CCD imaging of twenty nearby isolated irregular galaxies

Astronomy and Astrophysics Supplement Series, Jul 2018

We present B, V and I CCD stellar photometry for a sample of 20 field irregular dwarf galaxies. Their corrected radial velocity is 500 km s-1. Most of them have been resolved into stars for the first time. Based on photometry of their brightest blue stars we have derived the following distances: 5.9 Mpc (UGC 685), 5.4 Mpc (UGC 1281), 7.2 Mpc (UGC 3303), 7.0 Mpc (UGC 3476), 7.3: Mpc (UGC 3600), 7.2: Mpc (UGC 3698), 7.9 Mpc (NGC 2337), 8.6 Mpc (UGC 3817), 5.7 Mpc (UGC 3860), 5.6 Mpc (UGC 4426), ≥7.9 Mpc , 7.4: Mpc (UGC 5086), 7.1 Mpc (UGC 5272), 5.9 Mpc (UGC 5340), 7.1 Mpc (UGC 5427), 2.7: Mpc (UGC 5456), 6.6 Mpc (NGC 3274), 9.3 Mpc (UGC 5889), 5.2 Mpc (NGC 5238), and 8.0 Mpc (UGC 9405). Our sample exhibits diverse morphological properties evidently caused by their different starburst activity. The galaxy sample has a median integral absolute magnitude MB = -14.6 and a median integral colour . One dwarf, UGC 5340, stands out by its very blue colour, , and by its high M(HI)/L ratio, as expected for young galaxies. Four objects of the sample are IRAS sources. Being well isolated systems, the considered galaxies may be used to estimate a local value of the Hubble parameter, . For half of the sample galaxies their individual H- values are concentrated within [ 58-68] km s-1/Mpc with a median of 65 km s-1/Mpc.

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CCD imaging of twenty nearby isolated irregular galaxies

Astron. Astrophys. Suppl. Ser. CCD imaging of twenty nearby isolated irregular galaxies? L.N. Makarova 0 I.D. Karachentsev 0 0 Special Astrophysical Observatory, Russian Academy of Sciences , N. Arkhyz, KChR, Stavropolsky kraj 357147 , Russia We present B, V and I CCD stellar photometry for a sample of 20 eld irregular dwarf galaxies. Their corrected radial velocity is V0 < 500 km s−1. Most of them have been resolved into stars for the rst time. Based on photometry of their brightest blue stars we have derived the following distances: 5.9 Mpc (UGC 685), 5.4 Mpc (UGC 1281), 7.2 Mpc (UGC 3303), 7.0 Mpc (UGC 3476), 7.3: Mpc (UGC 3600), 7.2: Mpc (UGC 3698), 7.9 Mpc (NGC 2337), 8.6 Mpc (UGC 3817), 5.7 Mpc (UGC 3860), 5.6 Mpc (UGC 4426), 7.9 Mpc (F 565-v1), 7.4: Mpc (UGC 5086), 7.1 Mpc (UGC 5272), 5.9 Mpc (UGC 5340), 7.1 Mpc (UGC 5427), 2.7: Mpc (UGC 5456), 6.6 Mpc (NGC 3274), 9.3 Mpc (UGC 5889), 5.2 Mpc (NGC 5238), and 8.0 Mpc (UGC 9405). Our sample exhibits diverse morphological properties evidently caused by their di erent starburst activity. The galaxy sample has a median integral absolute magnitude MB = −14:6 and a median integral colour (B − V )T = +0:47. One dwarf, UGC 5340, stands out by its very blue colour, (B − V )T = +0:18, and by its high M (HI)/L ratio, as expected for young galaxies. Four objects of the sample are IRAS sources. Being well isolated systems, the considered galaxies may be used to estimate a local value of the Hubble parameter, H = V0=D. For half of the sample galaxies their individual H- values are concentrated within [58−68] km s−1/Mpc with a median of 65 km s−1/Mpc. galaxies; distances | content | galaxies; dwarf; irregular 1. Introduction This paper continues a series of publications (Karachentsev et al. 1997; Makarova et al. 1998; ? Tables 2{17 are only available in electronic form at the CDS via anonymous ftp to ( or via Karachentsev & Drozdowsky 1998) devoted to photom etry of the brightest stars in nearby late-type galaxies, using the CCD camera of the 2.5 meter Nordic telescope. Together with results obtained at the 6 meter telescope, these data consist a part of our general program aimed at the measurement of photometric distances for a complete sample of the nearest galaxies with radial velocities within 500 km s−1. Nearing completion, the program will yield a representative atlas of detailed images for about 150 dwarf irregular galaxies. The availability of such a volume-limited \gallery" allows the structural properties of dwarf galaxies to be classi ed in connection with diverse conditions of star formation in high and low density regions of the Local universe. Now we will consider 20 mainly isolated dwarf galaxies. Most of them have been resolved into stars for the rst time. 2. Observations and photometry All the galaxies were observed on February 4{7, 1995 at the 2.56-meter Nordic telescope (La Palma). A CCD camera with a TEC 1k 1k chip provided a eld of view of 3 3 arcmin with a resolution