on the infrared void in the lupus dark clouds
TRANSCRIPT
On the infrared void in the Lupus dark clouds
G. A. P. FrancoP
Departamento de Fısica - ICEx - UFMG, Caixa Postal 702, 30.123-970 - Belo Horizonte - MG, Brazil
Accepted 2001 November 27. Received 2001 November 17; in original form 2001 April 23
A B S T R A C T
Stromgren uvbyb photometry observations obtained for 205 stars in the general direction of a
void in the IRAS 100-mm emission from the Lupus dark cloud complex are presented and
analysed. The colour excess versus distance diagram confirms the existence of a region
depleted from interstellar material, which is also seen in the ROSAT soft X-ray background
emission map. The distance to the surrounding material is estimated as being within the
interval from 60 to 100 pc. This result is in disagreement with previous distance estimates to
the supposed supernova that has been suggested as responsible for clearing the region from
dust. As an alternative, the data presented support the suggestion that the void may have been
produced by the detachment of material from the interface between Loop I and the Local
Bubble as a consequence of hydromagnetic instabilities. Moreover, the distribution of colour
excess as a function of distance supports a value of ,150 pc as the most probable distance to
the dark cloud known as Lupus 1.
Key words: stars: distances – ISM: clouds – dust, extinction – ISM: individual: Lupus.
1 I N T R O D U C T I O N
The existence of a void in the 100mm emission from dust in a
region toward the Lupus dark cloud complex was reported by
Gahm et al. (1990). This void coincides in position and shape with
an extended soft X-ray source, H 1538232, previously discovered
with the HEAO A-2 experiment by Riegler, Agrawal & Gull (1980),
who, on the basis of the extended nature of the source and its
thermal spectrum, suggested that it could be an old supernova
remnant. This hypothesis has been further reinforced by the
remarkable correlation between the X-ray source, radio-continuum
emission and the 21-cm H I line (Colomb, Dubner & Giacani 1984,
cf. their fig. 4). Moreover, the H I line shows dynamical effects
which were interpreted as due to the interaction of the expanding
shock front with a cloudy surrounding medium.
The Lupus molecular clouds make up a large complex, which
consists of six subgroups designated as Lupus 1 to 6 (Schwartz
1977; Tachihara et al. 1996; Cambresy 1999). Evidence for
extensive low-mass star formation has been reported by several
authors (Krautter 1991, and references therein; Hughes et al. 1994),
and since recent distance estimates locates this complex at
150–170 pc (Franco 1990; Krautter 1991; Hughes, Hartigan &
Clampitt 1993; Rizzo, Morras & Arnal 1998; Crawford 2000),
these clouds are one of the nearest star-forming regions to the Sun.
To further refine the knowledge of reddening and distance to the
interstellar structure along the direction of the void, a photometric
programme has been carried out. The ‘void’, seen in a grey-scale
100-mm emission map from IRAS in Fig. 1 is characterized by a
region of very low infrared emission seen near the centre of the
figure. In order to keep close to the shape presented by Gahm et al.
(1990), its border-line was set at 18 MJy sr21. The void is
surrounded by a region of weak emission ð18 , I100 ,
28 MJy sr21; hereafter called ‘intercloud’), with a sharp increase
in emission towards the south-east. To the south of the void one
sees the infrared emission connected to the dark cloud designated
as Lupus 1 – it is instructive to compare the shape of this cloud
with its counterpart seen in an extinction map (e.g. Cambresy 1999,
cf. his fig. 8). Patches of higher infrared emission are seen to the
north-west of the void. The lines of sight showing infrared
emission I100 > 28 MJy sr21 will be referred to as ‘the cloud’
regions.
The analysis conducted in this paper shows that the infrared void
really corresponds to a region cleaned from dust; however, the
results do not support the interpretation that it was produced by the
action of a supernova event (Gahm et al. 1990). Instead, it is
suggested that the void is a hole in the interface between Loop I and
the Local Bubble, produced by the action of pressure differences
between these volumes, as proposed by Breitschwerdt, Freyberg &
Egger (2000).
2 O B S E RVAT I O N A L DATA
2.1 Stromgren uvbyb photometry
Four-colour uvby and Hb photometry was obtained for 205 stars
earlier than G0, having lines of sight along the observed void, and
almost covering the region of the sky displayed in Fig. 1. The
observing list was prepared from the SAO Star Catalog (togetherPE-mail: [email protected]
Mon. Not. R. Astron. Soc. 331, 474–482 (2002)
q 2002 RAS
