The Astronomical Journal, 123:1613–1628, 2002 March E # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A. X-RAY PROPERTIES OF THE YOUNG STELLAR AND SUBSTELLAR OBJECTS IN THE IC 348 CLUSTER: THE CHANDRA VIEW Thomas Preibisch Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, D-53121 Bonn, Germany; [email protected] and Hans Zinnecker Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany; [email protected] Received 2001 October 2; accepted 2001 November 20 ABSTRACT We explore the X-ray properties of the young stellar and substellar objects in the open cluster IC 348 as seen in our deep Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer image. First, we give identifications of all X-ray sources and determine upper limits for the X-ray luminosities of the undetected cluster members. Then we analyze the X-ray spectra of the young stellar objects, deriving plasma tempera- tures between 0.7 and 3 keV for the T Tauri stars in IC 348 and higher temperatures, between 3 and 7 keV, for flaring sources and two embedded young stellar objects. We find several large X-ray flares, in some of which a clear hardening of the X-ray spectra during the flare peak is seen. Next we use the exceptional opti- cal, infrared, and X-ray data set of this cluster to study various correlations and their implications, and to dis- cuss new answers to some long-standing questions related to X-ray emission from young (sub)stellar objects. The X-ray luminosities of the young low-mass stars are strongly correlated to the stellar bolometric luminosi- À4 ties (LX 10 Â Lbol). Also, a good correlation between X-ray luminosity and stellar mass is found 2 (LX / M ). For the weak-line T Tauri stars we find a tight correlation between X-ray activity and chromo- 0:8 spheric activity ðLX / LH Þ, supporting the hypothesis that the chromosphere is heated by X-rays from the overlying corona. The observed X-ray properties of the brown dwarfs (and brown dwarf candidates) are very similar to those of late-type stars; we explain this behavior as the consequence of the fact that very young sub- stellar objects are still warm enough to maintain partially ionized atmospheres, which are capable of sustain- ing electrical currents, while in the cooler neutral atmospheres of L and T dwarfs such currents are shut off (hence no X-ray emission). Finally, we explore the difference between the X-ray luminosity functions of clas- sical and weak-line T Tauri stars. We find that the classical T Tauri stars in IC 348 seem to be on average less X-ray luminous than the weak-line T Tauri stars. However, we suggest that this apparent difference is caused by a selection effect: there is a strong detections bias against those weak-line T Tauri stars that are optically faint and hence X-ray faint; the population of classical T Tauri stars, on the other hand, is essentially com- pletely known because of its very prominent H emission. This conclusion is corroborated by another new result: when using a photometrically selected, magnitude-limited, complete sample of T Tauri stars and tak- ing the KÀL infrared excess as a tracer of circumstellar material, we find no evidence in IC 348 for a difference in X-ray properties of young stars with and without circumstellar matter, i.e., classical and ‘‘ naked ’’ T Tauri stars. Key words: low-mass, brown dwarfs — open clusters and associations: individual (IC 348) — stars: coronae — stars: pre–main-sequence — X-rays On-line material: machine-readable tables 1. INTRODUCTION Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory, in which we detected IC 348 is a very young stellar cluster in the Perseus molecular cloud complex. The cluster is exceptionally 215 individual X-ray sources. First results of this obser- vation have been reported in Preibisch & Zinnecker well studied in optical and infrared wavelengths (see Her- (2001, hereafter PZ01), including a comparison of the big 1998, hereafter H98; Scholz et al. 1999; Lada & Lada optical, infrared, and X-ray images. In this paper we 1995; Luhman et al. 1998, hereafter L98; Luhman 1999, present detailed information on the identification of the hereafter L99; Najita, Tiede, & Carr 2000, hereafter X-ray sources and study the X-ray properties of the N00). Spectral types are known for more than 200 stars, young stellar and substellar objects in IC 348, including H emission has been detected for more than 100 stars, their implications. and numerous substellar objects have been identified. The distance to IC 348 is 310 pc (H98), the mean age of the stars is 1.5 Myr, and their mean extinction is 3.5 mag (H98; L98). ROSAT X-ray observations of IC 348 were 2. Chandra Observations and Data Analysis presented by Preibisch, Zinnecker, & Herbig (1996, here- The Chandra observation of IC 348 was performed on after PZH96) and led to the discovery of 116 X-ray 2000 September 25, utilizing ACIS in its imaging configura- sources in a 2 diameter field of view. We have recently tion. The total exposure time was 52,956.8 s. The results pre- obtained a deep X-ray image of IC 348 with the sented in PZ01 can be summarized as follows: 1613 1614 PREIBISCH & ZINNECKER Vol. 123 TABLE 1 Chandra X-Ray Sources in IC 348 Source Identification SpT AV W(H ) kT LX CXOPZ- (2) (3) (mag) (A˚) (keV) (1028 ergs sÀ1) (1) (4) (5) (6) (7) J034346.9+321320 ... 