Coronae in the Coronet: A very deep X-ray look into a stellar nursery
aa r X i v : . [ a s t r o - ph ] S e p Astronomy&Astrophysicsmanuscript no. rcraaa3˙aph c (cid:13)
ESO 2018November 4, 2018
Coronae in the Coronet:A very deep X-ray look into a stellar nursery
Jan Forbrich , ⋆ and Thomas Preibisch Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D–53121 Bonn, Germany Astrophysikalisches Institut und Universit¨ats-Sternwarte Jena, Schillerg¨aßchen 2-3, D–07745 Jena, GermanySubmitted: ; accepted:
ABSTRACT
Aims.
To study the X-ray properties of young stellar objects (YSOs), we analyze an exceptionally sensitive Chandra dataset of theCoronet cluster in the CrA star-forming region, achieving a limiting luminosity of L X , min ∼ × erg / sec for lightly absorbedsources. This dataset represents one of the most sensitive X-ray observations ever obtained of a star-forming region. Methods.
The X-ray data are used to investigate the membership status of tentative members of the region, to derive plasma temper-atures and X-ray luminosities of the YSOs, and to investigate variability on the timescale of several years.
Results.
46 of the 92 X-ray sources in the merged Chandra image can be identified with optical or near / mid-infrared counterparts.X-ray emission is detected from all of the previously known optically visible late-type (spectral types G to M) stellar cluster members,from five of the eight brown dwarf candidates, and from nine embedded objects (“protostars”) with class 0, class I, or flat-spectrumSEDs in the field of view. While the Herbig Ae / Be stars TY CrA and R CrA, a close companion of the B9e star HD 176386, and theF0e star T CrA are detected, no X-ray emission is found from any of the Herbig-Haro (HH) objects or the protostellar cores withoutinfrared source. We find indications for di ff use X-ray emission near R CrA / IRS 7.
Conclusions.
The observed X-ray properties of the Coronet YSOs are consistent with coronal activity; no soft spectral componentshinting towards X-ray emission from accretion shocks were found. The X-ray emission of the AeBe stars TY CrA and HD 176386originates probably from close late-type companions. The Ae star R Cra shows a peculiar X-ray spectrum and an extremely hot plasmatemperature. Finally, we discuss the di ff erences of the X-ray properties of YSOs in di ff erent evolutionary stages. Key words.
Stars: individual: R CrA, TY CrA, HD 176386, T CrA - stars: pre-main sequence - stars: activity - stars: magnetic fieldsX-rays: stars
1. Introduction
Young stellar objects (YSOs) generally show high levels of X-ray activity, exceeding the emission level of the Sun and late-type field stars by several orders of magnitudes (for a recent re-view see Feigelson et al. 2007). A good knowledge of the X-rayproperties of YSOs is of paramount importance not only for theunderstanding of the physical mechanisms that lead to the X-rayemission; the X-ray emission has also far-reaching implicationsfor the physical processes in the circumstellar environment, theformation of planetary systems, and the evolution of protoplane-tary atmospheres (e.g., Glassgold et al. 2005; Feigelson 2005b).In the investigation of the stellar populations of star-forming re-gions, X-ray studies are particularly e ff ective in discriminatingYSOs from unrelated fore- and background field stars. X-raystudies can give a census of the members of a star-forming re-gion that is independent of circumstellar material, allowing toovercome the bias in membership determinations based on in-frared excess criteria. Furthermore, since radiation at energiesabove ∼ ff ected by extinction than opticallight, X-ray observations can penetrate up to A V ∼
500 mag intothe cloud and allow a deep look at embedded YSOs.Recently, two very large observational projects provided un-precedented X-ray datasets on young stars. The
Chandra
Orion ⋆ now at: Harvard-Smithsonian Center for Astrophysics, 60Garden Street, MS 42, Cambridge, MA 02138, U.S.A., e-mail: [email protected] Ultradeep Project (COUP), a 10-day long observation of theOrion Nebula Cluster with
Chandra / ACIS (see Getman et al.2005) is the deepest and longest X-ray observation ever madeof a young stellar cluster. With a detection limit of L X , min ∼ . erg / sec for lightly absorbed sources, X-ray emission frommore than 97% of the ∼
600 optically visible and well character-ized late-type (spectral types F to M) cluster stars was detected(Preibisch et al. 2005a). The XMM-Newton Extended Survey ofthe Taurus Molecular Cloud (XEST) covered the densest stellarpopulations in a 5 square degree region of the Taurus MolecularCloud (see G¨udel et al. 2007a) and provided X-ray data on 110optically well characterized young stars. Despite the new dimen-sion in the quantity and quality of the X-ray data on these twostar-forming regions, sensitive X-ray observations of other star-forming regions are still useful to shed new or additional lighton several still open questions.The first question concerns the origin of the X-ray emis-sion in T Tauri stars (TTS). The hot ( > ∼ −
30 MK) plasmatemperatures typically derived from the X-ray spectra of TTSshow quite clearly that the bulk of the X-ray emission must berelated to coronal magnetic activity (Preibisch et al. 2005a), aconclusion that is also supported by the lack of correlated X-rayand optical variability (Stassun et al. 2006, 2007; Forbrich et al.2007). However, in some TTS, some fraction of the X-ray emis-sion seems to originate in accretion shocks at the stellar sur-face (e.g. Kastner et al. 2002) and / or in shocks in the inner-most parts of stellar jets (e.g. G¨udel et al. 2007b). Due to the Jan Forbrich and Thomas Preibisch: Coronae in the Coronet relatively low shock temperatures of at most a few MK, suchshock-related X-ray emission should be detected as a soft excess(at energies below ∼ ff erent spectral com-ponents (e.g. Telleschi et al. 2007), in some cases indications forvery soft spectral components have also been found in CCD low-resolution spectra of young stars (e.g., Flaccomio et al. 2006; seealso the model spectra in G¨unther et al. 2007).A related question concerns the origin of the observed X-rayemission from young intermediate-mass (Herbig Ae / Be) stars.As these intermediate-mass stars have neither outer convectionzones that may harbor a dynamo to produce magnetic activity,nor strong enough stellar winds to create X-rays in internal windshocks, the detection of X-ray emission from a large fractionof the observed Herbig Ae / Be stars still remains largely unex-plained (e.g., Damiani et al. 1994; Zinnecker & Preibisch 1994;Hamaguchi et al. 2005a; Stelzer et al. 2005, 2006). Chandra ob-servations with their superior spatial resolution revealed that insome of the X-ray–detected Herbig Ae / Be stars the true sourceof the X-ray emission is not the A or B star, but a nearby late-type companion. It remains unclear, however, whether late-typecompanions are the true source of the X-ray emission in all cases, or whether some Herbig Ae / Be stars may neverthelessbe intrinsic X-ray emitters. Some Herbig Ae / Be stars, e.g. ABAur and HD 163296, show very soft X-ray spectra that havebeen interpreted as emission from magnetically confined winds(Telleschi et al. 2007b) or accretion shocks Swartz et al. (2005).Obtaining good S / N X-ray spectra of further Herbig Ae / Be starscan help to investigate these possibilities.Another open question is how early in the protostel-lar evolution X-ray activity start. While class I protostarsare well established X-ray emitters (e.g., Grosso et al. 1997;Neuh¨auser & Preibisch 1997; Imanishi et al. 2001a), it is stillunclear whether class 0 protostars, which represent an evenearlier, extremely young evolutionary stage in which most ofthe mass resides still in the circumstellar environment, alsoshow X-ray activity. The detection of an X-ray flare from thecandidate class 0 protostar IRS 7E in the Coronet cluster byHamaguchi et al. (2005b) provided the first piece of evidencefor the presence of X-ray emission in extremely young ob-jects, but the exact evolutionary stage of IRS 7E is not yet fullyclear. This object clearly deserves further examination. Havingno near-infrared counterpart, this source was recently classifiedas a class 0 / I transitional object by Groppi et al. (2007), basedon mid-infrared detections and new submillimeter data.A final interesting issue is X-ray emission from HH objects.Since the X-ray detection of HH 2 (Pravdo et al. 2001), it isclear that the shock-heated material in some jets can actuallyproduce observable soft X-ray emission (e.g. Pravdo et al. 2004;Grosso et al. 2006; Favata et al. 2006). However, the vast major-ity of all HH objects remain undetected in X-ray observations.It is not clear whether this is due to the limited sensitivity of ex-isting X-ray observations, or whether X-ray emission is createdonly in a small fraction (the fastest ?) of all jets. Very deep X-ray data of nearby star-forming regions allow to investigate thispoint.The deep Chandra data discussed in this paper are wellsuited to address all these issues. The Corona Australis starformation region is one of the nearest (about 3.5 times closerthan the Orion Nebula Cluster) and most active regions of re-cent and ongoing star formation (e.g. Neuh¨auser et al. 2000;Neuh¨auser & Forbrich 2007). It contains a loose cluster of a few
Table 1.