Table 1. Stromgren photometry of stars earlier than G0 towards Lupus. The first five columns give the identification in the SAO Star Catalog, the HDnumber (when available), the Michigan two-dimensional classification (Houk 1982), and the right ascension and declination for the equinox 1950.0.Column 6 gives the computed V magnitude on the Johnson system. Figures in parentheses give the standard deviation of the measurements in units of0.001 mag. Columns 10 and 12 give the number of nights in which the star was observed in uvby and Hb, respectively.
SAO HD Spectral a1950 d1950 V ðb 2 yÞ m1
c1
n Hb ntype (h m s) (8 0) (mag) (mag) (mag) (mag) (mag)
183459 15 22 58 229 56 9.689 (04) 0.562 (00) 0.121 (07) 0.556 (10) 2 2.620 (09) 2183506 137798 G0 V 15 25 56 228 42 6.504 (02) 0.335 (02) 0.158 (03) 0.410 (04) 2 2.626 (05) 2183526 15 27 25 229 51 10.113 (07) 0.385 (09) 0.134 (09) 0.451 (05) 2 2.654 (10) 2183543 138270 F0 V 15 28 56 228 35 9.739 (02) 0.397 (02) 0.127 (02) 0.503 (21) 2 2.678 (03) 2183596 138791 A2mA7–F0 15 32 17 228 53 7.704 (00) 0.176 (02) 0.211 (02) 0.750 (04) 2 2.785 (02) 2183604 138874 F7 V 15 32 48 229 25 8.983 (07) 0.403 (00) 0.168 (03) 0.436 (02) 2 2.615 (06) 2183612 139020 A4 III 15 33 40 228 41 8.917 (01) 0.273 (08) 0.149 (12) 0.976 (02) 2 2.840 (09) 2183642 139291 F0 V 15 35 17 229 26 8.155 (03) 0.226 (00) 0.160 (02) 0.626 (05) 2 2.703 (05) 2183662 15 36 12 229 30 10.365 (07) 0.401 (09) 0.127 (20) 0.451 (30) 2 2.654 (15) 2183667 139500 Gþ F 15 36 28 228 21 9.332 (03) 0.401 (05) 0.156 (09) 0.417 (13) 5 2.619 (03) 2183676 139558 F6 V 15 36 50 229 35 8.415 (02) 0.337 (04) 0.142 (07) 0.351 (07) 5 2.617 (13) 2183678 139578 G0/2 V 15 36 55 229 47 9.835 (11) 0.418 (04) 0.152 (07) 0.379 (15) 5 2.626 (12) 2183680 139612 A0 V 15 37 02 229 03 9.192 (04) 0.110 (00) 0.098 (02) 1.049 (11) 3 2.857 (11) 2183684 139648 A3 V 15 37 16 229 48 8.638 (02) 0.147 (04) 0.162 (02) 0.980 (11) 3 2.819 (02) 2183695 139732 G2 V 15 37 47 229 14 9.201 (01) 0.373 (02) 0.170 (02) 0.412 (04) 2 2.609 (08) 2183711 15 38 50 229 58 9.985 (04) 0.381 (04) 0.144 (00) 0.502 (02) 2 2.659 (02) 2183714 139955 F5 V 15 39 05 228 43 9.287 (00) 0.356 (01) 0.163 (01) 0.460 (10) 3 2.643 (16) 2183740 140259 A9 V 15 40 37 228 26 9.543 (01) 0.676 (01) 0.428 (28) 0.347 (01) 2 2.589 (05) 2183741 140349 A9=F0þG=K 15 41 04 228 40 9.429 (01) 0.272 (02) 0.234 (08) 0.691 (06) 3 2.768 (09) 2183764 140635 A9 V 15 42 45 229 42 9.224 (12) 0.202 (04) 0.144 (06) 0.818 (13) 3 2.756 (00) 2183768 140673 F2 V 15 42 55 229 01 9.051 (05) 0.309 (04) 0.150 (01) 0.604 (05) 3 2.700 (24) 2183792 140992 A7/8 II 15 44 32 228 07 10.369 (01) 0.195 (03) 0.167 (09) 0.863 (09) 3 2.791 1183810 141107 F2 V 15 45 18 228 38 7.683 (04) 0.301 (02) 0.117 (01) 0.451 (08) 3 2.628 (07) 2183814 141148 F3 V 15 45 31 229 51 9.270 (04) 0.304 (04) 0.152 (05) 0.483 (06) 3 2.676 1183818 141190 A7 IV 15 45 46 229 19 7.964 (01) 0.160 (02) 0.164 (02) 0.831 (08) 3 2.772 (10) 2183830 141337 A8/9 III 15 46 33 229 16 9.087 (01) 0.187 (00) 0.200 (02) 0.948 (03) 3 2.854 1183841 141444 B9.5 V 15 47 07 228 33 8.957 (02) 0.081 (05) 0.110 (07) 1.103 (07) 3 2.858 1183842 141445 A0mF0–F0 15 47 08 229 16 9.674 (02) 0.259 (00) 0.225 (01) 0.762 (17) 2 2.811 1183844 141480 A3 III/IV 15 47 18 229 15 9.005 (07) 0.134 (04) 0.146 (08) 1.139 (02) 3 2.860 1183852 141576 A0 V 15 47 48 229 21 9.067 (04) 0.090 (00) 0.126 (00) 1.074 (01) 2 2.914 1183862 141664 F0 V 15 48 13 229 32 9.334 (01) 0.299 (07) 0.142 (02) 0.608 (05) 2 2.688 1183868 141756 G2 V 15 48 45 228 35 9.559 (05) 0.372 (04) 0.165 (16) 0.385 (30) 2 2.618 1183870 15 48 56 228 03 10.141 (00) 0.379 (07) 0.164 (05) 0.476 (04) 2 2.645 1183889 141960 A9 V 15 49 50 229 41 8.530 (06) 0.171 (00) 0.185 (04) 0.756 (05) 2 2.767 1183892 142058 F5 V 15 50 21 228 50 9.066 (00) 0.316 (01) 0.122 (09) 0.450 (02) 2 2.654 1183899 142147 B9 II 15 50 50 228 02 10.108 (01) 0.014 (00) 0.081 (05) 0.629 (23) 2 2.693 1183908 142251 F3 V 15 51 28 229 00 9.495 (07) 0.325 (02) 0.154 (07) 0.517 (15) 2 2.708 1183934 142457 F2 III=IVþ A6=8 15 52 40 228 12 8.028 (11) 0.193 (04) 0.191 (12) 0.855 (06) 2 2.792 (21) 2183943 15 53 06 228 03 9.939 (02) 0.357 (04) 0.137 (04) 0.529 (09) 2 2.646 1183951 15 53 23 228 35 9.904 (07) 0.325 (02) 0.145 (00) 0.471 (00) 2 2.653 1183957 142669 B2 IV/V 15 53 47 229 04 3.883 (04) 20.085 (01) 0.097 (04) 0.133 (03) 2 2.628 (00) 2183966 142727 F7 V 15 54 11 229 51 8.056 (03) 0.348 (00) 0.162 (02) 0.402 (10) 2 2.631 1183980 142954 F2 V 15 55 29 228 09 9.047 (02) 0.256 (05) 0.151 (07) 0.593 (00) 2 2.723 1183992 143050 F5 V 15 56 06 229 50 8.797 (02) 0.313 (02) 0.157 (03) 0.424 (01) 2 2.635 1183997 143113 A3/4 V 15 56 28 228 50 9.099 (07) 0.126 (07) 0.155 (01) 1.149 (07) 2 2.855 1184000 143115 A2mA7–F0 15 56 32 229 56 7.189 (04) 0.140 (07) 0.220 (14) 0.772 (08) 2 2.820 (00) 2184018 15 57 41 229 45 10.082 (07) 0.435 (08) 0.143 (10) 0.500 (12) 2 2.633 1184026 143403 A4 V 15 58 07 228 51 9.918 (01) 0.136 (01) 0.160 (02) 1.010 (09) 2 2.853 1206585 136780 F3/5 V 15 20 30 230 27 9.592 (01) 0.405 (02) 0.135 (01) 0.518 (12) 2 2.660 (04) 2206602 15 21 37 230 29 9.803 (01) 0.469 (02) 0.140 (02) 0.756 (15) 2 2.691 (09) 2206603 136988 F3 V 15 21 37 231 35 8.747 (00) 0.389 (02) 0.137 (00) 0.519 (09) 2 2.657 (00) 2206608 137055 F0 V 15 21 57 231 