2MASS ... 0.70 i ... ... 5.0 Æ 1.6 J034349.3+321040 ... H-IfA68 ... 1.74 i 2.0 ... 34.2 Æ 7.0 J034351.2+321309 ... L-22 G5 2.31 ... 2.1 Æ 0.3 472.0 Æ 40.3 J034351.5+321239 ... noCP ... ... ... ... ... J034353.3+320927 ... noCP ... ... ... ... ... Notes.—Table 1 is presented in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. Columns: (1) Chandra X-ray source name as defined in Preibisch & Zinnecker 2001; the name contains the J2000.0 coordinates in the format HHMMSS.S+DDMMSS. (2) Identification with optical and/or infrared counterparts from the catalog of Herbig 1998 (H-), Luhman et al. 1998 or Luhman 1999 (L-), or Najita et al. 2000 (N-). Stars known to be foreground or background objects are marked with ‘‘ FG ’’ and ‘‘ BG ’’, respectively. If no cataloged counterpart is known, the entry ‘‘ 2MASS ’’ means that the X-ray source has a counterpart in the 2MASS point-source catalog, while ‘‘ noCP ’’ means that no optical or infrared counterpart could be found. Source CXOPZ-J034424.6+321349 is identified with a faint infrared source not included in the 2MASS catalog. (3) Spectral type of the counterpart. (4) Visual extinction of the counterpart; ‘‘ i ’’ indicates that the value was estimated from the near-infrared colors, and a colon indicates that, owing to a lack of fur- ther information, an extinction of 3.5 mag has been assumed. (5) Equivalent width of the H emission line. Negative values denote H absorption. (6) Plasma temperature determined in the fit of the X-ray spectrum. (7) Extinction corrected 0.2–10 keV band X-ray luminosity. 1. The number of individual X-ray sources detected was be rather uniform, we are confident that they are back- 215. ground objects. 2. About 80% of all known cluster members with masses In Table 1 we present a list of all X-ray sources with corre- between 0.15 and 2 M are seen as X-ray sources in our sponding identifications and further information about the Chandra image. stellar and X-ray properties. The determination of the 3. X-ray emission at levels of 1028 ergs sÀ1 was discov- extinction corrected X-ray luminosities (integrated over the ered from four of 13 known brown dwarfs and from three of 0.2–10 keV band) was based on the analysis of the X-ray 12 brown dwarf candidates in IC 348. spectra as described in x 5. 4. X-ray emission was also detected from two deeply embedded young stellar objects, presumably Class I proto- stars, south of the optical cluster center. 4. UPPER LIMITS FOR NONDETECTED The data analysis presented in this paper was performed CLUSTER MEMBERS with the CIAO 2.1.2 software package provided by the In Table 2 we present a list of all those known cluster Chandra X-Ray Center and is based on the Level 2 proc- members that were not detected as X-ray sources in our essed event list provided by the pipeline processing at the Chandra image. We determined upper limits to the count Chandra X-Ray Center. rates of these objects by counting the observed number of photons in source regions centered at their optical/ infrared positions and comparing them with the expected 3. IDENTIFICATIONS OF THE CHANDRA number of background photons determined from several X-RAY SOURCES large source-free background regions. We used the Baye- We searched for counterparts of the X-ray sources in the sian statistics method described by Kraft, Burrows, & member lists published in H98, L98, L99, and N00. For X- Nousek (1991) to determine the 90% confidence upper ray sources without a cataloged counterpart, we searched limits for their count rates. From these count-rate upper for optical counterparts on the red DSS plate and on our limits we computed upper limits for the extinction-cor- own deep optical images, and for infrared counterparts in rected X-ray luminosities in the 0.2–10 keV band assum- the 2MASS source catalog, in the 2MASS images, and in ing thermal plasma spectra with a temperature of kT =1 our own deep J- and K-band images1 of IC 348.