of 00:0176 per pixel. The exposure times in lters of the B, V , I Johnson-Cousins system as well as the seeing (F W HM ) and the air masses are presented in Table 1. The CCD frames were debiased, flat elded and calibrated by the equatorial standards from Landolt (1992) . Stellar photometry was carried out with the DAOPHOT package (Stetson 1987) . For the analysis we selected, as a rule, starlike objects with standard photometric errors (V ) 0:2 mag and image parameters jSHARPj 2.5. The CCD frames for the 20 observed galaxies in the B, V or I lters are presented in Figs. 1{20. Our stellar photometry results are given in Tables 2{17, which are accessible from the CDS via ftp. Besides the detailed photometry of starlike objects, we made a surface photometry tor to a galaxy. The distance moduli were estimated from the relation 0(B) = 1:51 < B(3B) > −0:51BT − AB + 4:14; where AB is the extinction, while the numerical coe cients were calibrated (Karachentsev & Tikhonov 1994) from galaxies with distances derived from cepheids. It is well known that the probability for red supergiants to occur in a dwarf galaxy is rather low. In some cases when we found a suitable red supergiant candidate with an apparent magnitude V (R1) and a colour (B −V ) > 1:6, we estimated the distance modulus from the relation which was also calibrated from galaxies with distances NGC 2337 5.02.95 known from cepheids. Some basic parameters of the galaxies as well the reUGC 3817 7.02.95 sults of our photometry are presented in Table 18. Its columns contain: (1) | the galaxy name; (2) | the stanUGC 3860 6.02.95 dard diameter from the PGC (Paturel et al. 1992) in arcmin; (3), (4) | the total apparent magnitude and the UGC 4426 7.02.95 integrated colour index from our measurements; (5) | the F 565-v1 7.02.95 galactic extinction; (6), (7) | the heliocentric radial velocity from the PGC and the corrected velocity with respect UGC 5086 5.02.95 to the Local Group centroid according to Karachentsev & Makarov (1996) ; (8), (9) | the mean apparent magUGC 5272 7.02.95 nitude and the mean colour index for the three brightest blue stars in the galaxy, used for its distance estimation; UGC 5340 7.02.95 (10), (11) | the distance modulus and the corresponding distance in Mpc, (12) | the galaxy absolute magnitude. UGC 5427 7.02.95 In the cases where a galaxy was observed in the V and I bands, the (B − V ) colour was derived from the relation UGC 5456 7.02.95 f(B − V ) / (V − I)g in Fig. 21. Here the dots correspond NGC 3274 6.02.95 to the standard Landolt stars. The circles and crosses indicate integrated colours of 65 and 49 galaxies measured by UGC 5889 7.02.95 Reshetnikov & Combes (1996) and Heraudeau & Simien (1996) , respectively. In a linear approximation the colourNGC 5238 7.02.95 colour relation may be tted by (B − V ) = k(V − I) with kb = 0:83 and kr = 0:91 for blue and red stars, respecUGC 9405 7.02.95 tively. The integrated colours of the galaxies follow the relation (B − V )T = 0:85(V − I)T − 0:20, shifted to the right of the standard stars. This agrees well with the specof the galaxies in concentric circular apertures, measuring trophotometric data of Fukugita et al. (1995) . their total apparent magnitudes, VT, and integral colour Below we give some notes on the observed galaxies. indices, (B − V )T or (V − I)T. These data are listed in UGC 685 = KIG 45. As a very isolated system, the Table 18. galaxy is listed in the Catalogue of Karachentseva (1973) . Within its apparent boundary there no red stars. Based on three brightest blue stars (#11, 19 and 6 in Table 2) we 3. Distance estimates estimated the galaxy distance modulus to be 28.89 mag. UGC 1281 = FGC 195. This edge-on galaxy is listed in The photometric limit of our CCD frames was about the Flat Galaxy Catalogue by Karachentsev et al. (1993) . V = 23 mag, which allowed us to reveal only the bright- One arcmin to the East, there is a compact elliptical est stars (supergiants) in the galaxies. Following Sandage galaxy. UGC 1281 has a high colour index for a Sm, & Tammann (1974) and de Vaucouleurs (1978 ) we se- which may be caused by internal extinction in the galaxy. lected blue supergiant candidates under the condition Within its apparent boundary there is a red star, #18 in B − V < 0:4, and used the mean apparent magnitude Table 3, with V = 21:23 and V − I = +1:93. Identifying of three brightest stars, < B(3B) >, as a distance indica- it with a red supergiant, we derive a distance modulus 0(R1) = 28:91, which agrees well with