59 7.413 (00) 0.197 (03) 0.173 (06) 0.744 (03) 2 2.733 (02) 2206623 137132 F6 V 15 22 17 230 24 9.569 (01) 0.335 (01) 0.141 (09) 0.406 (06) 2 2.620 (15) 2206646 15 23 05 230 36 9.932 (02) 0.422 (04) 0.110 (14) 0.588 (00) 2 2.713 (02) 2206652 137340 F6 V 15 23 22 231 09 7.327 (02) 0.299 (00) 0.157 (02) 0.421 (02) 2 2.646 (03) 2206656 15 23 35 235 36 7.639 (02) 0.047 (02) 0.168 (00) 1.007 (02) 3 2.892 (04) 2206657 15 23 40 230 29 10.533 (01) 0.300 (04) 0.132 (10) 1.048 (13) 2 2.793 (06) 2206658 15 23 48 232 04 10.307 (02) 0.326 (03) 0.138 (14) 0.626 (13) 3 2.705 (06) 2206665 15 24 35 230 26 9.990 (04) 0.460 (00) 0.178 (09) 0.427 (19) 2 2.639 (07) 2206669 137595 B2 II/III 15 24 54 233 22 7.468 (11) 0.095 (03) 0.010 (05) 0.057 (01) 3 2.592 (02) 2206674 137616 B9.5/A0 IV/V 15 25 02 234 45 9.497 (06) 0.242 (06) 0.032 (08) 1.089 (06) 3 2.849 (14) 2206688 137728 A3 V 15 25 40 231 18 7.146 (05) 0.182 (00) 0.146 (00) 1.121 (09) 2 2.846 (03) 2206698 137834 F5 V 15 26 15 235 13 8.568 (07) 0.317 (03) 0.140 (02) 0.411 (02) 3 2.649 (10) 2206700 15 26 21 234 41 10.082 (01) 0.494 (03) 0.087 (14) 0.520 (08) 3 2.625 (12) 2206714 138044 F3 V 15 27 34 232 04 9.861 (03) 0.315 (00) 0.138 (07) 0.511 (00) 2 2.673 (17) 2206718 138089 F0 V 15 27 52 235 28 7.942 (05) 0.169 (03) 0.176 (04) 0.774 (02) 3 2.767 (07) 2206720 138138 A2/3 V 15 28 08 233 38 6.866 (07) 0.087 (03) 0.194 (06) 0.879 (04) 3 2.869 (10) 2206726 15 28 28 230 05 10.049 (11) 0.399 (06) 0.147 (09) 0.520 (14) 2 2.647 (04) 2
On the infrared void in the Lupus dark clouds 475
q 2002 RAS, MNRAS 331, 474–482
Table 1 – continued
SAO HD Spectral a1950 d1950 V ðb 2 yÞ m1
c1
n Hb ntype (h m s) (8 0) (mag) (mag) (mag) (mag) (mag)
206734 138221 B6/7 V 15 28 42 232 42 6.486 (02) 0.101 (03) 0.056 (04) 0.531 (03) 3 2.720 (09) 2206738 138297 A3 III/IV 15 29 12 233 06 8.606 (05) 0.176 (02) 0.196 (08) 0.971 (13) 3 2.891 (02) 2206739 138296 F2 V 15 29 14 230 58 8.630 (07) 0.280 (03) 0.155 (04) 0.591 (09) 2 2.724 (00) 2206746 15 29 43 233 22 10.124 (05) 0.480 (02) 0.108 (08) 0.588 (10) 3 2.668 (01) 2206749 138416 A1 V 15 29 55 233 57 9.659 (04) 0.272 (05) 0.088 (08) 1.149 (09) 3 2.900 (01) 2206754 138491 A5 V 15 30 22 234 11 7.999 (02) 0.257 (01) 0.120 (04) 1.146 (10) 3 2.828 (07) 2206765 15 30 52 231 22 9.925 (04) 0.372 (03) 0.169 (02) 0.440 (08) 2 2.650 (05) 2206768 138575 A0 V 15 31 01 233 00 6.986 (05) 0.001 (05) 0.156 (06) 1.003 (08) 3 2.905 (00) 2206774 15 31 11 233 28 10.133 (03) 0.525 (01) 0.113 (06) 0.836 (26) 3 2.769 (02) 2206795 138923 B8/9 V 15 33 02 232 55 6.259 (03) 20.031 (01) 0.112 (03) 0.744 (03) 3 2.782 (05) 2206798 15 33 18 233 14 9.237 (05) 0.427 (03) 0.080 (06) 1.118 (16) 3 2.877 (07) 2206801 139010 A0/1 V 15 33 35 232 28 8.025 (02) 0.067 (01) 0.143 (01) 1.093 (10) 3 2.896 (07) 2206806 139035 B8/9 III 15 33 50 232 39 8.185 (06) 0.059 (05) 0.090 (08) 1.059 (13) 3 2.787 (00) 2206810 139095 A9/F0 V 15 34 10 231 54 7.905 (12) 0.211 (04) 0.171 (03) 0.657 (08) 5 2.744 (09) 2206813 139124 F2 V 15 34 19 232 19 8.630 (07) 0.301 (04) 0.151 (02) 0.663 (03) 3 2.680 (00) 2206821 139184 F3/5 V 15 34 45 231 05 7.482 (03) 0.302 (00) 0.142 (01) 0.490 (11) 2 2.647 (02) 2206839 139449 F5 V 15 36 16 234 28 8.482 (03) 0.300 (06) 0.156 (09) 0.490 (13) 3 2.659 (03) 2206840 15 36 21 231 46 10.157 (07) 0.210 (09) 0.137 (10) 0.859 (03) 2 2.749 (18) 2206841 139501 F6 V 15 36 30 231 43 9.122 (08) 0.335 (02) 0.152 (03) 0.475 (08) 4 2.652 (09) 2206845 139520 F8/G0 V 15 36 40 230 46 8.846 (01) 0.354 (00) 0.164 (02) 0.343 (07) 2 2.627 (11) 2206846 139542 F3 V 15 36 41 234 29 8.840 (00) 0.274 (08) 0.164 (09) 0.479 (02) 2 2.671 (09) 2206849 139559 A7=9þ F=G 15 36 53 231 13 8.325 (06) 0.261 (03) 0.182 (06) 0.603 (06) 4 2.713 (10) 2206856 15 37 05 230 34 9.228 (04) 0.287 (02) 0.138 (02) 0.511 (12) 4 2.678 (12) 2206859 139649 A0 V 15 37 21 234 05 9.426 (01) 0.280 (03) 0.056 (10) 1.036 (19) 2 2.875 (00) 2206861 15 37 31 232 05 9.418 (04) 0.348 (02) 0.135 (05) 0.516 (12) 4 2.649 (03) 2206863 139676 A9 V 15 37 35 235 15 7.581 (02) 0.173 (01) 0.195 (03) 0.765 (05) 3 2.777 (09) 2206868 139766 A7/8 V 15 37 59 232 29 8.660 (09) 0.222 (06) 0.151 (06) 0.974 (07) 3 2.748 (03) 2206876 139899 F3 V 15 38 37 231 41 8.340 (09) 0.294 (04) 0.148 (05) 0.468 (05) 5 2.663 (11) 2206877 139883 F2 V 15 38 41 234 43 8.367 (10) 0.262 (09) 0.140 (08) 0.517 (03) 3 2.699 (09) 2206886 139979 F0 V 15 39 16 235 36 8.326 (08) 0.214 (07) 0.154 (06) 0.671 (05) 3 2.711 (08) 2206889 140008 B5 V 15 39 29 234 33 4.745 (02) 20.061 (02) 0.112 (04) 0.413 (07) 2 2.716 (00) 2206892 140037 B5 III 15 39 31 232 01 7.489 (04) 20.005 (02) 0.082 (04) 0.440 (05) 2 2.713 (08) 2206895 140075 A5/7 III 15 39 45 234 32 8.747 (06) 0.374 (04) 0.082 (10) 1.190 (18) 2 2.881 (04) 2206898 140105 A2 IV/V 15 39 58 230 55 9.556 (04) 0.267 (02) 0.230 (04) 0.678 (16) 2 2.744 (20) 2206899 140126 F3/5 V 15 40 01 233 11 9.294 (14) 0.383 (07) 0.151 (14) 0.499 (06) 2 2.663 (07) 2206901 140127 F5 V 15 40 04 235 05 8.509 (08) 0.303 (01) 0.158 (02) 0.424 (03) 3 2.656 (02) 2206906 140194 