Chandra observations of the CrA star-forming regionused in this study.
Obs Expo. Aimpoint Start Date[ksec] [J2000]19 19.96 19 01 50.6 −
36 57 30 2000-10-07 17:00:553499 38.13 19 01 50.6 −
36 57 30 2003-06-26 12:57:064475 20.17 19 01 48.9 −
36 59 23 2004-06-17 23:15:565402 15.39 19 01 45.0 −
36 58 09 2005-08-08 02:36:495403 15.15 19 01 45.0 −
36 58 09 2005-08-09 02:37:495404 15.17 19 01 45.0 −
36 58 09 2005-08-10 01:57:215405 15.17 19 01 45.0 −
36 58 09 2005-08-12 03:11:535406 14.76 19 01 45.0 −
36 58 09 2005-08-13 01:50:17 dozen known YSOs, which cover a wide range of masses (fromintermediate-mass Herbig AeBe stars down to very-low-massbrown dwarfs) and evolutionary stages (from pre-stellar coresthrough class 0 and class I protostars, class II T Tauri stars, toclass III objects that have already cleared their dusty environ-ment). The central part of the star-forming region, around thebright star R CrA, contains the densest clustering of very young,embedded objects, which is known as the “Coronet” cluster.Dozens of HH objects trace jets emanating from the YSOs. Werefer to the region covered by the X-ray data discussed here asthe Coronet region (see Fig. 1) because it is centered on theCoronet cluster and covers its surroundings. The distance to theCorona Australis star-forming region is relatively well known,based on distance determination of two members. The opti-cally brightest member, the B8e star TY CrA, is a well knowneclipsing spectroscopic multiple system, for which Casey et al.(1998) derived a distance of D = ±
11 pc. The B9e starHD 176386, which forms a common proper motion pair withTY CrA (Teixeira et al. 2000) and shows signs of strong accre-tion (Grady et al. 1993), has a Hipparcos distance of 136 + − pc,which is fully consistent with the distance derived for TY CrA.We thus adopt a distance of 130 pc for the CrA star-forming re-gion.The CrA star-forming region was the target of numerous X-ray observations prior to the present study, including observa-tions with EINSTEIN (Damiani et al. 1994; Walter et al. 1997),ROSAT (Zinnecker & Preibisch 1994; Neuh¨auser & Preibisch1997; Neuh¨auser et al. 2000), ASCA (Koyama et al. 1996),Chandra (Garmire & Garmire 2003), and XMM-Newton (seeHamaguchi et al. 2005b; Forbrich et al. 2006). All these obser-vations, however, were several times less sensitive than thedataset analyzed in the present paper.
2. Chandra observations and data analysis
We have performed a series of five individual ∼
15 ksec ob-servations (separated by about one day) of the Coronet re-gion with the ACIS camera on-board Chandra (Weisskopf et al.2002; Garmire et al. 2003). The main aim of these observationswas to monitor the X-ray emission of the YSOs simultaneouslywith optical, infrared, and radio observations, and the results ofthis multi-wavelength monitoring are described in Forbrich et al.(2007). In order to optimize the sensitivity of the X-ray data forthe present study, we included into our analysis three previousChandra observations retrieved from the public archive, whichhave exposure times between 20 and 40 ksec. Details of the in-dividual observations are listed in Table 1.Our data analysis was performed with the ChandraInteractive Analysis of Observations (CIAO) software package an Forbrich and Thomas Preibisch: Coronae in the Coronet 3 version 3.3, combined with CALDB 3.2.1. Merging all eight in-dividual observations results in a very deep dataset with a totalexposure time of 156 470 sec. In our analysis we consider onlyresults arising from the imaging array (ACIS-I) of four abut-ted 1024 × ′ × ′ onthe sky. As the aimpoints and the orientation of the camera onthe sky (the “roll angle”) di ff ered for the individual observations,the merged image covers a slightly larger area. Exposure mapsand aspect histograms were computed for the single datasets aswell as for the merged dataset, allowing for a determination ofthe e ff ective exposure time at each sky position, taking into ac-count the spatial variation of the detector quantum e ffi ciency,non-uniformities across the face of a detector, mirror vignetting,and bad pixels.The detection limit of the Chandra data was determined inthe following way: using the PIMMS software developed bythe NASA High Energy Astrophysics Science Archive ScienceCenter and assuming an intrinsic source spectrum of a 10 MKthermal plasma with a metal abundance of 0.4 times solar, as typ-ical for young stellar X-ray sources (e.g., Getman et al. 2005),we find that one detected count in 156.47 ksec corresponds toan X-ray flux of 4 . × − ergs / cm / s in the 0 . − L X , min ∼ . × erg / s at the as-sumed distance of 130 pc and for no intervening extinction.Our dataset thus represents one of the most sensitive X-ray ob-servations of a star-forming region ever obtained; it is aboutfour times more sensitive than the data of the Chandra OrionUltradeep Project (Getman et al. 2005), which provided a de-tection limit of L X , min ∼ × erg / sec for lightly absorbedsources (Preibisch et al. 2005a). Many of the YSOs in the CrAstar-forming region su ff er from substantial extinction, up to A V ∼
45 mag. The intervening extinction raises the e ff ective de-tection limit; for A V = [3 , , ,
45] mag the detection limitsare [1 . , . , , × erg / s. Note that these values are validfor the central ( ∼ ′ radius) area of the Chandra image; sourcedetectability degrades with o ff -axis angle, and is ∼ − ff axis an-gles 8 ′ − ′ ).For locating the X-ray sources in our image, we usedthe wavelet transform detection algorithm implemented as the wavdetect program within the CIAO data analysis system(Freeman et al. 2002; Getman et al. 2005).With a nominal threshold of identifying a pixel as belongingto a source (parameter ’sigthresh’) of 1 . × − and waveletscales between 1 and 16 pixels, the program located 91 sources.We manually added one clearly detected source with about 100counts (marked as such in Table A.1), which was missed by thealgorithm due to its location very close to the edge of the image.Thus, we consider a total of 92 sources.After reprocessing all observations with CIAO 3.3 yieldeda homogeneous set of ”level 2” event files (without pixel ran-domization), the acis extract package , version 3.94, was usedfor further analysis. Spectral extraction regions were defined ina way to include a specified fraction of the point-spread functionat the respective positions (90% at an energy of 1.5 keV), inde-pendently for each observation. In the framework of acis extract ,the background was determined from a region surrounding each PIMMS is the Portable, Interactive Multi-MissionSimulator provided by the HEASARC Online Service athttp: // heasarc.gsfc.nasa.gov / Tools / w3pimms.html see http: // / xray / docs / TARA / code / source, containing a minimum number of counts (100 in ourcase) and excluding neighbouring sources. Composite sourcespectra were constructed by summing the single-observationspectra, taking into account appropriately scaled backgroundspectra for each observation. Composite response matrix files(RMFs) and ancillary response files (ARFs) were constructedusing the FTOOLs addrmf and addarf, weighting the single-observation source-specific RMFs and ARFs by their respec-tive exposure times. Spectral fitting with Monte Carlo techniqueswas then performed with the CIAO tool Sherpa (see Sect. 4.1).Basic results of this analysis, namely a source list with netcounts, median energy, source significance, hardness ratios, aswell as incident flux estimates and information on the e ff ec-tive exposure times are listed in Table A.1. The errors for thenet counts contain the propagated 1 σ Gehrels errors (Gehrels1986) of the source and background counts. The uncertainty ofthe absolute energy calibration of ACIS is 0.3% . Also listed isthe source significance, i.e. the photometric signal-to-noise ratio.Hardness ratios compare the source counts in two energy bandsin the form HR = (Cts hard - Cts soft ) / (Cts hard + Cts soft ). We listthe three hardness ratios as defined in Getman et al. (2005) withthe following energy ranges: [0.5-2.0] keV vs. [2.0-8.0] keVfor HR1, [0.5-1.7] keV vs. [1.7-2.8] keV for HR2, and [1.7-2.8] keV vs. [2.8-8.0] keV for HR3. The uncertainties for thehardness ratios were calculated using a method described byLyons (1991) with a script supplementary to acis extract de-veloped by Konstantin Getman . In a few cases, no error couldbe estimated because divisions by zero occur, cases correspond-ingly marked by ’NaN’ (for ’not a number’). Finally, we listthe two acis extract flux estimates. Their di ff erence lies in thehandling of the ancillary response matrix which is done ei-ther channel-wise (for FLUX1) or using an averaged value (forFLUX2). For a detailed discussion of the algorithms used by acis extract , see Getman et al. (2005) and the online documen-tation .For those (putative) members of the CrA star-forming re-gion that remained undetected in the Chandra data, we deter-mined upper limits to the count rates and X-ray luminosities inthe following way. We counted the observed number of photonsin source regions centered at their optical / infrared positions andcompared them to the expected number of background photonsdetermined from several large source-free background regions.We used the Bayesian statistics method described by Kraft et al.(1991) to determine the 90% confidence upper limits for theircount rates. From these count rate upper limits we computed up-per limits for the extinction corrected X-ray luminosities in the0 . − N H ∼ A V · × cm − (Feigelson et al. 2005a).
