A Revision of the X-Ray Properties of MWC 297
Total Page：16
File Type：pdf, Size：1020Kb
A&A 447, 1041–1048 (2006) Astronomy DOI: 10.1051/0004-6361:20053630 & c ESO 2006 Astrophysics Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297 A revision of the X-ray properties of MWC 297 F. Damiani, G. Micela, and S. Sciortino INAF - Osservatorio Astronomico di Palermo G.S.Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: [email protected] Received 14 June 2005 / Accepted 11 October 2005 ABSTRACT We present a Chandra ACIS-I X-ray observation of the region near the Herbig early-Be star MWC 297, where we detect a tight group of point X-ray sources. These are probably physically associated to MWC 297, because of their obvious clustering with respect to the more scattered field-source population. These data are compared to earlier ASCA data with much poorer spatial resolution, from which the detection of strong quiescent and flaring emission from MWC 297 itself was claimed. We argue that this star, contributing only 5% to the total X-ray emission of the group, was probably not the dominant contributor to the observed ASCA emission, while the X-ray brightest star in the group is a much better candidate. This is also supported by the spectral analysis of the Chandra data, with reference to the ASCA spectra. We conclude that none of the X-ray data available for MWC 297 justify the earlier claim of strong magnetic activity in this star. The X-ray emission of MWC 297 during the Chandra observation is even weaker than that found in other Herbig stars with the same spectral type, even accounting for its large line-of-sight absorption. Key words. stars: early-type – stars: individual: MWC 297 – stars: emission-line, Be – stars: coronae – stars: pre-main sequence – X-rays: stars 1. Introduction reported the detection of a large flare in an X-ray observation of this object, made with the ASCA satellite, since this type MWC 297 is a Herbig Be star, a more massive analogue of of variability is associated with magnetic-related activity, or- very young, pre-main-sequence (PMS) stars like T Tauri stars. dinarily found in stars later than mid-F, but disappearing for It shows all “classical” features of the class, namely a B-type earlier-type stars, where surface convection is absent, and any spectrum with strong emission lines, and is deeply embedded dynamo-related magnetic activity should cease. Among Herbig in a nebulous region (Herbig 1960). Indeed, it is one of the Ae-Be stars, MWC 297 is the only star where such type of vari- earliest-type stars in its class. Its spectral type was not de- ability has been found in the ASCA survey of Hamaguchi et al. fined in detail until recently, being its spectrum dominated by (2005), with the insignificant exception of TY CrA, which has emission lines: the study of Finkenzeller & Mundt (1985) does a known later-type companion. not assign a spectral type, Hillenbrand et al. (1992) tabulate a type O9, while a dedicated study by Drew et al. (1997) con- Herbig stars like MWC 297 are particularly interesting cluded that this star is of type B1.5, with only half a subclass objects for studies of possible magnetic activity, since they uncertainty. All these studies were made more difficult by the are stars in a fast transition phase, between their formation large visual extinction towards the star, whose estimates by var- and their settling on the main sequence, and some models ious studies seem to converge towards the value AV = 8. This (Siess et al. 2000) predict a transient subphotospheric con- is also the value derived by Drew et al. (1997), who moreover vection zone, which might sustain magnetic activity for some argue that the most likely distance of this star is 250 ± 50 pc, time. This is indeed suggested by observations of optical/UV much nearer than previous estimates of about 500 pc (Cantó lines in some Herbig Ae stars (AB Aur, Praderie et al. 1986; et al. 1984; Bergner et al. 1988). HD 104237, Donati et al. 1997; HD 139614, Hubrig et al. As an early type star, the phenomena which can be ex- 2004). Therefore, a further study of the X-ray emission of pected from MWC 297 should be similar to those encountered MWC 297 is made here, using archive data taken with the in OB stars, and unlike those of late-type stars. It was there- Chandra X-ray Observatory ACIS-I imaging spectrometer. fore surprising that Hamaguchi et al. (2000, henceforth HTBK) This