A2 V 15 40 20 230 22 7.432 (07) 0.069 (02) 0.143 (04) 1.186 (03) 3 2.878 (07) 2206907 140216 A9 V 15 40 20 231 50 9.570 (06) 0.179 (03) 0.198 (04) 0.889 (08) 3 2.784 (07) 2206908 140195 B9.5 V 15 40 20 233 19 9.197 (04) 0.141 (06) 0.108 (10) 1.045 (12) 3 2.881 (10) 2206909 140217 A1/2 IV 15 40 29 234 51 9.606 (05) 0.501 (04) 0.098 (05) 1.222 (30) 2 2.878 (07) 2206912 140241 F3 V 15 40 34 235 19 9.144 (05) 0.315 (06) 0.151 (08) 0.423 (10) 3 2.664 (07) 2206914 140260 F5 V 15 40 39 235 12 9.456 (01) 0.305 (04) 0.151 (05) 0.410 (13) 3 2.649 (04) 2206924 140390 F0 V 15 41 15 230 02 8.845 (15) 0.221 (03) 0.160 (04) 0.630 (16) 3 2.734 (18) 2206928 140442 A1 V 15 41 40 230 31 7.429 (05) 0.054 (01) 0.152 (02) 1.086 (04) 3 2.895 (02) 2206931 140475 A2 V 15 41 53 234 56 7.714 (03) 0.047 (01) 0.186 (02) 0.954 (02) 3 2.893 (04) 2206933 140495 F3 V 15 41 56 232 40 9.258 (04) 0.299 (03) 0.143 (02) 0.468 (06) 3 2.668 (03) 2206934 140497 A9/F0 V 15 41 58 233 59 9.402 (04) 0.674 (01) 0.164 (10) 0.875 (45) 2 2.751 (05) 2206944 140601 A0 V 15 42 31 231 48 9.130 (02) 0.051 (04) 0.131 (05) 1.107 (04) 3 2.881 (06) 2206952 15 42 56 231 04 10.459 (08) 0.235 (03) 0.165 (05) 0.724 (15) 3 2.726 1206955 140703 F3 V 15 43 12 235 43 8.195 (07) 0.291 (07) 0.149 (12) 0.499 (12) 3 2.661 (01) 2206958 140735 F3 V 15 43 19 232 25 9.624 (06) 0.306 (05) 0.155 (10) 0.451 (05) 3 2.658 1206960 140734 A3/5 IV 15 43 22 230 30 9.606 (07) 0.126 (07) 0.166 (09) 1.100 (16) 3 2.816 1206961 140783 A7 III 15 43 31 231 45 9.383 (07) 0.154 (05) 0.181 (00) 0.903 (00) 2 2.839 (00) 2206962 140784 B8 V 15 43 32 234 31 5.621 (12) 20.044 (05) 0.106 (02) 0.567 (07) 2 2.722 (04) 2206963 140782 F5 V 15 43 36 230 44 8.405 (07) 0.331 (04) 0.152 (04) 0.522 (08) 2 2.651 (07) 2206966 15 43 40 230 10 10.232 (04) 0.355 (03) 0.146 (06) 0.411 (14) 2 2.649 1206968 140817 A0 V 15 43 51 235 21 6.838 (16) 0.025 (01) 0.134 (03) 0.867 (00) 2 2.857 (04) 2206969 140840 B9/A0 V 15 43 52 235 21 7.361 (10) 0.000 (08) 0.151 (04) 0.976 (00) 2 2.907 (04) 2206973 140875 A2 IV 15 44 03 230 38 9.409 (05) 0.142 (02) 0.130 (04) 1.115 (22) 2 2.828 (15) 2206974 140900 F3/5 V 15 44 06 232 27 8.639 (12) 0.302 (02) 0.149 (09) 0.444 (16) 2 2.670 (00) 2206975 15 44 07 234 23 10.211 (05) 0.446 (04) 0.121 (09) 0.543 (02) 2 2.669 1206978 140940 F7/8 V 15 44 18 233 18 8.732 (00) 0.339 (06) 0.162 (04) 0.400 (14) 2 2.641 1206994 15 44 53 235 52 9.726 (07) 0.354 (00) 0.134 (02) 0.486 (19) 2 2.674 1206996 141093 F5 V 15 45 10 232 24 9.425 (12) 0.306 (04) 0.145 (05) 0.435 (00) 2 2.650 1206999 141110 F2 V 15 45 21 234 35 9.412 (22) 0.383 (04) 0.114 (09) 0.591 (00) 2 2.683 1207000 141133 F2/3 V 15 45 21 231 55 8.162 (03) 0.257 (04) 0.152 (07) 0.556 (03) 2 2.691 1207008 141211 A0 V 15 45 59 235 52 9.334 (03) 0.237 (04) 0.067 (02) 1.035 (08) 3 2.893 (33) 2207010 141253 A5 V 15 46 07 230 03 8.204 (04) 0.106 (03) 0.196 (07) 0.876 (04) 3 2.847 (11) 2207016 141294 F5 V 15 46 22 235 43 9.144 (05) 0.320 (05) 0.138 (07) 0.423 (09) 3 2.621 (05) 2207017 141313 F3 V 15 46 24 231 06 8.893 (03) 0.283 (02) 0.154 (08) 0.539 (15) 3 2.707 (07) 2207018 141326 F3 V 15 46 29 230 05 9.289 (05) 0.271 (00) 0.157 (01) 0.687 (06) 3 2.710 (34) 2
476 G. A. P. Franco
q 2002 RAS, MNRAS 331, 474–482
Table 1 – continued
SAO HD Spectral a1950 d1950 V ðb 2 yÞ m1
c1
n Hb ntype (h m s) (8 0) (mag) (mag) (mag) (mag) (mag)
207020 141327 B9 V 15 46 33 232 39 7.482 (06) 0.021 (04) 0.116 (07) 0.955 (09) 3 2.847 (19) 2207025 141384 A0 V 15 46 50 233 58 9.613 (01) 0.159 (05) 0.084 (07) 1.164 (10) 3 2.877 (02) 2207026 141383 F6/7 V 15 46 51 230 19 9.687 (04) 0.379 (05) 0.121 (08) 0.401 (13) 3 2.627 (16) 2207034 141518 F3 V 15 47 33 233 31 8.578 (02) 0.276 (04) 0.150 (07) 0.466 (11) 3 2.686 (04) 2207039 141536 F3 V 15 47 42 234 53 8.359 (03) 0.313 (00) 0.123 (03) 0.430 (06) 3 2.641 (03) 2207040 141556 B9.5 III–IV 15 47 46 233 28 3.970 (04) 20.029 (01) 0.153 (01) 0.927 (01) 3 2.837 (03) 2207045 141641 Ap Si 15 48 06 231 21 8.925 (10) 20.021 (10) 0.102 (08) 0.377 (05) 3 2.683 (19) 2207046 141640 G0/2 V 15 48 12 230 27 9.901 (08) 0.374 (04) 0.171 (02) 0.341 (21) 3 2.619 (12) 2207057 141739 A5 III/IV 15 48 33 231 48 9.692 (15) 0.118 (07) 0.202 (13) 0.966 (07) 3 2.869 (02) 2207060 141759 F5 V 15 48 46 234 40 9.142 (11) 0.282 (08) 0.158 (07) 0.477 (08) 3 2.662 (05) 2207065 141815 F7 V (+F) 15 49 02 233 58 7.646 (04) 0.336 (01) 0.161 (00) 0.450 (02) 3 2.649 (01) 2207072 141834 B9.5 IV 15 49 15 235 16 10.291 (09) 0.233 (06) 0.059 (09) 1.133 (08) 3 2.873 (12) 2207074 141869 F5 V 15 49 23 233 12 9.169 (04) 0.318 (07) 0.153 (06) 0.390 (06) 3 2.634 (07) 2207075 141903 F2 V 15 49 32 233 06 10.034 (17) 0.366 (05) 0.127 (01) 0.663 (25) 3 2.698 1207081 141976 A(7) (II)(m) 15 49 54 230 06 9.427 (11) 0.174 (05) 0.212 (11) 0.736 (07) 3 2.790 (09) 2207084 142016 A4 IV/V 15 50 05 230 37 7.243 (02) 0.106 (02) 0.180 (01) 0.905 (05) 3 2.837 (13) 2207085 141998 A9 V 15 50 08 235 01 9.303 (07) 0.296 (04) 0.152 (08) 0.853 (11) 3 2.796 (07) 2207088 142041 A8/9 V 15 50 19 231 29 9.191 (03) 