3. X-ray sources and cluster members
In order to identify the X-ray sources, we inspected the sourcepositions on optical images from the Digitized Sky Survey, near-infrared images from 2MASS, and near- to mid-infrared imagesfrom the Spitzer observatory. see http: // cxc.harvard.edu / cal / http: // / users / gkosta / AE / accessory tools.htm http: // / xray / docs / TARA / ae users guide.html Jan Forbrich and Thomas Preibisch: Coronae in the Coronet HD 176386TY CrA S CrAHBC 679
IRS 2IRS 6R CrAIRS 7WIRS 7ET CrA IRS 1 IRS 5IRS 3 IRS 9 IRS 13
Fig. 1.
Above: 21 . ′ × . ′ detail of the Spitzer IRAC 3 . µ m image of the Coronet region. The white line mark the boundariesof the field of view of the merged Chandra data. The positions of the Chandra X-ray sources are marked by the boxes. The dashedblack line indicates the area shown in the image below. Below: The central 7 . ′ × . ′ region of the Spitzer IRAC 4 . µ m imagewith X-ray source positions marked by boxes.A set of reduced Spitzer-IRAC mosaic images of the CrAstar-forming region and a list of sources with IRAC photome-try and classifications of their broad-band infrared spectral en-ergy distributions (SEDs) was kindly provided to us by LoriAllen. The SED classification was performed as described inMegeath et al. (2004) and sorts the objects into the di ff erent in-frared classes 0 / I / II / III (with class 0 = protostar, class I = evolved protostar, class II = T Tauri star with disk, class III = T Tauri starwithout disk; for further information see Lada 1987; Andr´e et al.1993). The superb PSF of Chandra / ACIS and the high accuracyof the aspect solution, resulting in a positional accuracy of typ-ically better than 1 ′′ , allowed a clear and unambiguous identi-fication of 46 of the 92 X-ray sources with optical and / or in-frared counterparts. Figure 1 shows the location of the X-ray an Forbrich and Thomas Preibisch: Coronae in the Coronet 5 Table 2.
Identification of Chandra X-ray sources with optical or infrared counterparts in the CrA star-forming region. Columns 2-4provide information on counterparts to the X-ray sources in the optical DSS images, the 2MASS images, and the Spitzer images:”y” means that a counterpart exists, ”n” means that no counterpart can be seen. Column 5 gives the SED class derived from theSpitzer photometry, and the last 3 columns give names, spectral types, and stellar bolometric luminosities of the stars collected fromWilking et al. (1992); Walter et al. (1997); Casey et al. (1998); Olofsson et al. (1999); Neuh¨auser et al. (2000); Prato et al. (2003);Nisini et al. (2005); additional references are given in the discussion of the individual objects in Sections 3 and 4.
Source Counterpart SED Name SpT L bol CXO J DSS 2MASS Spitzer class [ L ⊙ ]190104.58–370129.6 n y y III CrA 453 M4190108.60–365721.3 y y y II S CrA K3 2.29190115.86–370344.3 n n y190116.26–365628.4 y y y II V667 CrA M5190118.90–365828.4 n y y II CrA 466 M4.5190119.39–370142.0 n n y190120.86–370302.9 y y y III CrA 4111 M5190122.40–370055.4 n n y190125.61–370453.9 n y y III ISO-CrA 133190125.75–365919.3 n y y II ISO-CrA 134190127.15–365908.6 y y y III ISO-CrA 135 M 0.12190128.72–365931.9 y y y ISO-CrA 136190129.01–370148.8 y y y III ISO-CrA 137190132.34–365803.1 n y y II TS 2.9 = ISO-CrA 139190133.84–365745.0 n y y II TS 2.8 = IRS 13 0.07190134.84–370056.7 y y y III V709 CrA K1 IV 1.55190139.15–365329.6 y y y HD 176386-B190139.34–370207.8 y y y III190140.40–365142.4 n y y II190140.81–365234.0 y y y TY CrA a / b / c / d B8e / K2 / F–K / M190141.55–365831.6 n y y I IRS 2 K2 4.3190141.62–365953.1 y y y II HBC 677190143.12–365020.9 n n y190148.02–365722.4 n y y I IRS 5 a / b K6 V / ? 1.6 / ?190148.46–365714.5 n n y I190149.35–370028.6 y y y III LS-RCrA 2 M6 (BD cand.)190150.45–365638.1 n y y II IRS 6A M2 0.5190150.66–365809.9 n y y I V710 CrA = IRS 1 = HH 100 IR K5-M0 3.1190151.11–365412.5 n y y II IRS 8190152.63–365700.2 n y y I IRS 9190153.67–365708.3 y y y R CrA A5 IIe var190155.31–365722.0 n n y I IRS 7 W ( = / II190155.85–365204.3 n n y190156.39–365728.4 n n y 0 / I IRS 7 E ( = The X-ray emission of the 0 . ′′ binary IRS 5a / b (Chen & Graham 1993; Nisini et al. 2005) is marginally resolved in the Chandra image. sources marked on the 3 . µ m and the 4 . µ m Spitzer images.Further information about the counterparts was obtained fromthe SIMBAD database and the literature; references are givenin the text describing the individual objects. The results of thesource identification are listed in Table 2.46 X-ray sources have either no counterparts in any of theoptical / infrared images mentioned above. Given the detectionlimits of K lim ∼
15 for the 2MASS image and our estimateof L lim > ∼
15 for the Spitzer 3 . µ m image, it is very unlikely that these objects are related to the CrA star-forming region:According to the Siess et al. (2000) pre-main sequence (PMS)models, a 5 Myr old 0 . M ⊙ star at a distance of 130 pc wouldhave un-reddened magnitudes of K = . L = .
