Astron. Astrophys. 331, 193-210 (1998)

4. X-ray point source identifications

4.1. Optical counterparts

4.1.1. Method

In order to find identifications with known optical objects, the position of each ROSAT point source was compared with published stellar data concerning the Monoceros and Rosette clouds. The result is given in the last columns of Tables 2 and 3. In our fields, we find that only a few O and/or B stars (belonging to the corresponding OB associations) are detected as X-ray sources: one B star in Monoceros (Table 2a), and one O star in the Rosette field (Table 3a). These OB identifications are rare because the fields were purposedly pointed away from the OB associations. Some bright late-type stars coincident with ROSAT sources are likely to be foreground field stars (Sect. 4.1.2). A search for positional coincidences with the GSC yields 11 (12) stars associated with reliable ROSAT sources in the Monoceros (Rosette) fields. Altogether, 27% (57%) of the respective ROSAT sources coincide with a cataloged star.

Fainter stellar counterparts were sought from Palomar Observatory Sky Survey (POSS I) plates POSS 923 (POSS 928) which includes the Monoceros (Rosette) cloud region. R band plates were used to minimize dust obscuration and increase the ability to find embedded counterparts. The plate digitization was made with the Machine Automatique à Mesurer pour l'Astronomie (MAMA) ¹ microdensitometer at Observatoire de Paris. The digitized images corresponding to the two PSPC fields appear on Figs. 3a and 3b. High resolution MAMA images (0 6@ were made for 3' 3' squares centered at the coordinates of each X-ray source, and catalogs of stellar objects with astrometric accuracy were obtained (see Appendix of FCMG for details). Comparison of SASS positions for well-resolved X-ray sources with unambiguous optical counterparts show a small systematic deviation, likely due to aspect boresight errors of the satellite. SASS positions are therefore corrected by shifts 4" east and 11" south for Monoceros, and 6" west and 5" north for Rosette. The sizes of these corrections are comparable to those found by FMCG for Cha I, and by CMFA for the Oph cloud core, and have been applied to the X-ray coordinates given in Tables 2 and 3.

Photometric calibration for of the MAMA scan intensities was based on a photometric sequence of a total of 25 standard stars in the two plates, from the JP11 (11-colour Johnson photometry) database provided in SIMBAD. In order to do an approximate photometry of the visible counterparts, we need to calibrate the correspondence between MAMA scan intensities and R -magnitudes (). For fainter magnitudes, we adopt the calibration curve derived by FCMG for the Chamaeleon I cloud from ESO-J plates, using photometric standards in the range 6 to 20. The offset necessary to align the JP11 and Cha I sequences was small. However, because the calibration is based on different sets of plates, the estimates here are likely to be less accurate than that found by FMCG; we estimate 0.5 mag.

4.1.2. Visible counterparts

Column [10] in Tables 2 and 3 gives the number of stars on the POSS plate lying within the error circle (col. [5]) of each boresight-corrected ROSAT source position. In regions of high stellar surface density, one or more stars may fall into the error circle coincidentally. To evaluate the probability of coincidence, we divide each 3' 3' MAMA image in N cells of size equal to the associated ROSAT error circle. We find number of stars within the cells, where (), and report the chance coincidence probability in column [10]. When a single object is found in a cell of a low P, this star is almost certainly the optical counterpart to the X-ray source. The apparent star surface densities derived from MAMA catalogs are given in the last column of Table 4.

Table 4. Number of ROSAT sources having optical counterparts and observed surface density of stars in the PSPC fields

The majority of ROSAT sources, 68% (81%) for Monoceros (Rosette), have at least one associated optically visible star. A significant number of ROSAT sources have no optical counterparts brigther than , the POSS plate limit. The results of this analysis are gathered in Table 4. We note that late-type stars brighter than cannot be TTS at the distance of the Monoceros (Rosette) clouds, since cloud members must be 3.7 mag (5.0 mag) fainter than TTS in the nearby Cha I, Oph and Taurus clouds (). Some of these counterparts (such as Mon X-38, X-39, and X-41, and Rosette X-4 and X-6) are brighter than and are likely to be foreground field stars. Other sources have brightnesses between and (13), including Mon X-1, X-3, X-9, X-12, X-40, and Rosette X-1, X-3, X-9, X-10, X-12, X-15, X-19, X-21, X-22 and X-26. These are likely intermediate-mass pre-main sequence (Herbig Ae/Be) stars associated with the clouds (Sect. 5.4).

