sign in sign up
<<
Home , AB7, HD 5980

arXiv:astro-ph/0208289v1 15 Aug 2002 .Naz´eY. eL`g,Ale u6Aˆt1,Bt 5,B40 Li`ege - 4000 B B5c, Bat. [email protected] Aoˆut(Belgium); 17, 6 Li`ege, All´ee du de imnhm dbso,Briga 1 T (UK); 2TT B15 irs@.sr.bham.ac.uk Birmingham [email protected], Edgbaston, Birmingham, adSaeFih etr rebl,M 07;corco- 20771; MD Greenbelt, Center, [email protected] God- Flight Center, Space Research Archive dard Science Astrophysics ergy re tet raa L681 [email protected] 61801; IL Urbana, Street, Green ..(eio;[email protected] Mexico (Mexico); 04510, D.F. 70-264, Postal Apdo. Mexico, Aut´onoma de ..62,Sc.Cnr-il,Mnra,Q,HC3J7 H3C QC, Montreal, Centre-Ville, moff[email protected] (Canada); Succ. 6128, C.P. nvria ainld aPaa ae del Paseo (Argentina); Plata Plata, La la B19000FWA de S/N, [email protected] Nacional Bosque Universidad nXryivsiaino h G 4 edi h M 1 the : (1) SMC the in field 346 NGC the of investigation X-ray An 1 2 3 4 5 6 7 8 ntttdAtohsqee eG´eophysique, de Universit´e et d’Astrophysique Institut colo hsc srnm,Uiest of University Astronomy, & Physics of School nvriisSaeRsac soito,Hg En- High Association, Research Space Universities srnm eatet nvriyo lios 02W. 1002 Illinois, of University Department, Astronomy nttt eAtoo´a nvria Nacional Universidad Astronom´ıa, de Instituto ´ preetd hsqe nvri´ eMontreal, de Universit´e physique, de D´epartement autdd inisAto´mcsyGeof´ısicas, y Astron´omicas Ciencias de Facultad eerhFlo F.N.R.S. Fellow Research hneainetwt D58,sc pta oniec a indicat W may headings: coincidence star. remnant. Subject massive spatial peculiar a a this is such to which related 5980, HD emission, indeed with X-ray eruption alignment diffuse 1994 the chance of with or region lum system X-ray a binary high the by The in winds cycle. colliding orbital with d either 19 complete a over variability X-ray sdtce o h rttm tXryeege.Tesa sXryb X-ray is star possibly The (or binary energies. a X-ray in at star time LBV first a of the 5980, HD for lies detected also is field the In cluster. hrceitc ftemi bet ftefil.TeNC36cluste 346 NGC The first field. this (with the In emission of SMC. X-ray the objects faint in main region the formation of star characteristics largest the and region ueosmsiesasadi epnil o h oiaino N66, of ionization the for responsible is and massive numerous epeetrslsfo a from results present We 1 , 8 ..Hartwell J.M. , L X unabs B D58 n h G 4 cluster 346 NGC the and 5980 HD LBV glxe: aelncCod–-as niiul(G 34 (NGC individual Clouds–X-rays: Magellanic (:) ∼ 1 2 . ..Stevens I.R. , 7 L × X unabs Chandra 10 ...Moffat A.F.J. 34 ∼ r s erg 1 . bevto fteNC36cutr hscutrcontains cluster This cluster. 346 NGC the of observation 5 ABSTRACT − × 2 1 .F Corcoran F. M. , u ed ob oioe ute oivsiaeits investigate to further monitored be to needs but , 10 34 6 1 ..Niemela V.S. , r s erg enmd ihthe with made been under- be to clarity. sources with X-ray taken low, the is of Clouds interstellar studies allowing Magellanic The the Both . pair towards own (LMC). a extinction our Cloud of forms satellites Magellanic that are Large 1986) (Mathewson, the kpc with 59 Visvanathan of & distance Ford, a at galaxy ular Introduction 1. a rae estvt n pta eouinthan resolution spatial a and with sensitivity SMC greater the study far of to opportunity launch an Sasaki recent provides & The (Haberl properties population the 2000). binary of X-ray studies al. the particular et of 2000; Yokogawa in 2000; and al. et al. 2000) Pietsch Haberl & et 1999; Haberl, (Kahabka Sasaki, al. population et source Schmidtke 1999; point the of and − 1 rvosXryosrain fteSChave SMC the of observations X-ray Previous irreg- an is (SMC) Cloud Magellanic Small The ,tgtycreae ihtecr fthe of core the with correlated tightly ), ROSAT 3 .H Chu Y.-H. , hs bevtosicue surveys included observations These . 7 tefsosol relatively only shows itself r ae,w ilfcso the on focus will we paper, ih,wt nunabsorbed an with right, ,H 5980) HD 6, nst a eassociated be may inosity D58 ssurrounded is 5980 HD . h otlmnu H luminous most the isenObservatory Einstein httermatis remnant the that e iei a eol a only be may it hile rpesse)that system) triple a 4 .Koenigsberger G. , Chandra however, , , ii ASCA 5 , ever before. 2. The Observations and Data Analysis