a modulus of 28.68 from three brightest blue stars. UGC 3303. A very bright red star is projected onto the central part of the galaxy, which shows some signs of spiral structure. An accurate photometry of the galaxy is not possible. We can nd only a few stars in its images. Derived from three brightest blue of them (#27, 40, and 44 in Table 4) the galaxy distance modulus is 29.27. As mentioned by Karachentsev & Musella (1996) , UGC 3303, together with the irregular dwarfs PGC 17716 and A0554+07, may belong to the scattered \Orion" group in the Milky Way zone of avoidance. UGC 3476. This galaxy without central condensation looks like a footprint. On the I frame, within the galaxy boundary there are about 50 faint stars and compact knots. The brightest blue object (#89 in Table 5) with V = 21:35 and V − I = −0:30 is probably not a single star. The next three brightest blue supergiant candidates give a distance modulus of 29.23 mag. UGC 3600. This is a very isolated irregular galaxy. On the frames with a F W HM = 10:04 we can distinguish only several bluish starlike objects. Their mean magnitude <B(3B)> ' 22.2 gives a rough distance modulus estimate of 29.3. UGC 3698. UGC 3698 was also observed with a poor seeing. Its distance modulus estimate via the brightest blue objects, (3B) = 29:3, also needs to be con rmed. NGC 2337 = UGC 3711 = IRAS 07066+4432. The galaxy has a signi cant brightness gradient. Its central region is shown separately on the insert of Fig. 7. Several blue compact knots are situated around its bright nucleus. In the outer parts of the galaxy one can see a faint ring-like polar feature. Being an IRAS source, the galaxy apparently has a large amount of dust. From the brightest blue starlike objects (#22, 24 and 29 in Table 6) its distance modulus is 29.48. Note that NGC 2337 and UGC 3698 are separated by 10 arcmin and have a radial velocity di erence of 11 km s−1 only. Their similar distances favour the idea of gravitational binding of these irregular systems. UGC 3817. This is a low surface brigtness galaxy with out central condensation. All its brightest blue stars are fainter than V = 22 mag. Via three of them (#40, 58, and 38 in Table 7) the galaxy distance modulus is 29.66. Within the galaxy boundary there is one red star, #37, with V = 21:62 and V − I = +2:28. Being a probable red supergiant, it gives a modulus 0(R1) = 28:91. Judging by the smoothed pro le of UGC 3817 and its yellowish integral colour, (B − V )T = +0:62, the galaxy may be a dwarf evolved system having lost massive blue stars. In such a case the distance obtained from the brightest blue stars may be overestimated. UGC 3860 = KIG 190 = DDO 43. Georgiev et al. (1997) have already resolved it into stars. UGC 3860 has no prominent blue complexes, except for one compact object, #39, with V = 21:34 and V − I = −0:41 at the NE-edge of the galaxy. Three blue supergiant candidates (#41, 33 and 22 in Table 8) yield a distance modulus of 28.79, which is lower than the estimate, 29.23, given by Georgiev et al. (1997) . UGC 4426 = DDO 52. This is a low surface brightness exceeds that of others by more than two magnitudes. This galaxy with a hint of a di use spiral structure. It does object may be an unresolved multiple system of young not contain blue stellar complexes, except for object #21 stars. Among two red stars with B−V > +1:6 the brighter with V = 21:21, B − V = −0:07 near the galaxy centre. one belongs apparently to the foreground, and the second The three brightest blue stars (#15, 29 and 24 in Table 9) (#10) with V = 21:70 and B−V = +1:71 yields a distance yield a distance modulus 0(3B) = 30:62, which seems modulus 0(R1) = 29:39. overestimated due to the apparent absence of other young It should be noted that UGC 5340 has an unusually massive stars in UGC 4426. As a more realistic estimate of blue integral colour, (B − V )T = 0:18, which agrees well the galaxy distance we give 0(R1) = 28:76, derived from with the colour (B − R)T = 0:32 according to Hopp the red star #10 with V = 21:26 and B − V = +1:63. & Schulte-Ladbeck (1995). This feature, as well as the F 565-v1. This tiny system was found by Schombert high hydrogen mass-to-luminosity ratio, M (HI)/LB > & Bothun (1988) in a search for low surface brightness 1 M =L , may indicate a young galaxy. It seems reagalaxies. Schombert et al. (1992) detected it in the HI sonable to study this system in more detail for investi21 cm line. The galaxy central part shows its irregular gating its velocity eld as well as