A near?infrared study of the Orion Nebula with DENIS. around the Trapezium obtained with the DENIS camera during the protosurvey period.

Transcription

1 A near?infrared study of the Orion Nebula with DENIS. E. COPET 1, D. ROUAN 1, N.EPCHTEIN 1 1 Observatoire de Paris-DESPA, Meudon, France ABSTRACT. We present new near-infrared star counts in the J and K s bands inside a large area around the Trapezium obtained with the DENIS camera during the protosurvey period. A sample of about 2000 sources has been extracted and has been used to draw out a high spatial resolution map of the extinction towards the Orion region. The slope of the K s luminosity function has been derived after dereddening. It is found to be consistent with the standard IMF slope proposed by Scalo (1986) 1. Introduction The Orion Nebula is one of the most nearby active star forming regions and has been already extensively studied at near{infrared wavelengths (see, e.g. Gatley et al and McCaughrean et al ). However, homogeneous IR surveys of the whole area just begin to be achieved (Ali et al. 1995). Thanks to the wide eld of the camera designed for the DENIS project, large zones of the sky can be covered in a relatively short elapse of time at two near{ir bands. The area selected for the present study consists of 3 adjacent strips covering approximately 2 0:5 o around the Trapezium. N Fig. 1. A mosaic image of the studied region at J band (1.25 m). The area is 30' 110', the integration time is 9 1:22 seconds. North is left, East is down.

2 2. Observations The observations have been carried out with the DENIS camera (e.g., Epchtein et al., 1994, Deul et al., 1995, this Conference) during the test period in November{December 1994, in the J and K s bands centered at 1.25 and 2.16 m, respectively. Each elementary image of results of the coaddition of 9 frames, each acquired in 1.22 sec. of integration time. In addition, several deeper images (co{addition of 25 elementary images) were taken around the center of the Trapezium in order to evaluate the completeness limit and to obtain some test elds aimed at improving the photometry of faint objects. Figure 1 presents a mosaic image of three \strips" of the studied region in the J band. 3. Data reduction method Sky subtraction and atelding of the \standard" images were done at the DENIS Paris Data Analysis Center (PDAC). We chose to use the software SExtractor developed by Bertin and Arnouts (in preparation) to extract sources and to produce their list after a convolution of the images by a \Mexican hat" function (Coupinot et al., 1994). This region is quite inhomogeneous due to the presence of nebulosities and sometimes very crowdy, thus, in order to improve our internal photometry, we used two extraction methods: a xed aperture and an \automatic aperture" derived from Kron's \rst moment" algorithm. We have rejected all the sources for which the dierence of magnitude between the two methods is larger than 0.1 magnitude. The zero point determination (21.8 at J and 20.3 at K s band) was determined using the JK measurements of Jones et al. (1994) in the OMC2 region. The cross-identication of both catalogs was achieved by pattern-matching and the GSC catalog was used to reduce the astrometric parameters R.A Declinaison Fig. 2. Map of the distribution of all stellar positions at K s. The Trapezium is located at the center of the large cluster. The coordinates system is in epoch 2000

3 R.A Declinaison Fig. 3. Iso stellar-density derived from Fig 1. using a 2'2' \counting" box size.the coordinates system is epoch Discussion 4.1. Star counts In the studied area, the number of sources detected and for which a photometric accuracy better than 0.1 magnitude can be achieved is Figure 2 shows a map of the distribution of stellar positions for all stars detected at K including those measured with a lower photometric accuracy. The iso-contour map of the K stellar density is plotted in Fig. 3. This star count was obtained by dividing the observed region into a grid of squares of 2' 2' extension. At this scale, no denite clustering eect, except in the Trapezium and OMC2 regions, can be seen but we note that half of the number of stars is found in 13 percent of the surface Extinction determination The colour-magnitude diagram (J? K) vs K magnitude is shown in Fig. 4a. with the plot of the Main Sequence stars transported at the distance of the Orion Nebula. We remark the clustering of stars along (J? K) = 0:8 for K > 9:5. There are very few stars on the ZAMS, and the scattering in (J? K) increases with the brightness of the stars. In an attempt to separate the intrinsic reddening and the extinction due to the local neighbourhood, we tried to deredden the sample. Towards this region, using a constant value for the extinction is not realistic, thus, we have constructed an extinction map using the following method: rst, assuming that all stars are main sequence stars, we have derived the visual extinction A V of each star by computing the distance to the MS sequence locus in the colour-magnitude diagram along the reddening vector. The computed values of A K and A J were converted into A V with the extinction law of Rieke et al. (1985). We have then used a neural network to derive the extinction map at each point of the studied eld. The purpose of using a neural network method,