0.167 (02) 0.179 (07) 0.771 (05) 3 2.786 (11) 2207100 142187 F6 V 15 51 09 233 54 9.067 (06) 0.355 (06) 0.163 (04) 0.392 (07) 3 2.646 (23) 2207103 142217 F5 V 15 51 20 230 16 8.186 (00) 0.315 (04) 0.131 (07) 0.391 (09) 2 2.636 (03) 2207110 142317 A0 V 15 51 57 234 05 9.040 (26) 0.303 (04) 0.098 (19) 1.083 (07) 2 2.909 (04) 2207115 142318 A2/3 III 15 52 01 235 28 10.000 (03) 0.326 (02) 0.081 (04) 1.156 (07) 2 2.837 (11) 2207121 142426 F5 V 15 52 31 232 57 9.605 (02) 0.413 (07) 0.129 (03) 0.408 (09) 2 2.605 (12) 2207124 15 52 37 234 39 10.188 (02) 0.268 (05) 0.111 (06) 1.159 (12) 3 2.800 (09) 2207129 142458 F2/3 V 15 52 47 235 16 9.352 (05) 0.394 (02) 0.119 (03) 0.665 (09) 2 2.683 (18) 2207133 142524 A0/1 V 15 53 02 232 56 9.619 (03) 0.105 (02) 0.128 (07) 1.089 (08) 2 2.902 (02) 2207134 142542 F3/5 V 15 53 04 231 38 6.293 (03) 0.291 (02) 0.153 (04) 0.493 (01) 2 2.665 (04) 2207135 142526 F0 V 15 53 05 233 18 10.078 (02) 0.330 (00) 0.130 (03) 0.627 (08) 2 2.676 (02) 2207140 142617 A5 IV 15 53 27 230 29 9.258 (07) 0.151 (02) 0.146 (11) 1.158 (05) 2 2.857 (03) 2207141 142602 A2 V 15 53 29 234 17 8.944 (10) 0.278 (00) 0.135 (04) 1.120 (01) 2 2.880 (19) 2207146 142643 A3 V 15 53 49 235 46 6.914 (04) 0.051 (00) 0.192 (01) 1.052 (07) 3 2.874 (05) 2207149 142690 F5 V 15 53 55 232 06 9.952 (04) 0.377 (09) 0.143 (09) 0.485 (02) 2 2.689 (15) 2207158 142785 F0 V 15 54 27 233 37 10.076 (07) 0.372 (04) 0.167 (01) 0.786 (10) 2 2.747 (02) 2207160 142786 A9 V 15 54 31 235 02 9.220 (12) 0.355 (00) 0.152 (04) 0.703 (11) 2 2.729 (12) 2207168 142851 A0 V 15 54 50 231 35 7.038 (03) 20.015 (00) 0.146 (02) 0.931 (02) 2 2.883 (03) 2207170 142832 F3 V 15 54 50 235 23 9.521 (18) 0.311 (01) 0.130 (07) 0.457 (06) 3 2.651 (04) 2207174 142887 A0 III/IV 15 55 03 233 55 9.097 (35) 0.285 (02) 0.020 (01) 0.953 (05) 2 2.740 (06) 2207176 15 55 11 235 34 10.463 (32) 0.383 (08) 0.146 (02) 0.339 (06) 2 2.619 (09) 2207189 142992 F0 V 15 55 40 231 54 8.685 (26) 0.239 (02) 0.158 (04) 0.528 (00) 2 2.707 (13) 2207195 143051 B9 IV/V 15 56 09 232 52 6.972 (26) 0.045 (01) 0.090 (04) 0.868 (02) 2 2.775 (02) 2207196 143070 F5/6 V 15 56 11 230 20 8.275 (24) 0.320 (01) 0.142 (09) 0.419 (03) 2 2.641 (14) 2207203 143148 A7 III/IV 15 56 40 231 41 7.393 (17) 0.184 (03) 0.150 (06) 0.774 (02) 2 2.737 (13) 2207205 143149 A0 V 15 56 43 233 14 6.829 (16) 0.006 (01) 0.173 (04) 1.023 (02) 2 2.910 (02) 2207216 143214 F2 V 15 57 03 231 19 9.737 (16) 0.317 (00) 0.141 (02) 0.558 (09) 2 2.684 (14) 2207222 143247 A3 V 15 57 22 231 57 8.664 (04) 0.109 (04) 0.180 (05) 0.975 (03) 3 2.892 (07) 2207231 143379 F6 V 15 57 56 230 26 8.718 (11) 0.330 (03) 0.148 (03) 0.467 (10) 3 2.639 (03) 2207236 143428 Fm 15 58 11 231 24 8.475 (04) 0.507 (02) 0.163 (05) 0.687 (05) 3 2.649 (22) 2207240 143443 F5/6 V 15 58 24 234 05 9.010 (00) 0.340 (02) 0.129 (03) 0.387 (09) 3 2.616 (08) 2207244 143487 Apec 15 58 35 230 46 9.428 (06) 0.386 (19) 0.258 (08) 0.393 (59) 3 2.683 (04) 2207248 143536 F5 V 15 58 51 230 34 9.260 (11) 0.335 (06) 0.151 (05) 0.403 (04) 3 2.646 (04) 2207250 143537 F3 V 15 58 56 231 58 8.826 (04) 0.370 (00) 0.147 (02) 0.748 (05) 3 2.706 (04) 2207252 143538 F0 V 15 59 03 235 07 8.640 (01) 0.241 (03) 0.157 (04) 0.524 (02) 3 2.702 (04) 2207255 143569 A0 V 15 59 09 233 30 8.367 (01) 0.107 (01) 0.136 (04) 1.074 (03) 3 2.904 (06) 2207258 143570 F2/3 V 15 59 17 235 40 9.216 (04) 0.411 (04) 0.105 (07) 0.658 (15) 3 2.693 (04) 2207259 143578 B8 IV/V 15 59 23 235 06 8.769 (11) 0.125 (02) 0.033 (05) 0.751 (03) 3 2.659 (01) 2207260 15 59 24 234 39 10.106 (02) 0.408 (03) 0.127 (03) 0.459 (06) 3 2.640 (13) 2207261 143592 Ap Si 15 59 28 235 04 8.651 (16) 0.077 (03) 0.083 (07) 0.746 (12) 3 2.791 (04) 2207263 143637 G1 V 15 59 36 230 31 9.076 (23) 0.374 (06) 0.168 (04) 0.364 (12) 3 2.609 (11) 2207266 143649 F7 V 15 59 43 230 48 7.785 (02) 0.353 (02) 0.159 (04) 0.437 (11) 3 2.616 (12) 2207270 143652 A0/1 IV 15 59 48 233 55 10.229 (09) 0.192 (08) 0.148 (05) 1.098 (11) 3 2.876 (03) 2207274 143696 A0 IV 15 59 58 233 58 10.070 (05) 0.247 (05) 0.134 (06) 0.963 (15) 3 2.873 (13) 2207275 143675 A5 IV/V 15 59 58 235 08 8.063 (06) 0.106 (03) 0.196 (05) 0.871 (05) 3 2.859 (04) 2
SA a1900 d1900 V ðb 2 yÞ m1
c1
n Hb n(h m s) (8 0) (mag) (mag) (mag) (mag) (mag)
155. 359 15 27 48 231 18 10.188 (03) 0.382 (00) 0.156 (01) 0.349 (04) 2 2.615 (13) 2155. 479 15 30 48 230 05 10.318 (02) 0.249 (04) 0.153 (00) 0.849 (02) 2 2.737 (22) 2155. 525 15 32 00 229 22 10.438 (01) 0.268 (05) 0.169 (03) 0.847 (08) 2 2.753 (09) 2155. 582 15 33 36 230 45 9.721 (00) 0.321 (04) 0.149 (07) 0.471 (02) 2 2.655 (35) 2
On the infrared void in the Lupus dark clouds 477
q 2002 RAS, MNRAS 331, 474–482
with four stars from the Kapteyn Selected Area 155 – Becker &
Bruck 1929–1938). The measurements were collected with the
Stromgren Automatic Telescope (Danish 50-cm telescope) at the
European Southern Observatory, La Silla, Chile. All stars were
measured at least twice in uvby, and most of them (84 per cent)
twice in Hb. The four-colour uvby measurements were used to