2.1. X-ray Observations We have obtained a Chandra observation of the giant H ii region N66 (Henize 1956), the NGC 346 was observed with Chandra for the largest star formation region in the SMC, in XMEGA9 consortium on 2001 May 15–16 for order to study the young cluster NGC 346 and 100 ks (98.671 ks effective, ObsID = 1881, its interaction with the surrounding interstellar JD∼2452045.2d). The data were recorded by medium. This cluster contains numerous massive the ACIS-I0, I1, I2, I3, S2 and S3 CCD chips stars (Massey, Parker & Garmany 1989), of spec- maintained at a temperature of −120◦C. The data tral types as early as O2 (Walborn et al. 2002). were taken with a frame time of 3.241s in the faint The large number of massive stars is not the only mode. The exposure was centered on the cluster, feature of interest in this field. On the outskirts of with HD 5980 lying 1.9′ to the north-east of the NGC 346 lies the remarkable star HD 5980, which aimpoint on ACIS-I3. Our faintest sources have underwent a (LBV)-type fluxes of about 2 × 1032 erg s−1 (see paper II), eruption in 1994. This massive binary (or triple?) assuming a distance of 59 kpc for the SMC. system has been monitored and analysed for its varying spectral and photometric properties in We excluded bad pixels from the analysis, us- visible and UV wavelengths since the early 80’s ing the customary bad-pixel file provided by the (Koenigsberger et al. 2000; Sterken & Breysacher Chandra X-ray Center (CXC) for this particular 1997, and references therein). Previous X-ray ob- observation. We have searched the data for back- servations of NGC 346 (Inoue, Koyama, & Tanaka ground flares, which are known to affect Chandra 1983; Haberl et al. 2000) detected a bright ex- data, by examining the lightcurve of the total tended source around HD5980 which has been count rate, but no flares were found. Event Pulse attributed to a (SNR); how- Invariant (PI) values and photon energies were ever, the crude instrumental resolution prohibited determined using the FITS Embedded File (FEF) unambiguous detections of a point source associ- acisD2000-01-29fef piN0001.fits. ated with HD5980. In addition to HD5980, the young massive stars of NGC 346 and their inter- For long exposures, removing the afterglow acting winds are likely to produce X-ray emission, events can adversely affect the science analysis but have not been detected previously. The sen- (underestimation of the fluxes, alteration of the sitive, high-resolution Chandra observation of the 10 spectra and so on) . We thus computed a new NGC 346 field thus provides an excellent oppor- level 2 events file by filtering the level 1 file and tunity to study a variety of phenomena involving keeping the events with ASCA grades of 0, 2, 3, emission at X-ray energies. 4 and 6, but without applying a status=0 filter. Throughout this paper, we will use this new file In this paper, we will describe in § 2 the ob- for all scientific analysis. servations used in this study, then discuss the available data on NGC346, HD5980, and the ex- Further analysis was performed using the CIAO tended emission in § 3, 4, and 5, respectively. v2.1.2 software provided by the CXC and also Finally, conclusions are given in § 6. The second with the FTOOLS tasks. The spectra were anal- part of this work (Naz´eet al., paper II) will de- ysed and fitted within XSPEC v11.0.1 (Arnaud scribe the properties of the other sources detected 1996). In Fig. 1, we show a 3 color image of the in the field. Chandra data of the NGC 346 cluster, HD 5980 and its surroundings. We will discuss the features of this image further in later sections.

9http://lheawww.gsfc.nasa.gov/users/corcoran/xmega/xmega.html 10see caveat on http://asc.harvard.edu/ciao/caveats/acis cray.html

2 We have investigated problems due to Charge NGC 346 cluster has not been detected in X- Transfer Inefficiency (CTI). To remove these CTI rays until this Chandra observation. Even though effects, we used the algorithm of Townsley et al. NGC 346 lies near the joint of the four ACIS-I (2000). Checking several sources on different chips CCD chips and part of the cluster is lost in the and/or at different positions on each CCD, we con- gaps between CCDs, the X-ray emission from the cluded that the impact of the CTI was small and core of NGC 346 is unambiguously detected in the that it did not change significantly the spectral corner of the ACIS-I3 chip. The X-ray emission parameters found by using non-corrected data. appears extended with multiple peaks correspond- When a difference was apparently found, we dis- ing to the bright blue stars MPG 396, 417, 435, covered that the best fits found on corrected and 470, and 476(Fig. 3). As only ∼300 counts are de- non-corrected data gave similar χ2. As the work tected and the X-ray peaks are not well-resolved, of Townsley et al. (2000)is not yet an official we will analyze only the overall emission from the CXC product11, we chose to present here only the cluster. results of the non-corrected data. The total count rate of this emission in the 0.3 − 10.0 keV band is 3.04 ± 0.36 × 10−3 cnts s−1. 2.2. HST WFPC2 Images The spectrum of the NGC346 cluster was ex- ′′ ′′ Two HST WFPC2 observations of the N66 tracted from a square aperture of ∼45 ×45 with / NGC 346 region in Hα are available from the a carefully chosen background region, as close as STScI archives. They were taken on 1998 Sept. 27 possible to the cluster. As we regard the clus- and 2000 August 7 for the programs 6540 and ter X-ray emission to be extended, we have used 8196, respectively. HD5980 is centered on the PC the calcrmf and calcarf tools to generate the ap- 2 in the observations from program 6540 (see Fig. propriate response files. The best fit (χν =0.90, 2), while program 8196 shows an area situated fur- Ndof =50, Z = 0.1Z⊙) to this spectrum was an ther south. The observations were made through absorbed mekal model (Kaastra 1992) with the the F 656N filter for 2070s (Prog. # 6540) and following properties (see Fig. 4): an absorption 0.83 22 −2 2400s (Prog. # 8196). This filter, centered at column density N(H)of0.420.17 × 10 cm and 2.23 6563.7 A˚ with a FWHM of 21.4A,˚ includes the a temperature kT of 1.010.61 keV. The luminosity 33 −1 Hα line and the neighboring [N ii] lines. in the 0.3 − 10.0 keV band was 5.5 × 10 erg s , which corresponds to an unabsorbed luminosity of 1.5 × 1034 erg s−1. The absorption column is The calibrated WFPC2 images were produced comparable to the value expected for NGC 346 by the standard HST pipeline. We processed them (see paper II), but the temperature is one of the further with IRAF and STSDAS routines. The lowest we found, except for the extended source images of each program were combined to remove around HD5980. cosmic rays and to produce a total-exposure map. No obvious correlation between X-rays features and the Hα emission is seen in the HST data. In the region of X-ray emission in NGC 346, Fig. 2 will be discussed more extensively in § 5. 30 blue stars were detected by Massey, Parker & Garmany (1989): MPG 435, 342, 470, 368, 476, 396, 487, 417, 370, 471, 467, 495, 451, 330, 455, 3. The NGC346 Star Cluster 454, 500, 445, 468, 375, 508, 395, 439, 371, 485, 561, 374, 366, 557, 400 (by increasing magnitude). The NGC346 cluster has a large number of Of these, 16 have known spectral types that we massive stars, containing almost 50% of all early- can use to convert magnitudes to bolometric lumi- type O stars in the SMC (Massey, Parker & Gar- nosities. The factors were many 1989). It is responsible for the ionization taken from Humphreys & McElroy (1984). We of N66, the most luminous H ii region in the SMC can then compute the expected X-ray luminosi- unabs (Ye, Turtle, & Kennicutt 1991). However, the ties using log(LX )=1.13 log(LBOL) − 11.89 (Bergh¨ofer et al. 1997). If the X-rays were com- 11 http://www.astro.psu.edu/users/townsley/cti/ ing only from these 16 stars, we should expect a