4 Color-Magnitude diagrams Orion Main sequence J-K K magnitude Orion dereddened Main sequence J-K Fig. 4. a.(up). (J? K) vs. K colour{magnitude diagram of our \photometric" sample. Fig. 4. b.(down) Same as Fig.4.b after dereddening by using the extinction map computed with a neural network method. The diamonds corresponds to main sequence stars transported at the distance of 460pc, the reddening vector represents A V = 10 magnitudes. is to extract from the A V distribution the opacity component most likely due to the molecular cloud. The visual extinction map plotted in Fig. 5 shows a distinct peak of extinction at 2000 = 5 h 35 m 19 s, 2000 =?5 o This map has been used to deredden the stars. The colour-magnitude diagram is displayed in Fig. 4b. It is clear from Fig. 4b, that we have a dereddening excess of (J 0? K 0 ) ' 0:2 magnitude, which corresponds to A V ' 1:2. This result is not surprising because to compute the initial A V, we made the assumption that the stars are on the Main Sequence, which is clearly not true for all stars in this young region. However, we remark that the scattering in (J 0? K 0 ) is large. Some contribution to this scattering is probably due to the dierent location of stars along the line of sight in the cloud, however extremely large intrinsic IR excesses are also evidenced : 158 sources out of the total sample, have a (J 0? K 0 ) larger than 1.0 even after dereddening Stellar colours. To make a classication of these stars using a colour-magnitude diagram alone is not easy and we cannot conclude that all the sources with a large reddening excess are not typical `normal' stars. For instance, if the uctuations of the cloud opacity are large at small scale, the large values of (J? K) may suggest the presence of small dense cores hidding several stars. We have represented on a colour{magnitude diagram the possible PMS stars in our region and the known young PMS stars transported at the distance of 460 pc in Fig. 6. Only one star belongs to the region of the diagram which contains the Class I embedded sources (e.g., Lada et al. 1984), 5 stars seem to be good candidates of

5 Av map 8.94 Trapezium Fig. 5. Visual extinction map derived after tting by neural network. The peak corresponds to ( 2000 = 5 h 35 m 19 s, 2000 =?5 o 13 0 ). The arrow points the location of the Trapezium. Herbig/Haro stars. The others can be interpreted as Classical T Tauri's stars according to their location in the diagram. A population of fainter stars (K 0 ' 12 to 13:5) could be interpreted as a low luminosity tail of the T Tauri's class, but we cannot exclude that they are faint eld red dwarfs. Figure 7 shows the locus of the stars for which the (J 0? K 0 ) colour exceeds 1 magnitude after dereddening. Two large \lamentary" structures seem to appear in this Figure, the most striking one displays a half-circle structure as if it was the frontier of some bubble, this may translate some eect of shock that would have triggered the star formation. Table 1 lists the coordinates and the magnitudes of stars with (J 0? K 0 ) > J 0 K 0 A V 05h 35m 23.8s -4 40' 12.1" h 35m 08.3s -4 54' 15.4" h 35m 19.5s -4 55' 37.0" h 35m 25.0s -5 06' 17.3" h 35m 28.7s -5 07' 42.9" h 35m 27.2s -5 10' 17.8" h 35m 35.9s -5 50' 32.3" h 34m 51.4s -5 46' 51.2" h 34m 05.2s -4 53' 15.8" TABLE I Coordinates, J 0, K 0 magnitudes and visual extinction value determined by the neural network for stars with (J 0? K 0) > 3 after dereddening. The asterisk indicates a source also detected by Ali et al. (1995)

6 6 5 Main Seq. Orion (J-K)>0.9 Class I T Tauri Hebig Ae/Be 4 (J-K) dereddened O6 O9 BO B2 B3B5 A0 A5A7 F0 F5 G0 G8 K0 K5 M K magnitude dereddened Fig. 6. Colour-magnitude diagram K 0 vs. (J 0? K 0) for the observed region and for known PMS stars transported at d = 460 pc. The `' corresponds to MS stars, the `+' to our sample, `2' to Class I emdedded sources, `' to T Tauri stars and `4' t o Herbig Ae/Be stars (J-K) > 3.0 (J-K) > 2.0 (J-K) > R.A Declinaison Fig. 7. Locus of the most reddened stars of our sample. Diamonds, crosses and open squares correspond to stars with (J 0? K 0) > 3, (J 0? K 0) > 2,and (J 0? K 0) > 1, respectively 4.4. Cumulative star counts and initial mass function. Figure 8 shows the J and K cumulative count diagrams for the whole studied region before and after dereddening. The turnovers of the magnitude distribution occur at K = 13 and J = 14, respectively. These values are close to our completeness limits, but it is unclear whether this eect is real or not. The slope of the K luminosity function (KLF) is very close to 0.34, the value found by Lada et al. (1991) in the L1630 molecular cloud. This value is in good agreement with the value = 1:70:5 found by Scalo (1986) for the slope of the IMF.

7 K J dereddened slope 0.33 slope 0.36 J J dereddened slope 0.35 slope Log N K magnitude J magnitude Fig. 8. Cumulative K and J magnitude distributions. The dotted lines represent the counts after dereddening. The values of the slope for each curve is ploted besides the corresponding symbol. Acknowledgements We thank all the DENIS members for their individual contribution to the success of the project and in particular E. Bertin for providing us with useful software tools. References Ali B., DePoy D.L. : 1995, Astron. J. 109, 709 Aspin C., Sandell G., Russel A.P.G. : 1994, Astron. Astrophys. Suppl. Ser. 106, 165 Coupinot G., Hecquet J., Auriere M., Futaully R. : 1992, Astron. Astrophys. 259, 107 Gatley I., DePoy D.L., Fowler A. : 1990, in Astrophysics with infrared arrays, R. Elston ed., p. 230 Jones T. J., Mergen J., Odewahn S., Gehrz, R. D. : 1994, Astron. J. 107, 2120 Kron R.G. : 1980, Astrophys. J. Suppl. 43, 305 Lada E. A., DePoy D. L., Evans II N.J., Gatley I. : 1991, Astrophys. J. 371, 171 Lada C.J., Wilkins B.A. : 1984, Astrophys. J. 287, 610 McCaughrean M.J., Satuer J.R. : 1994, Astron. J. 108, 1382 Rieke G. H., Lebofsky M. J. : 1985, Astrophys. J. 288, 618 Scalo J. M. : 1986, Fund. Cosmic Phys. 121, 161 Shu F.M., Adams F.C., Lizano S. : 1987, ARAA. 25, 23