2. An ex-tinction of A V ∼
50 mag would thus be required to prevent de-tection in the 2MASS K -band image, and an even higher valueof A V ∼
100 mag to prevent detection in the Spitzer 3 . µ mimage. We also note that the number of X-ray sources withoutcounterparts is in good agreement with the expected number of Jan Forbrich and Thomas Preibisch: Coronae in the Coronet (mostly extragalactic) background X-ray sources that has beenderived from the deep X-ray source counts . Furthermore, thebackground nature of these sources is also supported by theiruniform spatial distribution within the field of view.We also searched for, but could not find any X-ray emissionassociated with the protostellar cores in the CrA star-formingregion as listed by Nutter et al. (2005), unless these contain in-frared objects. Finally, we also searched for X-ray sources at thepositions of 43 HH objects in the CrA star-forming region aslisted in Wang et al. (2004). From none of these objects X-rayemission is detected .As discussed in the following subsections in more detail, wedetect X-ray emission from 9 class 0, class I, and flat-spectrumobjects, 14 class II objects, and 12 class III objects. The observed X-ray properties can provide crucial informa-tion for a clarification of the membership status of presumedmembers discussed in the literature. We can confidently expectto detect X-ray emission from (almost) any stellar member ofthe region, given the following considerations: The COUP datashowed that all young late-type stars in the Orion Nebula Clustershow strongly elevated X-ray emission as compared to the Sunand solar-like field stars (Preibisch et al. 2005a): at least 98% ofall late type YSOs in the Orion Nebula Cluster have fractional X-ray luminosities of log ( L X / L bol ) > −
5, and there are strong in-dications that the 2% of the stars below this value are not clustermembers but field stars (see discussion in Preibisch et al. 2005a).Assuming a lower limit to the fractional X-ray luminosi-ties of log ( L X / L bol ) > − L X / L bol ) = −
5, and ages of 5 Myr. For the case ofno extinction, the X-ray detections should then be 100% com-plete down to stars of ∼ . M ⊙ or spectral type ∼ M7. Foran extinction of A V = ∼ . M ⊙ or spectral type ∼ M5, and for A V =
10 magat ∼ . M ⊙ or spectral type ∼ M0. Note that the typical
X-rayemission level of young stars is log ( L X / L bol ) = − .
6, i.e. a factorof 25 higher than our assumed lower limit, so we expect to detectalso the majority of lower-mass objects, unless their extinctionis very high. Based on the X-ray source counts presented by Brandt et al. (2001)there should be ∼ −
70 extragalactic sources exceeding the flux limit ofour data. The number of actually detectable extragalactic sources mustbe somewhat smaller due to the extinction of the dark cloud, especiallyin the central part of our field of view. We note that the Chandra source 190205.84–365444.2 has neitheroptical nor infrared counterparts, but is located near a bow shock seenin the Spitzer images. However, a relation between the X-ray emissionand the bow shock seems very unlikely. First, the positional o ff set of8 ′′ is clearly much larger than the astrometric accuracy, and the o ff setis not along the flow direction, i.e. cannot be explained by the non-simultaneous nature of the X-ray and infrared images. Second, the dis-tribution of energies of the 21 detected source counts is quite hard (nophotons with energy ≤ − − Neuh¨auser et al. (2000) list 34 previously known or suspectedmembers of the CrA star-forming region. Among the objectsin the field of view of the Chandra data, only two remainundetected. 2MASS J19010586-3657570 ( = ISO CrA 113,spectral type G0) shows no near-infrared excess and theSpitzer photometry is consistent with purely photospheric emis-sion. Neuh¨auser et al. (2000) reported that no signs of 6708 ÅLithium absorption are seen in its spectrum, suggesting that itis not a young star. The second case is 2MASS J19014791-3659302 ( = TS 13.4), which also shows no infrared excess andhas a class III Spitzer SED. Together with the non-detection inthe Chandra data, these points strongly suggest that these twostars are not members of the CrA star-forming region but unre-lated field stars.The results of a ISOCAM survey of the CrA star-formingregion were reported by Olofsson et al. (1999). They identified21 infrared sources with mid-infrared excess, 10 of which arelocated within the field of view of the Chandra data. Eight ofthese 10 objects are detected as X-ray sources, only ISO CrA 140and ISO CrA 145 remain undetected by wavdetect. The 2MASScolors of ISO CrA 140 show a near-infrared excess, but theSpitzer photometry suggest a class III SED, and we note thatthis object was not detected in the ISOCAM 14 . µ m band, rais-ing doubts about the presence of excess emission. The near-infrared colors of this object suggest an extinction on the orderof A V ≈
10 mag. Since ISO CrA 140 is a relatively faint infraredsource, we suspect that its low intrinsic luminosity in combina-tion with strong extinction may have prevented the X-ray detec-tion. Alternatively, it may be an unrelated background AGB starrather than a young star in the CrA star-forming region. With 28counts in an area of radius 3 ′′ (18 . + . − . net counts), ISO CrA 145may actually be marginally detected although the source was notfound by wavdetect. The same region for ISO CrA 140 containsonly 10 counts.L´opez Mart´ı et al. (2005) identified 13 candidate very low-mass members of the CrA star-forming region by optical spec-troscopy. Six of these are in the field of view of the Chandraimage, and five of them, CrA 453, 466, 468, 4110, and 4111,all with estimated spectral types around M5, are detected as X-ray sources, consistent with the assumption that they are youngmembers of the CrA star-forming region. The X-ray detectionof all these very low-mass star reinforces our expectation thatour X-ray data should be complete for all stellar members of theCoronet region, unless they are particularly strongly obscured.The remaining object is CrA 465, a brown dwarf candidatewith estimated spectral type M8.5. It was not detected as an X-ray source by the automatic source detection, but as will be dis-cussed in § The bright infrared source IRS 3 (2MASS J19020491-3658564)is usually considered to be a YSO associated with the CrAstar-forming region. In a spectroscopic and photometric study,Nisini et al. (2005) derived a spectral type K5–M0 III, L ⋆ = . L ⊙ , A V =
10 mag, M ∼ . M ⊙ , and an age ∼ < . / ksec, that corresponds to an upper limit of L X < an Forbrich and Thomas Preibisch: Coronae in the Coronet 7 . × erg / sec and log ( L X / L bol ) < − .
6. This value wouldbe most unusually low if IRS 3 was a YSO: in the COUP datamore than 99.5% of all young late-type stars in the Orion NebulaCluster have fractional X-ray luminosities above this level. Thisnon-detection therefore suggests that IRS 3 is not a young star,and this argument is supported by several other pieces of evi-dence: First, IRS 3 shows no infrared excess. Second, the ageof ∼ not a YSOin the Coronet cluster, but rather a background giant behind thedark cloud. Spitzer IRAC colors were used to classify the infrared sourcesas described in Megeath et al. (2004). While class I or class IIobjects are probably YSOs, the class III objects are a mixture ofYSOs which have already dispersed their (inner) disks and unre-lated field stars. We thus consider objects with class I or class IISEDs and no previous identification as new YSO candidates. Asdiscussed in Megeath et al. (2004), several factors may lead toincorrect classifications. For example, some of the objects iden-tified as class I may be in fact strongly reddened class II objects,and background objects such as planetary nebulae, AGB stars,and galaxies may be misidentified as class I or class II objects.The detection of X-ray emission at levels typical for YSOs al-lows a clear distinction between YSOs and background objectsof the above-mentioned kind. We detect 14 of the 17 objects withSpitzer class II SEDs, and 9 of the 10 objects with Spitzer class(0)I SEDs.