3 total unabsorbed luminosity of 2 × 1033 erg s−1. surprising that the X-ray luminosity is lower than This represents only 13% of the detected luminos- in Arches, and, in fact, it may be too large to be ity. Several hypotheses could explain this discrep- explained solely with the cluster wind scenario. ancy: a effect (Bergh¨ofer’s relation was determined for Galactic stars but SMC stars have 4. The LBV HD 5980 weaker winds); X-ray emission from the other (un- classified) stars in this region, especially from the Among all stars in NGC 346, HD 5980 is unique low-mass stars (see e.g. Getman et al. 2002); ad- in its spectral variations in the last two decades, ditional emission linked to interactions in binary and warrants further examination. This eclips- systems (such as colliding winds); or an extended ing binary (or triple system?) was classified region of hot gas as the winds from the individual as WN+OB before 1980, but its spectral type stars combine to form a cluster wind (Ozernoy, changed to WN3+WN4 in 1980–1983, and to Genzel, & Usov 1997; Cant´o, Raga, & Rodr´ıguez WN6, with no trace of the companion in 1992. 2000). In 1994, this star underwent a LBV-type erup- tion, and presented at the same time a WN11 type. The eruption has now settled down and the spectrum is returning to its pre-eruption state (for Chandra has also observed X-ray emission from more details see Koenigsberger et al. 2000, and other very young (age < 5 Myr) stellar clusters, references therein). We will label the 1994 eruptor comparable to NGC 346. The Arches cluster in the as ‘star A’ and its companion as ‘star B’. Galactic Center region has been observed to have emission from several different regions, with a to- unabs 35 −1 Chandra is the first X-ray telescope to detect tal X-ray luminosity of LX ∼ 5 × 10 erg s (Yusef-Zadeh et al. 2002). NGC3603 has also this peculiar system, since the low spatial resolu- been observed with Chandra (Moffat et al. 2002) tion of previous X-ray observatories did not allow and the total unabsorbed cluster luminosity is the distinction of HD 5980 from the extended emis- unabs 35 −1 sion surrounding it. However, even with a 100 ks LX ∼ 1 × 10 erg s (though in this case it is clear that a substantial fraction of the emission exposure, the data possess a rather low signal-to- noise ratio. The Chandra ACIS-I count rate of is from point sources in the cluster). For both −3 −1 the Arches and NGC 3603 clusters the fitted tem- HD 5980 is only 2.98 ± 0.19 × 10 cnts s in the perature of the X-ray emission is higher than the 0.3 − 10.0 keV band or about 300 counts in the kT ∼ 1 keV value for NGC 346. total observation. This limits our ability to sensi- tively determine the spectral shape of the emission and to look for source variability. On the other It is important to note that the calculations hand, the relative faintness of the source means presented by Cant´o, Raga, & Rodr´ıguez (2000) that photon pileup will not affect our analysis to and Raga et al. (2001) for the Arches cluster as- any great degree. sume that there are about 60 stellar sources hav- −4 −1 ing mass-loss rates of 10 M⊙ yr . If we take the list of the bluest stars in NGC 346 given by We have extracted a spectrum of HD5980 using Massey, Parker & Garmany (1989), the first 60 the CIAO tool psextract. An annular background sources have spectral types in the range O3-B0. region around the source (but sufficiently far from Assuming mass-loss rates for 8 SMC stars given it) was chosen, in order to eliminate contamina- by Puls et al. (1996) to be typical, then all stars tion from the surrounding extended emission (see in NGC 346 (excluding HD 5980) have mass-loss § 5). This spectrum is shown in Fig. 5a. The −4 −1 rates smaller than 10 M⊙ yr . Only MPG best fit model to the spectrum has the follow- 0.35 22 −2 435 (O4III(n)(f)) has a mass loss rate as high as ing parameters: N(H) =0.220.14 × 10 cm −5 −1 13.35 mekal 9×10 M⊙ yr . However, the majority of these and kT = 7.044.35 keV for an absorbed 0.44 22 −2 60 stars are late O-type stars or model and N(H) = 0.280.19 × 10 cm and 1.89 cooler, which must have much smaller mass-loss Γ=1.741.53 for an absorbed power-law. These ab- −6 −1 rates (10 M⊙ yr or less), and they are more sorption columns are consistent with the value widely distributed than in Arches. Thus, it is not expected for NGC346 (see paper II) but part of