obtain the colour index ðb 2 yÞ, the colour indices differences
m1 ¼ ½ðv 2 bÞ2 ðb 2 yÞ� and c1 ¼ ½ðu 2 vÞ2 ðv 2 bÞ� on the
standard uvby system (Crawford & Barnes 1970; Grønbech, Olsen
& Stromgren 1976), and the visual photoelectric V magnitude on
the Johnson system. The Hb index is on the standard b system
(Crawford & Mander 1966). The mean standard deviations are
0.006, 0.003, 0.005, 0.008 and 0.008 mag in V, b 2 y, m1, c1 and
Hb, respectively. The observed stars are listed in Table 1,1 together
with their HD numbers (when available), spectral types (Houk
1982) and equatorial coordinates. The obtained standard deviations
and comparison with previously published photometry (Olsen
1983; Olsen & Perry 1984) for 23 stars in common (Table 2)
demonstrate the high quality of this observational material.
Intrinsic colours and absolute magnitudes can be estimated in
the uvbyb system for stars of spectral types ranging from B to
early/mid G-type stars, excluding A1 and A2 stars, using the
calibrations suggested by (Crawford 1975, 1978, 1979). In order to
obtain intrinsic values with a high degree of confidence, a set of
selection criteria were imposed on the observed sample. This
procedure has the purpose of avoiding objects with peculiar indices
in our final sample (a detailed description of the adopted selection
criteria is given by Franco 1989). A total of 150 stars fulfilled these
selection criteria. The accuracy of reddenings and distances are
estimated by propagation of the measurement errors into the
calibrations. Based on the above-mentioned standard deviations,
the mean accuracy of the obtained colour excesses was estimated
to be of the order of 0.014 mag. The distances to the A- and F-type
stars were estimated to be accurate to within 16–25 per cent, and
for the B-type stars to within 13–31 per cent.
2.2 Stellar distances
An independent test of the accuracy of the derived distances can be
obtained by comparison with parallactic distances obtained by the
Hipparcos satellite. Among the sample passing the selection
criteria, 66 stars possess absolute trigonometric parallaxes from
Hipparcos (ESA 1997). Fig. 2 shows a comparison between the
distances estimated from uvbyb photometry and those from
the trigonometric parallaxes measured by Hipparcos. In spite of the
very good agreement, some comments should be made before
the high quality of the photometric data and derived stellar
parameters are accepted. It is well known that systematic errors can
be introduced when parallaxes are used to derive stellar distances
and other physical quantities (e.g. Lutz & Kelker 1973; Koen
1992). For the specific case of the Hipparcos parallaxes, estimates
of these biases have been discussed by Brown et al. (1997, and
references therein) and Arenou & Luri (1999). The former authors
have shown that distances computed from Hipparcos parallaxes are
almost unbiased for small ratios of the observational error to the
true parallax (i.e., spH/p < 0:1Þ. On the other hand, the computed
distances may be overestimated by more than 100 per cent when
this ratio is equal to 1.
Although it is not easily seen, the scatter in Fig. 2 is partly due to
biased distance estimates. This effect is better shown in Fig. 3,
which presents the distribution of the relative differences between
computed parallactic and photometric distances, ð1=pH 2 rÞ/r. In
Figure 1. IRAS map of the Lupus region containing the void in 100-mm
flux. The void’s border-line contour was set at 18 MJy sr21, and successive
contour lines are spaced by 10 MJy sr21. The cloud designated as Lupus 1 is
seen just below the void.
Table 2. Comparison to earlier results obtainedby Olsen (1983) and Olsen & Perry (1984). Themean differences (x) represent the average ofthe results obtained in this paper minus theresults obtained by them for 23 common stars;s identifies the standard deviation. Results aregiven in units of 0.001 mag.
V b 2 y m1 c1 b
x 23.6 2.2 23.0 26.5 24.1s 6.2 5.4 8.7 11.8 10.1
Figure 2. Comparison between the parallactic distances obtained by
Hipparcos and the photometric distances based on uvbyb data. The denoted
errors for the photometric distances were estimated by propagation of the
measuring errors into the calibrations of each star.
1 Available in electronic form at the CDS via anonymous ftp to cdsarc.u-
strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cats/J.MNRAS.
htx
478 G. A. P. Franco
q 2002 RAS, MNRAS 331, 474–482
Fig. 3 the full line represents the distribution obtained for stars with
spH/pH < 0:2, and the dashed line represents the stars having
parallaxes with higher relative errors, respectively. The distribution
presented by the former group of stars is rather symmetric,
indicating an almost unbiased sample. The greater part of them
agree in distance within 25 per cent. On the other hand, stars having
higher parallatic errors are asymmetrically distributed, which is a
clear indication that there is a strong bias in the estimated
parallactic distances.