Class I objects:
Spitzer photometry reveals three objects in theChandra field of view with class I SEDs that were not iden-tified as YSOs before. Two of them, 190148.46–365714.5 and190155.61–365651.1 (both remained undetected in the 2MASSimages), can be identified with X-ray sources. Their X-ray de-tections strongly support the YSO status of these objects. With22 and 41 source counts, respectively, these X-ray sources aretoo faint for detailed spectral fitting, but the high median ener-gies of their source photons of 4.1 keV and 4.5 keV are fullyconsistent with the hard spectra expected for embedded class Iobjects.The third new object with Spitzer class I SED is theinfrared source B185836.1-370131 discovered originally byWilking et al. (1997). It is invisible in the 2MASS J - and H -bandimages but is seen as a very faint source in the 2MASS K -bandimage. This object coincides with sub-mm source SMM 2 fromNutter et al. (2005), but remains undetected in the Chandra data.If it is truly a protostellar member of the CrA star-forming re-gion, its non-detection in the Chandra data could be related tovery strong extinction, as suggested by the non-detection in the2MASS images. Since we cannot estimate the extinction to thisobject, no upper limit to the X-ray luminosity can be determined.However, we can ask how much extinction would be required tokeep it undetected in the Chandra data, if one assumes that it hasan X-ray luminosity of ∼ × erg / sec (the mean value forthe X-ray detected Coronet class I objects IRS 1, 5, and 2). UsingPIMMS and assuming a plasma temperature of 30 MK, we find that a hydrogen column density of 2 . × cm − , correspond-ing to an extinction of A V ∼ Class II objects:
Three objects with class II Spitzer SEDs re-main undetected in the Chandra data: LS CrA I and B185831.1-370456, two brown dwarf candidates which will be discussed in § ff ering from particularly strong extinction, or an unre-lated background AGB star.
4. X-ray properties of the YSOs
Since the short-term variability (as seen in the light curves ofthe individual observations) of the Coronet X-ray sources is dis-cussed in Forbrich et al. (2006) and Forbrich et al. (2007), wefocus here entirely on the long-term variability defined by thetemporal sequence of the Chandra observations, covering a pe-riod of nearly five years.For the present analysis we determined for each source themean count rates during each of the 8 individual Chandra obser-vations. While many sources show only small and often statisti-cally insignificant variations, strong variability is seen in some ofthe YSOs. The more interesting long-term lightcurves are shownin Fig. 2, and the variability of individual objects will be dis-cussed below.A detailed analysis of the X-ray spectra of all sources withoptical / infrared counterparts was performed with the Sherpapackage in CIAO. The spectra were fitted with one- and two-temperature optically thin thermal plasma models plus an inter-vening absorption term. We used the XSPEC models “apec”, as-suming a uniform density plasma with 0.3 times solar elementalabundances, and “wabs” for the absorption model. Spectral fitswere carried out ignoring energy bins outside an energy range of0.2–10 keV. As X-ray spectral fits sometimes su ff er from ambi-guities in the spectral parameters, special emphasis was placedon a careful scanning of the parameter space in order to find thebest fit model. For this, we employed the monte-lm algorithmimplemented in Sherpa, which performs hundreds of fitting runsper spectrum, each one using a di ff erent set of randomly chosenstarting values for the fitting parameters.Spectra of sources with less than 1000 counts were generallywell fitted with a single-temperature plasma model, for strongersources and sources for which the single-temperature model didnot provide an acceptable fit, a two-temperature model was em-ployed. The spectral analysis yielded plasma temperatures andhydrogen column densities and was also used to compute theintrinsic (extinction-corrected) X-ray luminosity by integratingthe model source flux over the 0 . − Jan Forbrich and Thomas Preibisch: Coronae in the Coronet
S CrA V709 CrA TY CrAIRS 2 IRS 5 190148.46–365714.5IRS 1 IRS 9 R CrAIRS 7W 190155.61–365651.1 IRS 7E
Fig. 2.
Long-term evolution of the X-ray emission of selected sources in the Coronet region derived from the Chandra observationsobtained in October 2001 (epoch 1), June 2003 (epoch 2), June 2004 (epoch 3), and August 2005 (epochs 4 to 8). The dots show themean count rates during each of the individual observations, which have exposure times between 15 and 40 ksec. The count rateswere determined from counts between 0.2 keV and 8 keV and were corrected for e ff ective exposure times using exposure maps.listed in Table A.2. Some representative examples of spectra areshown in Fig. 3. For X-ray sources with less than ∼
50 counts,the spectral fitting procedure often does not allow to reliably de-termine the spectral parameters. In these cases, the incident fluxat the telescope aperture as determined by acis extract (see TableA.1) provides at least a rough estimate of the observed (i.e. not extinction-corrected) source luminosity.
TY CrA:
The B8e star TY CrA is (at least) a quadruple sys-tem: in addition to the two spectroscopic companions with sep-arations of 0.07 AU and 1.2 AU and estimated spectral typesof ∼ K2 and late F to early K (Casey et al. 1995; Corporon et al.1996; Casey et al. 1998), a visual companion at a projected sep-aration of 41 AU (0 . ′′ ) and of spectral type ∼ M4 was found by Chauvin et al. (2003). With 21026 counts, the TY CrA sys-tem represents the second-brightest X-ray source in the field ofview. Since the individual components cannot be resolved in theX-ray data, their contributions to the observed X-ray emissioncannot be discerned. However, it is interesting to note that theobserved X-ray properties agree very well with the expectedX-ray emission of the three late-type companions, assumingthat these stars have X-ray characteristics similar to those ofother stars with similar age and mass: the median X-ray lu-minosities of young G- and early K-type stars in the COUPsample are 2 . × erg / sec, and for young M-type stars3 . × erg / sec (Preibisch & Feigelson 2005). Thus, the ex-pected combined X-ray luminosity of the three companions ofTY CrA is 5 . × erg / sec, a value that is very close to theobserved system X-ray luminosity of 5 . × erg / sec. Also,the plasma temperatures of T =
10 MK and T =
27 MK de-rived from the fit to the X-ray spectrum of the TY CrA systemare in the prototypical range observed for G- and early K-type T an Forbrich and Thomas Preibisch: Coronae in the Coronet 9
Fig. 3.
Chandra X-ray spectra of selected YSOs in the Coronet region. The solid dots with error bars show the observed spectra,while the histogram lines show the best fit models. The first row shows two pairs of class III / II objects: V 709 (class III) & S CrA(class II), and 190140.40 (class III) & 190139.34 (class II), which illustrate the systematically harder spectra of the class II objects.The first plot in the second row shows the very hard spectrum of the Herbig Ae star R CrA; the dotted line shows the one-temperaturefit model, the solid line corresponds to the “two temperature – two absorption” model (for details see text). The next three panelscompare the spectrum of the companion to the B star HD 176386 and the spectrum of the TY CrA multiple system to the spectrumof the class III T Tauri star HBC 687; the similarity of these spectra suggest that the true sources of X-ray emission apparentlyobserved from these B stars are most likely young late-type stellar companions. The third row shows three class I objects and finallythe class 0 / I protostar IRS 7E.Tauri stars (see, e.g., Preibisch et al. 2005a). The observed X-rayemission from the TY CrA system can thus be very easily under-stood as originating from the late-type companions of TY CrA;the data provide no direct hint towards possible X-ray emissionfrom the B8e star. Of course, we cannot rule out the possibilitythat (some fraction of) the observed X-ray emission may never-theless come from the Be star, but the data provide no indicationsfor this assumption.
HD 176386:
This B9 IVe star has a visual companion at ∼ ′′ separation (Je ff ers et al. 1963). The strong decreaseof the brightness ratio between primary and companion from optical to near-infrared wavelengths of ( ∆ [ V , J , H , K ] = [6 . , . , . , .