The results derived from the analysis of the optical images are given in the last columns of Tables 2 and 3. The offsets between MAMA and ROSAT coordinates are given in cols. [11] and [12] for the ROSAT sources having at least one optical counterpart; if there are multiple candidates we give the coordinates of the one closer to the ROSAT source position. Col. [13] provides the as estimated above, and col. [14] the GSC number. In the "Notes" , individual remarks are given: catalog identifications, spectral types and visual magnitudes. Some associations with near- and/or far-IR sources are also indicated (Sect. 4.2).

4.1.3. Blank fields

Thirteen (four) reliable ROSAT sources in the Monoceros (Rosette) field are without optical counterpart. The incidence of blank fields is unlikely to be due to POSS plate limits. If the empirical correlation between log and established in FCMG for the Cha I cloud (where the TTS typically have ) holds here (Sect. 5.3), the faintest X-ray source of the Rosette cloud should have an optical counterpart with , considerably above the plate limit. The large number of blank fields and the absence of bright infrared counterparts rule out the possibility that most are deeply embedded hot stars, although a few cases (such as the ROSAT detection of the embedded A star WL16 in the Oph core) cannot be excluded. A few could also be X-ray luminous Class I protostars (Koyama et al. 1996; Grosso et al. 1997). But most likely they are heavily obscured CTTS and WTTS, which dominate the embedded population in the Oph cloud core (CMFA).

4.2. Infrared counterparts

Inspection of the Infrared Sky Survey Atlas and associated catalogs in the Monoceros and Rosette fields reveals many sources: 114 from the Point Source Catalog (PSC) and 101 from the Faint Source Catalog (FSC) for the Monoceros field, and 157 PSC for the Rosette field. However, only a few IRAS sources in each field coincide with an X-ray source, as indicated in the notes to Tables 2 and 3, and most of these have fluxes too uncertain to establish reliable infrared colors. Most of the IRAS sources have poor quality measured fluxes at 12, 25 and 60 µm. Only Rosette X-9, with and has colors clearly consistent with classical T Tauri stars.

The Monoceros cloud is reasonably well-studied from ground-based observatories. Early 10 and 20 µm images in a 1 sq. arcmin. area (Beckwith et al. 1976; Hackwell et al. 1982) uncovered 5 sources, and high spatial resolution 2.2 µm images of the same region found 12 additional sources (Aspin & Walther 1990). A JHK survey of a larger area discovered an IR cluster with 190 members (Carpenter et al. 1997), and a 2.11 µm map by Hodapp (1994) reveals a complex morphology. Fig. 4 shows these latter surveys superposed on a [1.0 - 2.4 keV] band image of the X-ray `hot spot' (Sect. 6.1). Only IR sources having (the magnitude associated with the ROSAT sentivity limit, see CMFA) are plotted on this hard-band image. The J -band has been chosen because its extinction cross-section is almost the same as for keV X-rays (see CMFA and Ryter 1996). Eleven coincidences between X-ray and infrared sources are found (Table 5).

Fig. 4. X-ray emission associated with the Monoceros IR cluster. The contour map is obtained from the ROSAT 1.0 - 2.4 keV image smoothed to 5"/pixel. Levels are: 0.08, 0.10, 0.12, 0.14, 0.18, 0.30, 0.6, in units of counts/pixel. Stars indicate sources with in the area indicated by dashed lines (from Carpenter et al. 1997), and the grey-scale image is the 2.11 µm map in the region indicated by dotted lines (from Hodapp 1994).

Table 5. Monoceros X-ray counterparts to near-infrared sources with

The Rosette field has little analogous near-IR information. A image of an IRAS source coincident with a CO peak resolves it into five sources (Block et al. 1993; Hanson et al. 1993). This near-IR cluster lies very close to the X-ray "hot spot" associated with the CO core (see below, Sect. 6.2). Like in the Monoceros case, the sources are however not detected; a study including the J band would be very interesting. The visible GSC source associated with ROSAT source X-27 is also a near-IR and IRAS source (Table 3b). On a broader spatial scale (almost a sq. deg.), Phelps & Lada (1997) found seven embedded clusters in a survey overlapping the complex associated with the Rosette nebula, and studied by Williams et al. (1995). These IR clusters are typically several arcminutes in extent, and neither astrometry nor photometry are currently available for their members. Still, for five of them (see Table 6), a broad correspondence exists between their locations and those of X-ray hot spots or of fainter extended X-ray emission. These probable unresolved X-ray emitting clusters are all situated in the Rosette main cloud (see below, Fig. 14).

Table 6. Rosette infrared clusters possibly associated with X-ray emission (see Fig. 13)

© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998
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