Three stars (SAO 183862, 207018 and 207045) are outside the
borders of Fig. 2, all of them having very high parallactic relative
errors. For SAO 183862 and 207018 the uvbyb photometry resulted
in distances of 228þ60250 and 249þ211
298 pc, while their Hipparcos
parallaxes are 0:76 ^ 1:73 and 0:14 ^ 1:39 mas, respectively. The
large uncertainty in the photometric distance of SAO 207018 is
caused by the rather large standard deviation of the observed Hb
index. Taking into account their spectral types (Table 1), it is most
likely that the Hipparcos distances are highly uncertain.
3 T H E D I S T R I B U T I O N O F R E D D E N I N G
The reddening versus distance relation based on the 150 selected
stars is shown in Fig. 4(a). At first sight the observed diagram
shows a rather complex distribution of points. This complexity is
caused by lines of sight passing through a heterogeneous
interstellar medium, and the analysis is facilitated by dividing
the stellar sample into three groups: (i) stars having lines of sight
towards the infrared void (Fig. 4b); (ii) stars towards the intercloud
medium (Fig. 4c), and (iii) stars towards the dark clouds (Fig. 4d).
The low reddening of stars with lines of sight towards the void
supports the hypothesis suggested by Gahm et al. (1990) that the
region has been cleared of dust.
24 stars in the selected sample were found to be closer than
100 pc. The average colour excess shown by these stars is
Eðb 2 yÞ ¼ 0:011 mag. Only one star (SAO 207065) seems to show
signs of significant reddening ½Eðb 2 yÞ ¼ 0:027 mag at an
estimated distance of 70 pc]; however, taking into account the
standard deviation of its measurements, the obtained colour excess
may drop to Eðb 2 yÞ ¼ 0:013 mag. In the vicinity of 100 pc one
Figure 3. Distribution of the relative differences between estimated
parallactic and photometric distances ð1=pH 2 rÞ/r. The full line represents
the sample stars with spH/pH < 0:2, while the dashed line represents the
sample stars with spH/pH . 0:2. Two stars having high spH
/pH value
(SAO 183862 and 207018) yield large relative differences, and are outside
the interval represented in the diagram.
Figure 4. Colour excess Eðb 2 yÞ as a function of stellar distance: (a) all selected stars; (b) stars towards the IRAS void. The horizontal dotted lines appear at
Eðb 2 yÞ ¼ 0:025 and 0.062 mag, (c) Stars towards the intercloud medium; and (d) stars towards the dark clouds. The horizontal dotted line was set at
Eðb 2 yÞ ¼ 0:100 mag. Note that no stars having colour excess larger than 0.1 mag occur closer to the Sun than 150 pc (vertical line).
On the infrared void in the Lupus dark clouds 479
q 2002 RAS, MNRAS 331, 474–482
notices the rise of reddening for stars both in the void and the
intercloud region (Figs 4b and c). The void group, however, does
not exceed the value of Eðb 2 yÞ ¼ 0:062 mag, while stars with
lines of sight passing through the intercloud medium continue
rising to higher values.
Though rather uncertain, the colour excess distribution may be
interpreted as the combined effect of two components. The first one
is located at a distance of about 100 pc or closer, and is supposed to
be associated with the interaction zone between Loop I and the
Local Bubble (see Section 5). The second contribution comes from
the dark molecular clouds, causes a colour excess larger than
Eðb 2 yÞ ¼ 0:1 mag, and, as will be discussed below, seems to be
located behind the previously mentioned interface between Loop I
and the Local Bubble. This hypothesis is supported by earlier
investigations. The 13CO maps of Tachihara et al. (1996) show that
the dense parts of Lupus 1 occupy the local standard of rest (LSR)
velocity range from 4.5 to 7.0 km s21. On the other hand, the
existence of material flowing outwards from the Scorpio-Centaurus
association, generally occurring at negative LSR velocities, has
been reported by several authors (e.g. de Geus 1992, and references
therein). Recently, Crawford (2000) analysed the interstellar Na I D
lines towards 29 early-type stars in the general direction of the
Lupus molecular clouds. Seven of them have lines of sight towards
the region investigated here, and all of them are closer than 150 pc.
None of these stars shows a strong Na I absorption component; on
the contrary, five of them show a neutral sodium column density
smaller than 1011 cm22. For the remaining two stars Crawford
detected diffuse components with negative LSR velocity.
4 D I S TA N C E T O T H E L U P U S M O L E C U L A R
C L O U D S
Several efforts have been made to estimate the distance to the
Lupus dark cloud complex, some of which are based on distance
estimates of individual stars that are supposed to be associated to
any of the clouds composing the complex. This is the case of the
binary stars HR 5999/6000, a visual pair with line-of-sight passing
through the central part of Lupus 3. Earlier attempts to obtain the
distance to this pair were complicated by uncertainties in defining
their absolute magnitudes: HR 5999 is a Herbig Ae star, and its
companion HR 6000 a peculiar B-type star, and values ranging
from 170 to 300 pc were proposed (e.g. Bessell & Eggen 1972; The
& Tjin A Djie 1978; Eggen 1983). Hipparcos trigonometric
parallaxes (ESA 1997) are now available for both stars, providing
distances of 208þ46232 and 241þ60
240 pc to HR 5999 and 6000,
respectively. These stars have spH/pH < 0:2, which indicates
that the parallactic distance may be rather unbiased; nevertheless,
as will be seen below, this distance interval seems to be rather
overestimated when compared with other distance estimates to the
Lupus dark clouds. It has already been pointed out by Bertout,
Robichon & Arenou (1999) that HR 5999/6000 may be
unassociated with Lupus 3.
Murphy, Cohen & May (1986) noted that the Lupus dark clouds
are projected on to a gap between two subgroups of the Sco-Cen
OB association. Based on this fact, they proposed that the distance
to these clouds should be in the range from 130 to 170 pc.
The first reliable distance to the Lupus clouds has been provided
by the analysis of the reddening suffered by field stars towards
Kapteyn Selected Area 179 (SA 179), which lies close to the border
of Lupus 4 (Franco 1990). The obtained value of 165 ^ 15 pc was
in good agreement with the upper limit proposed by Murphy et al.
(1986). It could perhaps be argued that SA 179, located in the
neighbourhood of Lupus 4, covers a rather small area (<16 deg2),
compared to the extent of the Lupus complex. The observed
reddening could be due to interstellar material not related to the
supposed macroscopic structure defined by the molecular complex.
However, a subsequent study by Hughes et al. (1993) analysed the
interstellar reddening towards Lupus 1 to 4, obtaining a distance of
140 ^ 20 pc, which gave support to the earlier result by Franco
(1990).