the abundance of heavy structure (Fig. 11), however, the galaxy itself seems to be elements. unresolved into stars down to Blim = 22:5 mag, which UGC 5427. With its brightness gradient and its chain yields as a rough lower limit 0(3B) > 29:5. of knots resembling a spiral arm, this dwarf galaxy may UGC 5086 = F 565-v3. This galaxy of low surface easily be confused with a distant spiral one. Via the three brightness is separated by 8 arcmin from the nearby gi- brightest blue starlike objects we have estimated its disant spiral NGC 2903. Thuan & Seitzer (1979) ascribe to tance modulus as 29.26. it a radial velocity of +543 km s−1. However, this detec- UGC 5456 = IRAS 10046+1036. As an IRAS source, tion may be spurious, due to the confusion with strong HI this isolated knotty galaxy of high surface brightness conflux from NGC 2903, because the HI flux and linewidth tains a considerable amount of dust. Its blue stars are presented by them for NGC 5086 are not consistent with clustered in nests. In such a crowded eld the image paits luminosity and structure. According to our data the rameter SHARP does not guarantee reliable separation of galaxy looks unresolved into stars, having the regular starlike objects from nonstellar ones. Via three brightest shape and the yellow colour typical of elliptical dwarf sys- blue objects (#35, 13 and 21 in Table 13) we derived a tems. Based on the physical association of NGC 5086 with distance modulus of 27.19. Here we disregard the brightest NGC 2903, we adopt for it the same distance modulus, object #10 with V = 19:15 and B − V = 0:14, situated 29.36 (Drozdowsky & Karachentsev 1998) . in the bright northern nest. The distance D = 2:74 Mpc UGC 5272 = DDO 64 = IRAS 09472+3147. Hopp & seems to be in disagreement with the radial velocity of Schulte-Ladbeck (1991) have already resolved this dwarf the galaxy, V0 = +368 km s−1, as well with the rate of into stars, but have not estimated its distance. The very its resolution into stars. Being at a stage of active star disturbed shape of the galaxy, the presence of many blue formation, this nearby galaxy would be a suitable target knots and the considerable IR flux suggest an active for a detail study with the Hubble Space Telescope. starburst stage. It is rather di cult to distinguish be- NGC 3274 = UGC 5721 = IRAS 10294+2755. The tween compact blue objects and single stars in the galaxy. galaxy has a considerable radial brightness gradient and Adopting the starlike objects: #38, 10 and 9 as blue super- many nonstellar bluish knots. Its central region is shown giant candidates, we derive the galaxy distance modulus on the insert of Fig. 17. Three brightest blue supergiant of 29.26. Within the galaxy boundary there are no red candidates, #34, 40, and 35 in Table 14, give us a disstars above our photometric limit. tance modulus of 29.08, which is lower than the estimate UGC 5340 = DDO 68 = VV 542. Among other ir- 0(3B) = 29:51 derived by Georgiev et al. (1997) . We have regular dwarfs this galaxy stands out for its disturbed not found red stars in this galaxy. structure. It is listed by Vorontsov-Velyaminov (1977) in UGC 5889 = NGC 3377A = DDO 88. This is a difthe Atlas of interacting galaxies as #542. However, in the fuse irregular galaxy with a red colour (B − V )T = 0:60. wide surroundings of UGC 5340 there are no other galaxies All blue stars in it are fainter than B = 21:5. The three which could be responsible for the strong tidal disturbance brightest of them yield a distance modulus of 29.85. of this system. NGC 5238 = UGC 8565 = CPG 384 = VV 828 = I Zw UGC 5340 has been resolved into stars for the rst time 64 = Mkn 1479. As a blue compact object with strong H by Hopp & Schulte-Ladbeck (1995) . The results of our emission, NGC 5238 is in the lists of Zwicky (1971) and photometry of the brightest stars are presented in Table 11 Markarian (1967) . Karachentsev (1972) and Vorontsovand as a CM diagram (Fig. 22). The galaxy is mainly Velyaminov (1977) classi ed it as an interacting double populated by blue stars. Three brightest ones, #38, 49, system. According to Karachentsev (1996) , this dwarf and 30 with < B(3B) > = 21:32 yield a distance modulus galaxy, resembling an intergalactic HII region, belongs to of 28.85. Here we have ignored the brightest object #41 the outskirts of the group around M 101. The mean colour with V = 19:37 and B − V = 0:03, whose luminosity index of NGC 5238 increases smoothly from 0.27 in the Fig. 21. (B − V ) vs. (V − I) diagram for the standard