4 these columns are probably due to wind absorp- and post-eruption to confirm the pre- tion and/or absorption from the 1994 ejecta. The dicted luminosity enhancement. derived observed X-ray luminosity of HD 5980 is 34 −1 LX =1.3×10 erg s in the 0.3−10.0 keV band The fact that HD5980 is a close binary sys- for both models. Using the normalisation factor tem containing two very massive stars, suggests of the mekal model, we found a volume emission that colliding winds may provide another source 57 −3 measure of ∼10 cm for this source. for the observed X-rays. We can thus estimate the expected X-ray luminosity. For the sys- We can compare HD5980 with other WR stars tem, we assume that the stellar winds of the detected in X-rays. The unabsorbed X-ray lu- stars have returned to their pre-eruption values 34 −1 −5 −1 minosity of HD 5980 is 1.7 × 10 erg s in the and we assume that M˙ A=1.4× 10 M⊙ yr , 33 −1 −1 0.3 − 10.0 keV energy range and 9 × 10 erg s v∞(A)=2500 km s for the O-star, and M˙ B= −5 −1 −1 in the ROSAT range, i.e. 0.2 − 2.4 keV. This 2×10 M⊙ yr , v∞(B)=1700 km s for the places HD5980 amongst the X-ray brightest single WN Wolf-Rayet star (Moffat et al. 1998). Thus WN stars (Wessolowski 1996) and the brightest the momenta of each star’s wind are very similar, WR+OB binaries (Pollock 2002) of the Galaxy. and the total wind kinetic energy is 5 × 1037 erg s−1. These wind parameters lead to a momen- Two factors could explain this high X-ray lu- tum ratio of η = (MBv∞(B))/(MAv∞(A))=0.97 minosity. First, the fast wind from the post- (Usov 1992) and the wind-wind collision shock eruptive phases (from 1300 kms−1 in 1994 to will lie halfway between the stars. In this config- ∼2000 kms−1 in 2000) should now collide with uration we expect about 1/6 of the wind kinetic the slow wind (ejected with a velocity as low as energy to pass perpendicularly through the two 200 kms−1 during the eruption, see e.g. Koenigs- shocks and be thermalised (Stevens, Blondin, & berger et al. 2000), which will increase the post- Pollock 1992), and because the systems are rela- 12 eruptive X-ray luminosity. X-ray emission from tively close much of this energy will be radiated . colliding ejecta has already been observed for an- other LBV, i.e. η Carinae. An elliptically shaped Consequently, we would expect the X-ray lumi- extended emission of 40′′×70′′(i.e. 0.4 pc×0.8 pc) nosity of HD5980 to be ∼ 1036 erg s−1 or more, in size surrounds the star in the X-ray domain rather than the observed value of ∼ 1034 erg s−1. (Seward et al. 2001). It is correlated to the high The discrepancy between the predicted and ob- velocity ejecta of the Homunculus (Weis, served X-ray luminosities could be due to a range Duschl, & Bomans 2001), which was created af- of factors, e.g. the winds may not be at the pre- ter the last great eruption. For HD 5980, however, eruption levels - lower mass-loss rates or lower the ejecta from the 1994 eruption has not had wind velocities will translate to a lower X-ray lu- sufficient time to form a detectable LBV nebula minosity. Radiative braking, wind clumping, or around the star, and no ‘Homunculus-like’ nebula the of the colliding wind region by the from a previous eruption is visible around HD 5980 thick wind of star B could also reduce the lumi- (see Fig. 2): any X-ray emission from the colliding nosity. Alternatively, the wind momenta may not ejecta should thus still be blended with the stellar be so nearly equal, as a low value of η also tends emission from HD 5980 and that may contribute to to lower the luminosity. explain the high X-ray luminosity. Unfortunately, no X-ray detection of HD5980 before the 1994 Using the ephemeris of Sterken & Breysacher eruption is available. A ROSAT all-sky survey im- (1997), we have computed a phase of φ = 0.24 − age taken in 1990 (exposure number 933001) pro- 0.30 for our Chandra observation: this is close to 36 −1 vides only an upper limit of LX ∼10 erg s at the eclipse of star A by star B (φecl=0.36). If the the position of HD5980. Wang & Wu (1992) gave eccentricity is e ∼ 0.3, a simple adiabatic model a 3σ upper limit of 2×1034 erg s−1 for all SMC WR stars, except AB7. These limits are compat- 12In terms of the cooling parameter χ defined by Stevens, ible with the detected luminosity of HD 5980 and Blondin, & Pollock (1992), for the WN wind χ ≪ 1 and χ ∼ do not allow us to quantitatively compare the pre- for the O-star wind 1.