instrument has a roughly similar bandpass and effective Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053630 1042 F. Damiani et al.: X-ray observations of MWC 297 Fig. 1. Chandra ACIS-I image of the MWC 297 region, slightly smoothed to highlight the sources. The (windowed) field of view has a size of 16.9 × 8.7. The out- lined rectangle (of size 2.3 × 2.0) contains MWC 297, and is zoomed in Fig. 2. The group of X-ray sources in this region was not resolved by ASCA. North is up and East to the left. area as the (combined) ASCA SIS and GIS detectors but much region have a large HR1, and are probably highly absorbed. better spatial resolution, of order of 0.5, while that of ASCA A more detailed analysis is made in Sect. 4. was of order of 3. Our X-ray source #12 coincides closely with the position of MWC 297 (from the SIMBAD database), and a zoom of the ACIS image near the star is shown in Fig. 2a. The very good 2. Chandra X-ray observation and source spatial resolution of Chandra is able to resolve the MWC 297 detection X-ray source from a neighboring one, only about 3 arcsec apart. The field shown in Fig. 2a is only 2.3 × 2.0 in size, and there- MWC 297 was observed with the ACIS-I CCD imaging fore comparable to the ASCA PSF. The nine X-ray sources we spectrometer onboard the Chandra X-ray Observatory on detect in this region were merged into one when observed with − September 21 22, 2001. The exposure time was 37 330 s. A ASCA. A proper comparison between these new data and the “windowed” observing mode was used, yielding a field of view older ones from ASCA should involve the entire group of nine . × . (FOV)of169 8 7 (Fig. 1). ACIS sources (henceforth called as “MWC 297 group”), not We have detected X-ray sources in the field using just the MWC 297 source. It turns out that MWC 297 is not the PWDetect, a wavelet-based detection algorithm developed at brightest X-ray source in its group: sources #5 and #6 are much INAF-Osservatorio Astronomico di Palermo, and available as brighter (they are the two sources in the upper-right quadrant 1 a Chandra contributed software ; it inherits basic features of Fig. 2a). Table 2 lists the relative contributions to the to- of its previous ROSAT version, described in Damiani et al. tal X-ray count rate among members of the MWC 297 group. (1997a,b). Using this method, we detected 27 point sources Note that since some of the sources are underexposed (falling (with less than one expected spurious), listed in Table 1. in inter-chip gaps, barely visible in Fig. 1) the actually detected We have applied a Kolmogorov-Smirnov test for source counts do not scale with the same proportion, but the count rate variability, and we have found only one source (#1 in Table 1) accounts properly for this, yielding the “true” intensity of the with significant variability above 99% confidence level. This sources. source is about 8 arcmin West of MWC 297 (the source at far The close spatial association of X-ray sources in the right in Fig. 1), and even with the poorer ASCA spatial resolu- MWC 297 group (1/3 of all sources in the field, in 1/32 of the tion could not have been confused with MWC 297 itself. area) suggests that this group is a physical association, not a We also report in Table 1 two hardness ratios2 chance one. The existence of a small cluster of infrared objects (columns HR1 and HR2), each defined as (H − S )/(H + S ), around MWC 297 was already reported by Testi et al. (1998), where H and S are the source counts in two energy bands, here and by Bica et al. (2003). Therefore, we may safely assume that chosen as [1.3−2.5] and [0.3−1.3] keV for HR1, and [2.5−8.0] all objects in the group have nearly the same age and distance and [1.3−2.5] keV for HR2. HR1 describes roughly the low- as MWC 297, when trying to determine their nature. energy cutoff caused by heavy absorption of soft X-rays, while a large value of HR2 is generally indicative of a high-energy component (high temperatures). Most X-ray sources in this 3. X-ray source identification To obtain some information on the nature of the detected 1 See http://www.astropa.unipa.it/progetti_ricerca/ PWDetect/ sources, none of which (except for MWC 297) can be associ- 2 We selected the energy bands used to compute these hardness ra- ated with objects in the SIMBAD database, we have searched tios in such a way as to maximize the average observed source counts the 2MASS catalog (Cutri et al.