6 6.3 654.3 1.00 0.10 0.62 0.25 23.3 1.02 22.00 42.55

RXJ 0427.5+0616 67.2 11.4 612.2 0.37 0.16 -0.06 0.20 75.5 0.90 18.00 9.45

RXJ 0427.8+0049 17.8 6.0 495.9 0.84 0.23 -0.05 0.34 17.7 0.80 20.25 4.04

RXJ 0429.9+0155 15.1 5.1 534.7 1.00 0.11 0.33 0.30 18.5 0.63 21.95 53.69

RXJ 0433.7+0522 15.1 6.3 817.3 1.00 0.18 0.31 0.39 10.2 0.59 21.95 37.34

RXJ 0434.3+0226 33.6 8.1 775.6 1.00 0.04 0.11 0.24 39.0 0.56 21.80 36.33

RXJ 0435.5+0455 19.6 6.0 746.2 1.00 0.08 0.37 0.26 25.4 0.58 22.00 53.09

RXJ 0441.9+0537 96.3 13.6 582.9 1.00 0.01 0.50 0.12 123.1 0.89 22.00 242.90

RXJ 0442.3+0118 19.8 7.1 528.9 0.27 0.44 0.34 0.43 14.1 1.31 19.15 3.76

RXJ 0442.5+0906 22.0 6.4 532.0 0.36 0.28 -0.03 0.29 24.2 0.91 18.00 3.56

RXJ 0442.6+1018 83.9 12.0 537.8 0.90 0.07 0.07 0.14 142.0 0.91 20.55 19.83

RXJ 0442.9+0400 17.4 5.9 532.6 1.00 0.11 0.11 0.34 21.3 0.62 21.75 26.06

RXJ 0444.3+0941 62.2 10.4 568.1 0.58 0.14 0.11 0.17 95.5 0.97 19.55 10.84

RXJ 0444.4+0725 12.6 4.9 514.6 1.00 0.15 -0.30 0.39 12.6 0.35 21.30 7.16

RXJ 0444.7+0814 18.9 6.6 571.0 0.58 0.33 0.32 0.35 19.2 1.25 20.00 4.27

RXJ 0445.2+0729 10.1 5.0 568.8 1.00 0.21 0.19 0.46 8.0 0.59 21.85 21.86

RXJ 0445.3+0914 9.5 4.7 600.6 1.00 0.21 0.43 0.51 8.9 0.70 22.00 28.40

RXJ 0445.5+1207 9.4 4.4 617.3 0.78 0.50 0.60 0.40 11.6 2.02 21.40 5.01

RXJ 0448.0+0738 88.8 12.8 535.0 0.28 0.14 -0.13 0.18 93.9 0.44 18.00 12.52

RXJ 0450.0+0151 20.6 6.3 535.6 0.95 0.21 -0.08 0.31 21.1 0.74 20.80 5.16

RXJ 0451.6+0619 9.5 4.3 558.8 1.00 0.18 -0.04 0.45 8.9 0.58 21.55 5.33

RXJ 0459.9+1017 29.4 7.7 603.5 1.00 0.05 0.32 0.23 27.8 0.62 21.95 98.47

RXJ 0511.2+1031 18.4 6.4 661.5 1.00 0.13 0.26 0.37 15.7 0.62 21.90 34.24

RXJ 0511.9+1112 12.9 4.8 618.1 0.71 0.36 0.29 0.37 12.8 1.18 20.20 2.87

RXJ 0512.0+1020 23.6 6.7 651.2 0.78 0.21 0.13 0.29 23.4 0.99 20.20 4.46

RXJ 0513.6+0955 9.2 3.9 593.2 0.43 0.42 -0.45 0.17 14.4 0.29 20.25 1.98

 

(s) (keV) (cm^-2) (erg s^-1 cm^-2)

RXJ 0516.3+1148 24.9 7.3 658.6 0.98 0.24 -0.03 0.29 23.8 0.54 21.60 15.87

RXJ 0523.0+0934 8.3 4.1 429.6 0.74 0.52 -0.25 0.51 8.0 0.54 20.15 1.92

RXJ 0523.5+1005 28.7 6.9 466.5 0.93 0.15 0.15 0.22 49.7 0.98 20.70 9.10

RXJ 0523.9+1101 6.8 3.3 498.8 0.28 0.40 -0.10 0.24 16.2 0.78 18.00 1.14

RXJ 0525.7+1205 15.6 6.3 846.4 1.00 0.15 0.62 0.13 9.9 1.02 22.00 24.91

RXJ 0528.4+1213 33.8 7.9 661.9 1.00 0.05 0.06 0.23 43.3 0.61 21.70 28.14

RXJ 0528.5+1219 11.0 4.4 626.0 0.66 0.29 0.30 0.37 17.0 1.19 20.10 2.32

RXJ 0528.9+1046 13.2 5.0 639.2 1.00 0.16 0.64 0.25 21.4 1.05 22.00 26.86

RXJ 0529.3+1210 23.2 6.9 631.9 0.15 0.29 0.03 0.42 24.2 1.20 18.00 3.40

RXJ 0530.9+1227 28.2 7.5 693.4 1.00 0.06 0.34 0.25 23.2 0.51 22.00 94.99

RXJ 0531.8+1218 10.5 4.8 688.5 0.61 0.30 -0.02 0.46 7.9 0.84 19.40 1.42

In Table 2 (click here) we list the closest optical counterpart to each X-ray source which is sufficiently bright to be a potential PMS star at the distance of Taurus-Auriga (e.g., having V brighter than mag). All but two of the counterparts listed in Table 2 (click here) are GSC stars. This is a consequence of the selection process, as we have searched for GSC counterparts and used their V magnitudes to select PMS candidates. The two non-GSC stars selected are listed in Simbad but, before our observations, it was not known whether they were TTS or not. In some cases, we have performed optical spectroscopy for several possible counterparts, when more than one star was found close to the X-ray position. As their V magnitudes have not been used to estimate the likelihood that the relevant X-ray source may be a PMS star, we do not list these additional counterparts in Table 2 (click here), but in Table 3 (click here). If several optical counterparts are observed, we distinguish them by the letters at the end of the relevant ROSAT source designations (identifying the relative positions of the different counterparts). In Table 2 (click here) we list also the offset between the X-ray and optical positions (X - optical), the V magnitude, the X-ray to optical flux ratio, and the discrimination probability P, i.e. the value that reflects to which degree a source resembles typical TTS properties.