5 predicts that the X-ray luminosity will vary in- 14814 in Haberl et al. 2000). The Chandra image versely with separation. We may expect a change reveals the extended emission in much more detail in the intrinsic X-ray luminosity of a factor of ∼ 2 than previous X-ray observatories. The overall through the and by ∼ 10% during the Chan- shape of the emission region is more or less rect- dra observation. We have detected a variation angular, with an extension to the northeast. It of apparently larger amplitude (factor ∼2) in the contains a few bright or dark arcs, but apart from count rate of HD5980 (see Fig. 5b). This still sug- these, the brightness is rather uniform, with no ob- gests we are seeing orbital variability associated vious limb-brightening. It has a size of 130′′×100′′, with colliding winds, since the additional effects i.e. 37pc×29pc at the SMC’s distance. HD5980 of changing absorption13 on the observed luminos- lies towards the top center of the emission region. ity can either reduce or enhance this variability. An X-ray bright filament extends from 9′′ west to This observed variability is much more likely to 23′′ south of HD5980. An X-ray dark feature ap- be due to colliding winds than wind-blown bubble pears some 30′′ at the southeast of HD5980. The type emission, where no short timescale variability total count rate of this source in the 0.3−10.0 keV would be expected. The high fitted temperature energy range is 7.25 ± 0.17 × 10−2 cnts s−1. It is of HD5980 kT ∼ 7 keV also suggests that we are the softest source present in the field. likely seeing colliding wind emission, as this tem- perature corresponds to a shock velocity of ∼ 2500 The spectrum of this extended X-ray emis- −1 km s , comparable to the wind speed of star A sion was extracted in a rectangular aperture of (v∞(A) above). ∼160′′×110′′ with a carefully chosen background region, located as close as possible to the source The detailed characteristics of the X-ray prop- and in a region on the ACIS-I1 CCD where there erties of HD 5980 need to be studied further, in are no point sources. A circular region of di- conjunction with detailed colliding wind models ameter ∼5′′ containing HD 5980 was removed (c.f. the case of η Carinae; Pittard & Corcoran from the extraction. As the X-ray emission is 2002). An XMM-Newton observation is scheduled extended the reponse matrices were calculated us- at phases φ = 0.09 − 0.10, just after periastron: ing the calcrmf and calcarf tools. The best fit 2 the comparison between these two datasets may (χν =1.03, Ndof =244) to this spectrum was an enable us to detect further the signature of the col- absorbed mekal model with the following proper- 0.15 22 −2 liding wind region, and constrain it more precisely. ties (see Fig. 6): N(H)=0.120.10 × 10 cm , 0.68 0.21 kT = 0.660.64 keV and Z = 0.170.15Z⊙. The Finally, we note that the unabsorbed X-ray observed flux in the 0.3 − 10.0 keV band was −13 −2 −1 luminosity of the central source in η Carinae, 3.35 × 10 erg cm s , i.e. an observed lu- 35 −1 if we assume a distance of 2.3 kpc, varies be- minosity of 1.40 × 10 erg s . The low value tween (0.6 and 2.5)×1035 erg s−1 (Ishibashi et al. of the absorption column, compared to HD 5980 1999). Such luminosities are much higher than for and the cluster, suggests that this X-ray source HD 5980. lies between NGC 346 and the observer, but still in the SMC (see paper II). 5. The Extended Emission around HD5980 Using the normalisation factor of the mekal Around the position of HD 5980 lies a region model, we found a volume emission measure of bright extended X-ray emission. It was first 58 −3 R nenH dV of ∼3×10 cm . Considering a con- detected by the Einstein Observatory (source IKT stant density for the hot gas, a sperical geometry 18 in Inoue, Koyama, & Tanaka 1983), and sub- of diameter ∼33 pc for the extended emission and sequently observed by ROSAT (source [HFP2000] assuming a pure H composition, the density of the hot gas is roughly 0.2 cm−3 and the total mass of 13The colliding wind region is viewed alternatively through the atmosphere of stars A and B. The absorption thus the hot gas is ∼100 M⊙. changes with the orbital phase. 14This ROSAT source may encompass some of the X-ray emission from the cluster.