45] mag according to Turon et al. (1993) andthe 2MASS point-source catalog) suggests that the companionis of substantially later spectral type and thus most likely a low-mass ( M < ∼ M ⊙ ) star.A strong X-ray source with 7720 counts is perfectly centeredon the position of the companion to HD 176386, whereas thereis no evidence for emission from the B star in our data. Thespectral fit yields parameters ( T = T =
21 MK, L X = . × erg / sec) which are very typical for K- and early M-type T Tauri stars.In order to derive an upper limit to the possible X-ray emis-sion from the primary B-star HD 176386, we extracted counts in a 1 ′′ radius aperture centered on its optical position. There are21 photons in this region, which however is strongly a ff ected bythe wings of the PSF of the X-ray emission from the companion.Comparison with nearby “background regions” at the same ra-dial distance from the strong X-ray source yielded an expectedbackground of 22 counts in a 1 ′′ radius aperture. The 90% con-fidence upper limit to the number of counts from HD 176386is 8.4 source counts. After correction for the small extractionregion, which would contain only about 30% of the flux froma point source at the o ff -axis angle of ∼ ′ , we derive an up-per limit of < .
21 counts / ksec for the source count rate; with A V = L bol = L ⊙ (Bibo et al. 1992) we derive upperlimits of L X < . × erg / sec and log ( L X / L bol ) < − . R CrA:
The Herbig Ae star R CrA shows an ex-tremely strong infrared excess. According to the analysis ofAcke & van den Ancker (2004), the stellar luminosity, derivedby fitting and integrating a model for the stellar photosphere tothe de-reddened photometry, is only a very small fraction of thebolometric luminosity, i.e. the total integrated luminosity of theSED. The fact that the derived stellar luminosity would placeR CrA below the main-sequence in the HR-diagram, shows thatthe stellar parameters are very uncertain. Perhaps, the central staris deeply embedded in circumstellar material and seen only inscattered light. In that case, the stellar luminosity and also thederived extinction of A V = .
33 mag would be considerably un-derestimated.The X-ray emission from R CrA is strongly variable andhas risen to considerably higher levels in the last two epochsof our monitoring Chandra observations (Fig. 2, see also X-ray lightcurves in Forbrich et al. 2006). The lightcurves ofthe individual August 2005 Chandra observations (shown inForbrich et al. 2007) reveal numerous flare-like peaks, suggest-ing that the source is more or less continuously flaring. Ourspectrum of R CrA contains 981 source counts. The extremehardness of the spectrum, providing clear evidence for the pres-ence of very hot plasma, was already noted in Forbrich et al.(2006) and is clearly confirmed. A one-temperature model can-not produce an acceptable fit to the observed spectrum; whilethe hard part ( > χ r = . T >
660 MK)can be established, the fit yields an extremely strong and coollow-temperature component of 1 . N H = . × cm . The emission measure of this al-leged low-temperature plasma component would be more than2000 times larger than that of the high-temperature compo-nent, and its (extinction-corrected) X-ray luminosity would be1 . × erg / sec, orders of magnitude higher than the luminos-ity of the high-temperature component of 4 . × erg / sec. Thisextremely high X-ray luminosity makes it very unlikely that thisvery soft spectral fit component represents soft X-ray emissiondue to accretion shock emission. Rather, we strongly suspect thatthis fit result is an example of the highly nonlinear interaction ofvery low-temperature plasma components and strong extinction(see, e.g., the discussion in Getman et al. 2005). For high hy-drogen column densities, such a very low-temperature plasmacomponent is almost entirely absorbed, and thus any uncertain- ties in the lowest energy bins of the spectrum can lead to largeoverestimates of the extinction-corrected X-ray luminosity.In an attempt to find a spectral fit solution that avoids thiskind of inference of a very luminous, but heavily absorbed, ultra-soft component, we considered a spectral model in which bothof the two plasma components have individual extinction fac-tors rather than a common extinction factor for both compo-nents (model = wabs × apec + wabs × apec , instead of the“usual” model = wabs × [apec + apec ]). This model providesa very good fit ( χ r = .
66) for N H , = (1 . ± . × cm − ( A V = .
25 mag), T = (9 . ± .
8) MK, and N H , = (4 . ± . × cm − ( A V =
20 mag), T >
607 MK, and is shown by thesolid line histogram in the spectrum in Fig. 3. The extinction-corrected X-ray luminosities for the two spectral components are L X , = . × erg / sec and L X , = . × erg / sec.The requirement of di ff erent extinction values for the twotemperature components in the spectrum of R CrA is similar tothe case of FU Ori, for which Skinner et al. (2006) found thatthe hot plasma component also requires a considerably largerhydrogen column density than the low-temperature componentto fit the observed spectrum. They argued that the hot compo-nent represents coronal emission that is strongly absorbed in ac-cretion streams or a strong stellar wind. Why the cooler spectralcomponent is less absorbed, remains unclear; perhaps it origi-nates from a di ff erent location. An alternative explanation wouldbe that the X-ray emission comes from two di ff erent, close andthus unresolved objects (perhaps late type companions to R CrA)for which the extinction along the line-of-sight is di ff erent.Although these explanations remain quite speculative, it is inter-esting that R CrA and FU Ori are both very strongly accreting,and their peculiar X-ray spectra may therefore be in some waya ff ected by accretion processes. If the X-ray emission originatesfrom R CrA (and not from an unresolved companion) the extinc-tion derived from the X-ray spectrum ( A V ≥ A V = .
33 mag is aserious underestimate.
T CrA:
This F0e star is particularly interesting because itsspectral type is very close to the upper boundary for starswith convective envelopes. For the stellar parameters of T CrAas listed in Acke & van den Ancker (2004), the models ofSiess et al. (2000) suggest an extremely shallow convective en-velope with relative thickness of ∆ R env / R ⋆ = . = ± A V = .
45 mag as derived by Acke & van den Ancker (2004),suggests a plasma temperature of ∼
11 MK and yields an X-rayluminosity of ∼ × erg / sec. This F0 star is thus clearly amuch weaker X-ray emitter than the G type T Tauri stars in theCoronet and in other young clusters (see, e.g., Preibisch et al.2005a). This suggests that T CrA is the hottest object in theCoronet region with coronal X-ray activity driven by a dynamoin a very shallow convection zone. an Forbrich and Thomas Preibisch: Coronae in the Coronet 11 S CrA :
This classical T Tauri star of spectral type K3 has afaint companion ( ∆ K = . . ′′ from S CrA(Prato et al. 2003; McCabe et al. 2006). The Chandra data yield3020 source counts, and their spatial distribution is consistentwith a single source at the position of the primary; there is noelongation along the direction toward the companion (along po-sition angle 149 ◦ ). The spectral parameters are prototypical forT Tauri stars. HBC 679 :
This weak-line T Tauri star of spectral type K5has a companion of spectral type M3 at a separation of 4 . The infrared source IRS 7E has many properties that are char-acteristic for class 0 protostars. Its SED is dominated by thestrong submillimeter emission (Nutter et al. 2005), and possi-bly the source has a disk and an outflow (Anderson et al. 1997;Groppi et al. 2004), although a definitive attribution is currentlydi ffi cult due to the high source density in the region and the com-parably low angular resolution. Harju et al. (2001) find evidencefor a radio jet emanating from IRS 7E. IRS 7E is detected in allfour IRAC bands of the Spitzer images. Based on this and newhigh-angular resolution submillimeter data, Groppi et al. (2007)conclude that IRS 7E (their source SMA 1) is a transitionalclass 0 / I source, thus the youngest of the sources discussed here.This result supports the notion that IRS 7E is di ff erent (i.e. in anearlier evolutionary state) from the class I objects.The X-ray detection of IRS 7E (originally reported byHamaguchi et al. 2005b) represents up to now the only reliablecase for high-energy emission from this early evolutionary stage.The long-term lightcurve (see Fig. 2) suggests considerable vari-ability, but (probably due to the low count rate) no individualflares are detected in the individual Chandra observations. TheX-ray spectrum of IRS 7E (see Fig. 3) is very hard; the spec-tral fit suggests an extinction of A V ∼
74 mag, yields a plasmatemperature of 80 MK, and gives an extinction-corrected X-rayluminosity of ∼ × erg / sec. These parameters are roughlyconsistent with those derived by Hamaguchi et al. (2005b) fromtheir XMM data for the phase of “constant” emission before theflare. The very high plasma temperature clearly shows that theX-ray emission is dominated by magnetic processes, suggestingthat magnetic activity starts already in extremely early stages of(proto)stellar evolution. X-ray emission from seven class I protostars is detected inour dataset: IRS 1, 2, 5, 7 W, 9, 190148.46–365714.5, and190155.61–365651.1. The X-ray properties of these objects aresimilar to those of other X-ray detected class I protostars.The class I object IRS 5 is especially interesting. The ob-ject is a close binary with a separation of 0 . ′′ (Chen & Graham1993; Nisini et al. 2005). The X-ray emission is marginally re-solved in the Chandra data, with the infrared brighter componentbeing also the stronger X-ray source. Due to the PSF overlap, Fig. 4.