stars (dots), and galaxies (open circles and crosses) Fig. 22. V vs. (B − V ) diagram for UGC 5340. Open squares correspond to stars measured within the galaxy boundary and crosses indicate the foreground stars 4. Conclusions centre to <B − V >T = 0:40 within the faintest isophote, which points out to the presence of an older stellar population in the galaxy. Via the brightest three blue stars the galaxy distance modulus is 28.57, exactly the same as the previous estimate by Karachentsev et al. (1994) . In determining the distance, we have not taken into account the central bright starlike object with V = 18:27 and B − V = −0:44, which may be the galaxy nucleus or a blue compact association. Taking advantage of the high quality of the Nordic telescope optics, we carried out a detailed two-colour imaging of 20 irregular galaxies. Most of them were resolved into stars for the rst time. We made photometry of the brightest stellar population and also measured the integral magnitudes and colour indices of the galaxies. Based on the magnitudes of the brightest blue stars we estimated distances to the galaxies, which lie in the interval of [5:1 − 9:4] Mpc. Besides UGC 3698 and UGC 5086, the studied galax UGC 9405 = DDO 194. Like the previous galaxy, this ies are well isolated systems, whose crossing time with di use dwarf system of regular shape is a probable outer respect to their nearest massive neighbours is of the order member of the group around the giant spiral M 101. A dis- of the cosmological time, 1010 years. Evolution of such tant elliptical galaxy is projected at the West of its central isolated dwarf galaxies should not be a ected by tidal dispart. In the outer regions of UGC 9405 one suspects a very turbances. Nevertheless, these objects show impressively faint spiral arm. Such a feature is seldom met in dwarf diversi ed morphologies, which are likely to result from galaxies. The central part of UGC 9405 has been resolved their starburst activity. into stars by Karachentsev et al. (1994) , who have made The absolute magnitudes of the galaxies lie in the a rough distance modulus estimate, 0(R1) = 29:4(:). interval [−12:5, −16:9] with a median of −14:6, and their integral colours, (B − V )0 , are concentrated within T The results of our photometry for 67 brightest stars are [0.30, 0.70] with a median of 0.47. Outside this colour presented as a CM diagram (Fig. 23). The stars situated index interval there is only one galaxy, UGC 5340, with outside the visible boundary of the galaxy are indicated (B −V )0T = 0:17. It has also a high M (HI)/LB ratio, being by crosses. From the three brightest blue supergiant candi- probably a young system. dates with < B(3B) > = 22:00 the galaxy distance modu- Note that among these 20 nearby irregular dwarfs only lus is 29.89. Identifying the star #24 in Table 17 with a red 4 objects are IRAS sources. If we rank them according to supergiant, we derive another estimate, 0(R1) = 29:16. their apparent magnitude, they occupy positions #3 − 6 The true distance of the galaxy is rather uncertain; we in our sample, whereas their classi cation by the mean adopt for it the average of the two determinations above. surface brightness give them positions #2 − 5. Therefore, the IR-flux from dwarf galaxies seems to exhibit a correlation both with their optical flux and with their surface brightness. Besides, the dwarf galaxies detected in the IR have a slightly bluer integral colour. Because the radial velocities of the considered galaxies can not be much a ected by neighbours, one can use them to estimate the local value of the Hubble parameter, H = V0=D. According to the data of Table 18, individual estimates of H have a median of 65 km s−1/Mpc. This value is in reasonable agreement with other estimates of the Hubble parameter. Acknowledgements. We are indebted to Leo Takalo and Pekka Heinamaki, who took part in the observations, and Sera m Kajsin, who made preliminary processing of the data. This work is partly supported by INTAS-RFBR grant No. 95{1390. de Vaucouleurs G. , 1978 , ApJ 224 , 14; 224 , 710 Drozdovsky I.O. , Karachentsev I.D. , 1998 , A& AS (in preparation) Fukugita M. , Shimasaku K. , Ichikawa T. , 1995 , PASP 107 , 945 Georgiev Ts .B., Karachentsev I.D. , Tikhonov N.A. , 1997 , Astron. 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L. N. Makarova, I. D. Karachentsev. CCD imaging of twenty nearby isolated irregular galaxies, Astronomy and Astrophysics Supplement Series, 181-196, DOI: 10.1051/aas:1998315