6 arises. UV analyses with IUE (de Boer & Savage Using the available data, a deeper analysis was 1980), HST STIS (Koenigsberger et al. 2001) and conducted, searching for the existence of a temper- FUSE (Hoopes et al. 2001) have also confirmed ature gradient throughout the source area, with the presence of an expanding structure. While de color and temperature maps. For color maps, Boer & Savage (1980) and Hoopes et al. (2001) −1 we generated a set of narrow-band images, e.g. found only absorptions at vLSR=+300kms , 0.3 − 0.9 keV, 0.9 − 1.2 keV and 1.2 − 2.0 keV, Koenigsberger et al. (2001) detected both ex- that enable us to distinguish the harder compo- panding sides of the object, with components at −1 nents from the softer parts. In addition, temper- vLSR = +21, +52, +300, +331kms . However, −1 ature maps were constructed from spectral fits the absorptions at +21 and +52 kms were in small regions of the SNR. The only obvious small, and need a future confirmation. The pres- trend in both the color and temperature maps is ence of components from both approaching and the softness of the northeastern extension of the receding sides of the expanding structure has now SNR. However, it is not possible to draw any firm been confirmed by Danforth et al. (2002) from conclusions regarding the presence of any temper- deep echelle spectroscopic integrations at optical ature gradient. wavelengths of the N66 region. Absorptions at any of these velocities are not seen in the spectra We have also constructed images of the ex- of Sk 80, a close neighbor of HD5980 situated on tended emission in narrow energy bands, each the edge of the X-ray source. A fast expanding correspond to a specific ion. The energy bands structure close to the position of HD5980 is thus used for each ion are shown in Fig. 7. The data present. in each band were binned to obtain 4.9′′ × 4.9′′ pixels (see Fig. 7). In contrast to N132D (Behar Unfortunately, even the high-resolution HST et al. 2001), no significative differences between WFPC2 images of the close neighborhood of the morphology of highly ionized species (Mg10+, HD5980 do not show any clear nebula associated Ne9+, Si12+, Fe19+) and lower ionization species with the X-ray extended emission (see Fig. 2). A were detected. lot of filamentary structures - generally indicative of a SNR (Chen et al. 2000) - are seen throughout the field, but are not limited to the exact location 5.1. The Nature of the Extended Emission of the X-ray source. There is thus no obvious Hα feature directly correlated with the extended X- Since its discovery the extended X-ray emis- ray emission. sion has been attributed to a supernova remnant (SNR). Further evidence was subsequently ob- Considering all the evidence (non-thermal emis- tained to support this hypothesis; for example, sion, high velocity expanding shell etc), the X-ray Ye, Turtle, & Kennicutt (1991) found a non- / radio source should thus be regarded as a SNR thermal radio source at this position that they located in front of HD 5980 but still belonging called SNR 0057 − 7226. Using the lowest con- to the SMC. The SNR hypothesis is further sup- tours of Ye, Turtle, & Kennicutt (1991), the ported by the comparison of the size of this fea- radio source is 56 pc×52 pc, slightly larger than ture to its X-ray and radio fluxes (Mathewson et the X-ray source. al. 1983). Moreover, evidence of high velocity motions in As HD5980 is projected more or less at the cen- this region were observed in the visible and UV ter of the diffuse X-ray emission and it underwent ranges. Using echelle spectra centered on Hα, a LBV-type eruption in 1994 (which is unlikely to Chu & Kennicutt (1988) found clumps moving −1 be the only one undergone by the system), it is at vLSR ∼ 300kms , redshifted from the main −1 tempting to associate the diffuse X-ray emission quiescent component by +170kms : only the with HD 5980. To investigate whether this asso- receding side of the expanding object is seen, sug- ciation is likely, we first compare HD 5980 with gesting a low density where the approaching side η Carinae, the most well-known LBV that have

7 gone through multiple eruptions. η Car is also A+B+D, then the high velocity redshifted UV surrounded by bright, extended X-ray emission absorptions could be produced by star C shining that appears to be assciated with the Carina Neb- through the receding back wall of the SNR. The ula: it is comparable in X-ray temperature (kT ), presence of a compact object might explain the size and morphology to the X-ray emission around ∼6 h variations seen in the lightcurve of HD 5980 HD 5980 (Seward & Mitchell 1981; Fig.8). But it by Sterken & Breysacher (1997). Accretion onto is generally assumed that this Nebula has been this compact object might also constitute another formed through the collective action of all stars of source of X-rays, thus contributing to understand the Carina cluster, not only by the single action the high X-ray emission from the system. of η Carinae.

We have also searched for clues of an interac- 6. Summary tion between HD5980 and its surroundings (e.g. a In this series of articles, we report the analysis LBV nebula). Walborn (1978) noted evidence of of the Chandra data of N66, the largest star for- such interaction: an arc of radius ∼ 30′′ centered mation region of the SMC. In this first paper, we on the star is actually visible in Hα and [O iii] (but have focused on the most important objects of the not in [S ii]). However, this arc is rather diffuse field: the NGC 346 cluster and HD 5980. (see Fig. 2), indicating a lack of shock compres- sion. Moreover, its velocity coincides with that of the quiescent region, and no line-splitting re- The cluster itself is relatively faint, with a total unabs 34 −1 gion is seen beginning at the arc and extending luminosity of LX ∼ 1.5 × 10 erg s in the towards the star (Danforth et al. 2002): this 0.3-10.0 keV energy range. Most of this emission arc is certainly not the rim of an expanding shell. seems correlated with the location of the brightest This optical arc has different size, shape, and lo- stars of the core of the cluster, but the level of X- cation from an X-ray bright arc (Fig. 2), therefore ray emission probably cannot be explained solely we conclude that they are probably two distinct by the emission from individual stars. features unrelated to each other. Echelle spectra of this area also do not show any enhancement In this field lies another object of interest: of the [N ii]/Hα ratio which usually indicates the HD 5980, a remarkable star that underwent a presence of circumstellar material surrounding the LBV-type eruption in 1994. Chandra is in fact star. The comparison between X-rays and visible the first X-ray telescope to detect HD5980 indi- data thus do not show any clear indication of an vidually. In X-rays, the star appears very bright, interaction between HD 5980 and its environment. comparable only to the brightest WR stars in the We also note that no circumstellar nebula pro- Galaxy, but still fainter than η Carinae. The com- duced by HD 5980 could explain the redshifted UV parison of our results with future X-ray observa- absorptions in the spectra of the star, or the non- tions will enable us to better understand HD 5980: thermal radio emission from the extended source. for example, phase-locked variations will be anal- ysed in the perspective of the colliding wind be- The last possibility is that the source is indeed haviour of this binary, while other variations may a SNR, but that HD 5980 is still directly respon- be related to the recent LBV eruption. Follow- sible for it. Several authors have proposed the up observations are thus needed to complete the existence of a third component in HD 5980, ‘star study of this system. C’, to interpret the visual light curve (Breysacher & Perrier 1991) and the presence of stable pho- A bright, extended X-ray emission is seen to tospheric absorptions in UV (Koenigsberger et al. surround HD5980. It is most probably due to a 2000). To explain the existence of a SNR, an ad- SNR whose progenitor is unknown. The spatial ditional, fourth component of the system, ‘star D’, coincidence of this extended X-ray emission with is needed. It would now be a compact object, a the peculiar massive star suggest an association remnant of the star that exploded in SN long ago. between these two objects. We also note the close If star C is much farther away than the system resemblance of this X-ray emission to the Carina