Detail of the X-ray spectrum of IRS 5 around the 6 . ±
15 eV. The quality of the spectral fit with this6.4 keV line (reduced χ χ F -test shows that the null hypothesis, i.e. the assumption that theobserved 6.4 keV excess is due to random noise, can be rejectedwith 99% certainty. Considering only the 5 − χ where this cool, fluorescing materialis located. One obvious possibility would be fluorescence fromirradiated material in a circumstellar disk, but the emission mayin principle also originate from interstellar material somewherealong the line of sight. As discussed in Tsujimoto et al. (2005),the width of the 6.4 keV line can help to distinguish betweenthese di ff erent possibilities (see also Favata et al. 2005). For flu-orescence emission from interstellar material along the line ofsight, one would expect a line width that should be considerablysmaller than 10 eV (formula (4) in Tsujimoto et al. 2005). Theobserved line width of 44 eV therefore clearly suggests that thefluorescent emission comes from dense local, i.e. circumstellarmaterial of higher column density, most likely from the irradi-ated circumstellar disk of IRS 5. The strong X-ray irradiation ofcircumstellar disk material has important consequences for thephysical processes in the circumstellar dust and gas and the evo- lution of proto-planets (e.g., Glassgold et al. 2005; Wolk et al.2005). X-ray emission has been detected from numerous young
BDs (e.g., Neuh¨auser & Comer´on 1998; Imanishi et al.2001b; Preibisch & Zinnecker 2001, 2002; Tsuboi et al. 2003;Preibisch et al. 2005b), but the origin of their activity isstill not well understood. The CrA star-forming region con-tains a number of very low-mass objects, some of whichare most likely young brown dwarfs (BDs). Several objectshave been identified as BD candidates in di ff erent studies(Wilking et al. 1997; Neuh¨auser et al. 1999; Olofsson et al.1999; Fern´andez & Comer´on 2001; L´opez Mart´ı et al. 2005),but, unfortunately, in no case a fully reliable spectroscopicclassification as BD is available, because either the spectraltypes were estimated from (narrow-band) photometry andare thus quite uncertain, or the objects are very close to thestellar / sub-stellar boundary. Therefore, all objects discussedhere are BD candidates, not bona-fide BDs.There are eight BD candidates in the field of view of theChandra image. Three of them (B185839.6-365823, LS-RCrA2, and ISO CrA 143) are among the X-ray sources detected bywavdetect. For the remaining five BD candidates we have per-formed a detailed investigation of the corresponding positions inthe Chandra image. For each object we defined a source regioncentered at its optical position with a radius of 3 ′′ and a corre-sponding background region as annulus with inner radius 5 ′′ andouter radius 10 ′′ . Then we determined the numbers of detectedcounts in the source and background regions and compared thenumber of background counts scaled by the corresponding areato the number of counts detected in the source regions. For twoobjects (B185840.4-370433 and B185831.1-370456) the num-ber of counts in the source region exceeded the number of ex-pected background counts with at least 90% confidence, tenta-tively indicating the presence of very weak X-ray emission. Forthe remaining 3 objects, upper limits to their count rates and (ifinformation on extinction was available) also to their X-ray lu-minosities were determined as described above. The results ofthis analysis are listed in Table 3.The X-ray luminosities and fractional X-ray luminosities ofthe young BD candidates in the CrA star-forming region are sim-ilar to those of the low-mass stars, and thus there is no evidencefor changes in the magnetic activity around the stellar / substellarboundary. In two of the three objects which yielded enoughcounts for spectral analysis, the derived plasma temperatures arein the lower range of plasma temperature found for young stars,consistent with previous findings. On the other hand, the BDcandidate B185839.6-365823 shows a rather hard spectrum andthe fit suggests a very high plasma temperature. Although theS / N of the spectrum is quite low and thus no reliable determina-tion of the spectral parameters is possible, the median photon en-ergy of 3.4 keV already suggests a relatively hard spectrum. Wealso note that this objects shows strong variability in the long-term lightcurve, so its hard spectrum may be related to X-rayflaring.
In Fig. 5 we plot the plasma temperatures versus X-ray lumi-nosities for the YSOs in the Coronet cluster. We first consider
Fig. 5.
Plasma temperatures versus X-ray luminosities for theCrA members with su ffi cient counts for X-ray spectral fitting.The di ff erent SED classes are shown with di ff erent symbols: theclass 0 / I protostar IRS 7E as a dark dot, class I objects as greydots, class II objects as open boxes, class III objects as asterisks,and objects without SED classification as crosses. Brown dwarfcandidates are surrounded by diamond symbols. For objects with2T fits, the high and low-temperature values are connected by thedashed lines.the derived plasma temperatures, which yield information aboutthe X-ray emission process in the sense that hot ( > ∼
10 MK) tem-peratures require magnetic processes while the expected signa-ture of emission from accretion shocks would be at much cooler( < ∼ ff erent evolutionary stages. While the source numbersare too low for statistically sound conclusions, the plot revealsthat Class II objects tend to show systematically hotter plasmatemperatures than class III objects. This e ff ect is also illustratedin the comparison of the spectra for V709 CrA (class III) toS CrA (class II) and 190139.34 (class III) to 190140.40 (class II)in Fig. 3. We note that a similar spectral di ff erence was foundby Flaccomio et al. (2006) for the YSOs in the NGC 2264 star-forming region.Another interesting aspect is that the class I objects in turntend to show systematically higher plasma temperatures thanclass II objects. This is especially interesting in the context ofthe recent debate whether class I and class II objects are trulyin di ff erent evolutionary stages, or whether the classification isa ff ected by other factors such as the inclination under whichthe YSO is seen (see discussion in White & Hillenbrand 2004;Eisner et al. 2005; Doppmann et al. 2005). In addition to the dif-ference in plasma temperatures of class I and II objects, we alsonote that our long-term lightcurves (see Fig. 2) suggest that the an Forbrich and Thomas Preibisch: Coronae in the Coronet 13 Table 3.
Results for the Brown Dwarf candidates (see Section 4.2.5).
Name Ref SpT log (cid:18) L bol L ⊙ (cid:19) M A V source T X L X log (cid:18) L X L bol (cid:19) [ M ⊙ ] [mag] counts [MK] [erg / sec]B185815.3-370435 1 ∼ < . < . × B185831.1-370456 1,2 M8.5 − . ∼ . ∼ ∼ . × ∼ − . − . ∼
13 47 > ∼ . × ∼ − . − . ∼
12 6 ∼ × ∼ − . − . ∼ < . < . × < − . − . ∼ . − . ∼ . < < . × < − . − . ∼ . ∼ . . × − . ≈ M8: − . ∼ . ∼ . . × − . IRS 7E IRS 7W R CrAIRS 9190155.59190155.76
Fig. 6.
Chandra X-ray image of the central region (89 ′′ × ′′ field of view), showing the di ff use emission between IRS 7Wand R CrA. The greyscale shows the 3 − . . ′′ pixels. The contours show the same im-age smoothed with a 10 pixel FWHM Gauss filter. Contours aredrawn at levels of 0.07, 0.08, 0.09, 0.1, 0.11 (to trace the di ff useemission) and 0.6, 0.7, 0.8, 0.9, 1.0 (to outline point sources.)The box indicates the source region used to characterize the dif-fuse emission while the two circles show the background re-gions.class I objects display stronger levels of variability than class IIobjects. Similar di ff erences in the plasma temperatures and lev-els of variability between class I and class II / III objects werefound by Imanishi et al. (2001a,b) for the ρ Ophiuchi dark cloud.These di ff erences in the characteristics of the X-ray emissionsupport the notion that class I and class II objects are truly dif-ferent.