Rizzo et al. (1998) used polarization as a function of stellar
distances to estimate lower limits to the distances of Lupus 1
(>140 pc) and Lupus 4 (>125 pc). Note that HD 141978, which
seems to appear in a discrepant position in their polarization versus
distance diagram (cf. fig. 6a of Rizzo et al. 1998), now possesses a
trigonometric parallax from Hipparcos (ESA 1997). The obtained
distance of 161þ55232 pc may remove the observed discrepancy,
locating the star beyond the distance where onset of polarization
occurs. It should be noted, however, that for this star
spH/pH < 0:25, which means that its computed parallactic
distance may be rather overestimated. The analysis performed by
Crawford (2000) for the interstellar Na I D lines towards the Lupus
molecular clouds suggests that the distance to this complex is most
likely in the vicinity of 150 ^ 10 pc. Note that four of the five stars
mentioned before as showing neutral sodium column density
smaller than 1011 cm22 have lines of sight passing through
Lupus 1. These stars are estimated to be at distances in the
approximate range 120–130 pc.
The colour excess versus distance diagram given in Fig. 4(d)
shows that at about 150 pc the colour excess suddenly jumps to a
minimum value of Eðb 2 yÞ < 0:1 mag, strongly suggesting that
150 pc seems to be best estimate for the distance to Lupus 1, in
close agreement with the most recent result obtained by Crawford
(2000).
It is worthwhile noting that Knude & Høg (1998) proposed a
distance of 100 pc to the Lupus dark cloud complex. This
discrepant value was obtained from reddening versus distance
diagrams built by the combination of data contained in the
Hipparcos/Tycho Catalogues (ESA 1997). Due to the excellent
accuracy of the Hipparcos distances, this contradictory result is
most likely caused by uncertainties in the estimated stellar
reddening. Their colour excesses are estimated from the Tycho
photometry and stellar classification from the Michigan Catalogs,
so uncertainties of the order of EðB 2 VÞ , 0:1 mag may be
expected due to errors in the photometric colours and
misclassification in the Michigan Catalog. In addition, these
authors were unable to distinguish between the contribution
coming from the diffuse component due to the interface Loop I–
Local Bubble and the one caused by the dark clouds.
5 I N T E R FAC E B E T W E E N L O O P I A N D T H E
L O C A L B U B B L E
There is now considerable evidence that the Sun lies within a
cavity filled with hot rarefied gas, the ‘Local Bubble’ (hereafter
LB), which has very irregular shape, with radii ranging from 65 to
250 pc (Sfeir et al. 1999). At present there are several opinions
concerning the possible history of the structure of the LB that are
somewhat contradictory. In one scenario the LB is thought to be an
isolated supernova remnant that has interacted with the adjacent
Loop I superbubble, which is the volume bounded by the
expanding shell associated with the cumulative stellar winds and
supernova explosions originating in the Sco-Cen OB association.
In the interaction zone of these two bubbles has formed a dense
480 G. A. P. Franco
q 2002 RAS, MNRAS 331, 474–482
wall of interstellar material bounded by a H I ring (Egger &
Aschenbach 1995). Evidence of the existence of this wall is given
by the observation that the interior of the neutral ring is almost
opaque in the ROSAT R1 band. Due to the ongoing supernova
activity in Loop I, its pressure is higher than that in the LB.
Detailed calculations performed by Breitschwerdt et al. (2000)
have shown that due to this overpressure in Loop I, instabilities on
parsec scales may occur. Under some conditions the instability
become fully non-linear and loosen up material from the wall
forms neutral blobs that travels ballistically towards the direction
of the Sun. This mechanism is proposed by Breitschwerdt et al. as
the one responsible for producing the local neutral cloudlets which
predominantly flow away from the Sco-Cen association.
The region of the sky analysed in the present work has lines of
sight towards the interaction zone between Loop I and LB. Fig. 5
shows the ROSAT soft X-ray background ð0:1–0:4 keVÞmap of the
investigated region. For comparison, the IRAS 18 MJy sr21
contours were overplotted. The correlation between the infrared
and X-ray maps is rather good. The shadowing effect of Lupus 1 is
clearly visible on the X-ray map, as well as effects caused by the
clumpy IRAS clouds seen to the north and to the west of the void.
The X-ray map shows a bright spot centred around
a1950 ¼ 15h36m; d1950 ¼ 2318220, which corresponds to the source
detected by Riegler et al. (1980), surrounded by a ‘kidney’-shaped
emission area which almost follows the southern rim of the IRAS
contour of the void. This map also shows a stripe of higher
emission east of the void, which to the north-east coincides with
regions of lower reddening of the intercloud area.
The colour excess diagrams shown in Figs 4(b) and (c) strongly
suggest that the void is a region cleared of dust located at a distance
somewhere in the range 60–100 pc. This range is in complete
disagreement with the estimated distance to the SNR candidate by
Riegler et al. (1980) of 340 pc, and it is even worse when compared
to the 950 pc estimated by Colomb et al. (1984). On the other hand,
the distance obtained here is in very good agreement with the value
proposed by Breitschwerdt et al. (2000) for the distance to the
interface Loop I–LB, suggesting that the void may be located in
such an interface, and in this case could have been caused by the
mechanism of Breitschwerdt et al.
The size of the IRAS void projected on to the sky is
approximately 48:5 £ 38, which would correspond to an average
diameter of 4–6:5 pc at the distance of 60–100 pc. However, it
may be that the size of the detached blob is rather smaller than that.
The observed IRAS void corresponds to a depletion of material in
the wall, but the exact size of the hole may correspond to the region
of lower X-ray absorption, which, estimated from the soft X-ray
image, gives a value of about 38 £ 18:7. That is, the detached blob
would have a diameter of 2:4–4 pc, which is rather close to the
proposed size of the typical cloudlets formed by the interaction
between the Loop I and the LB (Breitschwerdt et al. 2000).
6 C O N C L U S I O N S
From analysis of the distribution of reddening towards the IRAS
void in the Lupus dark clouds, a distance of 60–100 pc is deduced
to the medium containing the void. This value indicates that the
region cleared from dust may be located in the interface Loop I–
LB, and it may have been caused by the action of the mechanism
proposed by Breitschwerdt et al. (2000), in which, due to the
overpressure in Loop I, instabilities may become fully non-linear
and fragment the interface into small cloudlets. In this scenario, the
X-ray source H1538232, first reported by Riegler et al. (1980)
using the HEAO A-2 experiment and further confirmed by the
ROSAT all-sky survey, is in fact an enhancement of the emission
that originates in the hot interstellar medium permeating the Loop I
bubble, due to a region of lower column density in the interface
between these bubbles. This hypothesis excludes H1538232 as a
candidate supernova remnant.
Moreover, the distribution of reddening towards regions of the
sky showing higher infrared emission shows a sudden jump at a
distance of about 150 pc. This distance is in perfect agreement with
the recent value proposed by Crawford (2000) for the distance to
the Lupus dark clouds. Considering the present result and different
distance estimates by several authors, a distance of about 150 pc
seems to be the best choice for the dark cloud known as Lupus 1.
AC K N OW L E D G M E N T S
This paper is based on observations collected at the European
Southern Observatory (ESO), La Silla, Chile. The author thanks
the Brazilian agencies CNPq and FAPEMIG for partially support-
ing this investigation. This research has made use of the ROSAT
All-Sky Survey, the IRAS Sky Survey Atlas, and the Hipparcos/
Tycho Catalogues. The author is also grateful to the IPAC staff for
providing the software used for processing and ‘mosaicking’ the
IRAS and ROSAT images, in particular to Dr. Booth Hartley for his
great help.
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