8 Nebula in which the LBV η Carinae lies.

We are grateful to Dr Martin A. Guerrero Ron- cel for useful suggestions on data analysis tech- niques. Support for this work was provided by the National Aeronautics and Space Administra- tion through Chandra Award Number GO1-2013Z issued by the Chandra X-Ray Observatory Cen- ter, which is operated by the Smithsonian Astro- physical Observatory for and on behalf of NASA under contract NAS8-39073. Y.N. acknowledges support from the PRODEX XMM-OM and Inte- gral Projects, contracts P4/05 and P5/36 ‘Pˆole d’attraction Interuniversitaire (SSTC-Belgium) and from PPARC for an extended visit to the University of Birmingham. IRS and JMH also ac- knowledge support from PPARC. AFJM thanks NSERC (Canada) and FCAR (Quebec) for fi- nancial aid. This research has made use of the SIMBAD databse, operated at CDS, Strasbourg, France and NASA’s Astrophysics Data System Abstract Service.

9 REFERENCES Koenigsberger, G., Georgiev, L., Barb`a, R., Tzve- tanov, Z., Walborn, N.R., Niemela, V.S., Mor- Arnaud, K. 1996, ASP Conf. Ser. 101, eds G. Ja- rell, N., & Schulte-Ladbeck, R. 2000, ApJ, 542, coby & J. Barnes, p.17 428 Behar, E., Rasmussen, A.P., Griffiths, R.G., Den- Koenigsberger, G., Georgiev, L., Peimbert, M., nerl, K., Audard, M., Aschenbach, B., & Walborn, N.R., Barb`a, R., Niemela, V.S., Mor- Brinkman, A.C. 2001, A&A, 365, L242 rell, N., Tsvetanov, Z., & Schulte-Ladbeck, R. Bergh¨ofer, T.W., Schmitt, J.H.M.M., Danner, R., 2001, AJ, 121, 267 & Cassinelli, J.P. 1997, A&A, 322, 167 Massey, P., Parker, J.W., & Garmany, C.D. 1989, Breysacher, J. & Perrier, C. 1991, IAUS 143, 229 AJ, 98, 1305 Cant´o, J., Raga, A.C., & Rodr´ıguez, L.F. 2000, Mathewson, D.S., Ford, V.L., Dopita, M.,A., ApJ, 536, 896 Tuohy, I.R., Long, K.S., & Helfand, D.J. 1983, ApJS, 51, 345 Chen, C.-H. R., Chu, Y.-H., Gruendl, R.A., & Points, S.D. 2000, AJ, 119, 1317 Mathewson, D.S., Ford, V.L., & Visvanathan, N. 1986, ApJ 301, 664 Chu, Y.-H., & Kennicutt, R.C., Jr 1988, AJ, 95, 1111 Moffat, A.F.J., Marchenko, S.V., Bartzakos, P., Niemela, V.S., Cerruti, M.A., Magalhaes, Danforth et al., 2002, in preparation A.M., Balona, L., St-Louis, N., Seggewiss, W., & Lamontagne, R. 1998, ApJ, 497, 896 de Boer, K.S., & Savage, B.D. 1980, ApJ, 238, 86 Moffat, A.F.J., et al., 2002, ApJ, (in press) Getman, K.V., Feigelson, E.D., Townsley, L., Bally, J., Lada, C.J., & Reipurth B. 2002, to Naz´eet al., in preparation (paper II) be published in ApJ, 575 Ozernoy, L.M., Genzel, R., & Usov, V.V. 1997, Haberl, F., Filipovic, M. D., Pietsch, W., & Ka- MNRAS, 288, 237 habka, P. 2000, A&AS, 142, 41 (HFP2000) Pittard, J.M., & Corcoran, M.F. 2002, A&A, 383, Haberl, F., & Sasaki, M. 2000, A&A 359, 573 636 Henize, K.G. 1956, ApJS, 2, 315 Pollock, A.M.T. 2002, Interacting winds from massive stars, eds A.F.J. Moffat & N. St Louis, Hoopes, C.G., Sembach, K.R., Howk, J.C., & ASP Conf. Series, 260, p 363 Blair, W.P. 2001, ApJ, 558, L35 Puls, J., Kudritzki, R.-P., Herrero, A., Pauldrach, Humphreys, R.M. & McElroy, D.B. 1984, ApJ, A.W.A., Haser, S.M., Lennon, D.J., Gabler, R., 284, 565 Voels, S.A., Vilchez, J.M., Wachter, S., & Feld- Inoue, H., Koyama, K., & Tanaka, Y. 1983, IAUS meier, A. 1996, A&A, 305, 171 101, 535 (IKT) Raga, A.C., Vel´azquez, P.F., Cant´o, J., Masciadri, Ishibashi, K., Corcoran, M.F., Davidson, K., E., & Rodr´ıguez, L.F. 2001, ApJ, 559, L33 Swank, J.H., Petre, R., Drake, S.A., Damineli, Sasaki, M., Haberl, F., & Pietsch, W. 2000, A., & White, S. 1999, ApJ, 524, 983 A&AS, 147, 75 Kaastra, J.S. 1992, An X-Ray Spectral Code for Schmidtke, P.C., Cowley, A.P., Crane, J.D., Tay- Optically Thin Plasmas (Internal SRON-Leiden lor, V.A., McGrath, T.K., Hutchings, J.B., & Report, updated version 2.0) Crampton, D. 1999, AJ, 117, 927 Kahabka, P., Pietsch, W., Filipovic, M. D., & Seward, F.D. & Mitchell, M. 1981, ApJ, 243, 736 Haberl, F. 1999, A&AS, 136, 81