5. Diffuse X-ray emission
Inspection of the Chandra image reveals an excess of counts ina region north of the class I object IRS 7 W and east of R CrA(see Fig.6). The box-shaped region contains 102 counts, whereasthe local background, estimated from two nearby source-freeregions, should contribute only ∼
38 counts. The excess of 64counts apparently represents di ff use X-ray emission. Comparingthe distribution of photon energies within this region to that in the background regions reveals a statistically significant excessin the 3 − . ff use X-ray emission. However, as there is muchstronger and extended [S II] emission to the east of this patch,it is not clear whether the X-ray emission is actually related tothis [S II] emission. Furthermore, if the X-ray emission were re-lated to the jets and outflows in this region, one would expect aquite soft X-ray spectrum, with most of the detected photons atenergies of < ∼ > ∼
6. Conclusions and Summary
The main results of our very deep X-ray study of the Coronetcluster can be summarized as follows:The observed X-ray properties of the YSOs in the Coronetregion are fully consistent with coronal magnetic activity. Wefind no indications for X-ray emission from accretion- or jet-shocks: in our X-ray spectral analysis, we find neither signif-icant plasma components at temperatures below ∼ < ∼ ff ering from more than a few magni-tudes of visual extinction would hardly be detectable in the data.The observed tendency that the class I objects exhibit ahigher degree of X-ray variability than the older class II and IIIobjects may be a consequence of magnetic reconnection eventsin the proto(star)-disk magnetic fields, causing frequent strong flares (see, e.g., Montmerle et al. 2000). The apparent di ff er-ences in the X-ray properties of class I versus class II / III ob-jects supports the assumptions that class I objects are truly in anearlier evolutionary stage. Finally, the high plasma temperaturesof the class 0 and class I protostars clearly show that the X-rayemission of these extremely young objects must be dominatedby magnetic processes.The Coronet class I objects with relatively well determinedspectroscopic ages as young as 0.1 Myr are clearly detected asX-ray sources. IRS 7E is so far the only case of a reliable X-raydetection of an object in earlier evolutionary stage than class Iobjects. The X-ray properties of this object are similar to those ofthe class I objects, showing that hot coronae and thus magneticactivity exist already in extremely young protostellar objects.Concerning now the origin of the X-ray emission from youngintermediate-mass stars, we first show that the X-ray emissionfrom HD 176386 originates not from the Be star, but from alate type companion. Then we demonstrate that the characteris-tics of the observed X-ray emission from the TY CrA multiplesystem agree very well with the expected X-ray emission of thethree late-type companions; there is no need to assume that theBe star itself emits any X-rays. The case of R CrA remains un-clear; its extremely hard X-ray spectrum clearly suggests a mag-netic origin of the emission. One or several yet undiscovered andspatially unresolved late-type companions may provide the moststraightforward explanation, although even in that case the ex-traordinarily high plasma temperature is very unusual.Finally, we point out that none of the numerous HH objectsin the CrA star-forming region is detected in the Chandra datadespite the very high sensitivity. As X-ray emission at levels sim-ilar to those of the X-ray detected HH objects in the other star-forming regions should have been easily detected in our data,this null-result suggests that observable X-ray emission from HHobjects is not very frequent.The further results can be summarized as follows: X-rayemission is detected from about half of the brown dwarf can-didates in the observed region. Di ff use X-ray emission is tenta-tively detected in the central part of the Coronet cluster, but itsnature and origin remain unclear. Acknowledgements.
We are grateful to Lori Allen for providing us with theSpitzer IRAC images of the CrA star-forming region and the classification of thesources prior to publication. We would like to thank Stefan Kraus for help withthe Spitzer images. This work made extensive use of NASA’s Astrophysics DataSystem Bibliographic Services and the SIMBAD database (CDS, Strasbourg,France). This publication makes use of data products from the Two Micron AllSky Survey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center / California Institute of Technology,funded by the National Aeronautics and Space Administration and the NationalScience Foundation.
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Appendix A: Detailed source lists
Table A.1.
Complete source list with parameters determined by acis extract for the energy range of 0.5–8 keV, i.e. total counts, median photonenergy, source significance, hardness ratios, as well as estimates of the observed flux. Also given are the ratio of the summed exposure map valuesat the source position relative to the maximum value, i.e., an information on the e ff ective exposure time, and the number of observations in whicha source was detected (N obs ). This number can be lower than eight in the outer parts of the map. The manually added source is marked by anasterisk.Src ID Net Med. E Source HR1 HR2 HR3 FLUX1 FLUX2 rel.exp.counts [keV] signif. [cm − s − ] [cm − s − ] (N obs )190104.58-370129.6 46 . + . − . − . + . − . − . + . − . − . + − NaN − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + − NaN − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . Table A.1. cntd.Src ID Net Med. E Source HR1 HR2 HR3 FLUX1 FLUX2 rel.exp.counts [keV] signif. [cm − s − ] [cm − s − ] (N obs )190150.45-365638.1 259 . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . − . + . − . − . + . − . − . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − . + . − . − . + . − . − . + . − . ∗ . + . − . . + . − . − . + . − . . + . − . Table A.2.
X-ray spectral fitting results for one- and two-temperature models. The fitting parameters are the absorbing hydrogencolumn density N H , the plasma temperatures T , and the normalization factors from XSPEC (defined as 10 − / (4 π D ) R n e n H dV ).We also list the extinction-corrected (unabsorbed) X-ray luminosity L X (integrated over the 0.2–8 keV band) derived from the modelparameters, the goodness-of-fit measure χ , and the number of degrees of freedom (DOF). The fitting parameters for R CrA aregiven in the text. Source N H T T norm norm L X χ DOF[10 cm − ] [MK] [MK] [erg s − ]190108.60–365721.3 0.72 + . − . + . − . + . − . (3 . + . − . ) 10 − (1 . + . − . ) 10 − . × + . − . + . − . (8 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . + . − . (1 . + . − . ) 10 − . × + . − . + . − . (4 . + . − . ) 10 − . × + . − . + . − . + . − . (4 . + . − . ) 10 − (2 . + . − . ) 10 − . × + . − . + . − . + . − . (3 . + . − . ) 10 − (4 . + . − . ) 10 − . × + . − . + . − . + . − . (3 . + . − . ) 10 − (1 . + . − . ) 10 − . × + . − . + . − . + . − . (6 . + . − . ) 10 − (4 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . + . − . (6 . + . − . ) 10 − (1 . + . − . ) 10 − . × + . − . + . − . + . − . (2 . + . − . ) 10 − (2 . + . − . ) 10 − . × + . − . + . − . + . − . (3 . + . − . ) 10 − (4 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . + . − . (1 . + . − . ) 10 − (8 . + . − . ) 10 − . × [ ] + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . + . − . (3 . + . − . ) 10 − (5 . + . − . ) 10 − . × [ ] + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . (3 . + . − . ) 10 − . × [ ] + . − . + . − . (7 . + . − . ) 10 − . × + . − . + . − . (6 . + . − . ) 10 − . × + . − . + . − . (3 . + . − . ) 10 − . × + . − . > − (9 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . + . − . (9 . + . − . ) 10 − (2 . + . − . ) 10 − . × + . + . − . + . − . (4 . + . − . ) 10 − (6 . + . − . ) 10 − . × + . − . + . − . (1 . + . − . ) 10 − . × + . − . + . − . + . − . (2 . + . − . ) 10 − (4 . + . − . ) 10 − . × + . − . + . − . (3 . + . − . ) 10 − . × + . > − (2 . + . − . ) 10 − . × [ ] iron abundance adjusted to fit the 6.7 keV line; [ ]]