10 Seward, F.D., Butt, Y.M., Karovska, M., Prest- wich, A., Schlegel, E.M., & Corcoran, M. 2001, ApJ, 553, 832 Sterken, C. & Breysacher, J. 1997, A&A, 328, 269 Stevens I.R., Blondin J.M., Pollock, A.M.T. 1992, ApJ, 386, 265 Townsley, L.K., Broos, P.S., Garmire, G.P., & Nousek, J.A. 2000, ApJ, 534, L139 Usov, V.V. 1992, ApJ, 389, 635 Walborn, N.R. 1978, ApJ, 224, L133 Walborn, N.R., Howarth, I.D., Lennon, D.J., Massey, P., Oey, M.S., Moffat, A.F.J., Skalkowski, G., Morrell, N.I., Drissen, L., & Parker, J.W. 2002, AJ, 123, 2754 Wang, Q. & Wu, X. 1992 ApJS, 78, 391 Weis, K., Duschl, W.J., & Bomans, D.J. 2001, A&A, 367, 566 Wessolowski, U. 1996, MPE report No. 263, 75 Ye, T., Turtle, A.J., & Kennicutt, R.C., Jr. 1991, MNRAS, 249, 722 Yokogawa, J., Imanishi, K., Tsujimoto, M., Nishi- uchi, M., Koyama, K., Nagase, F., & Corbet, R.H.D. 2000, ApJS, 128, 491 Yusef-Zadeh, F., Law, C., Wardle, W., Wang, Q.D., Fruscione, A., Lang, C.C., Cotera A. 2002, ApJ, 570, 665

A This 2-column preprint was prepared with the AAS L TEX macros v5.0.

11 Fig. 1.— Chandra color image of the region close to HD5980. Three energy bands were used to create this color image : red corresponds to 0.3- 1.0 keV, green to 1.0-2.0 keV and blue to 2.0- 10.0 keV. Before combination, the images were smoothed by convolution with a gaussian of σ=1′′.

Fig. 2.— HST WFPC2 Hα data of the region sur- rounding HD 5980 (which is the star located at the center of the PC chip). The X-ray data overlaid as contours on the HST image were first binned by a factor of 2 and then smoothed by convolution with a gaussian of σ=2′′.

Fig. 3.— The Chandra X-ray data of the NGC346/HD5980 region superimposed on the DSS image, showing the extended nature of the X-ray source associated with the cluster. The lo- Fig. 8.— X-ray images of the cation of the brightest stars in the cluster are in- (24 ks ; ROSAT PSPC observation #900176) com- dicated. The X-ray data were first binned by a pared to the extended emission around HD5980. factor of 2 and then smoothed by convolution with If we assume a distance of 2.3 kpc for the Ca- a gaussian of σ =2′′) rina Nebula and 59 kpc for the SMC, both images are ∼55 pc×55 pc. ROSAT PSPC support rings Fig. 4.— The X-ray spectrum of the NGC346 and Chandra ACIS-I CCD gaps can be spotted cluster, shown along with the best-fit absorbed on these images. Both images were smoothed by mekal model, with N(H)=0.42 × 1022 cm−2 and convolution with a gaussian (σ=15′′ for the Ca- kT =1.01 keV (see § 3). rina Nebula and σ=2′′ after binning by a factor of 2 for HD5980). Fig. 5.— a. The Chandra X-ray spectrum of HD5980, shown along with the best-fit absorbed mekal model, with N(H)=0.22 × 1022 cm−2 and kT =7.04 keV. b. The Chandra X-ray lightcurve of HD5980, in the 0.3-10 keV range and with 10 bins of 10 ks each.

Fig. 6.— The Chandra X-ray spectrum of the extended emission around HD5980, shown along with the best-fit absorbed mekal model, with N(H)=0.12 × 1022 cm−2, kT = 0.66 keV and Z =0.17Z⊙ (see § 5).

Fig. 7.— Narrow-band X-ray images of the ex- tended emission surrounding HD 5980. Each im- age is labeled with the principal line-emitting ion and the energy range used to generate the image.

12 This figure "f1.jpg" is available in "jpg" format from:

http://arxiv.org/ps/astro-ph/0208289v1 This figure "f2.jpg" is available in "jpg" format from:

http://arxiv.org/ps/astro-ph/0208289v1 HD5980 MPG396 MPG435 MPG368 MPG470,476

MPG370 Sk80 MPG417

0.005

0.004

0.003

0.002

0.001 0 20 40 60 80 100

This figure "f8.jpg" is available in "jpg" format from:

http://arxiv.org/ps/astro-ph/0208289v1