{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,14]],"date-time":"2026-10-14T03:48:25Z","timestamp":1791949705846,"version":"4.3.4"},"reference-count":46,"publisher":"American Association for the Advancement of Science (AAAS)","issue":"5322","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Science"],"published-print":{"date-parts":[[1997,7,4]]},"abstract":"<jats:p>\n            The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO\n            <jats:sup>+<\/jats:sup>\n            emission. These molecules probably orbit the star in a solar-system\u2013sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike stars.\n          <\/jats:p>","DOI":"10.1126\/science.277.5322.67","type":"journal-article","created":{"date-parts":[[2002,7,27]],"date-time":"2002-07-27T09:37:56Z","timestamp":1027762676000},"page":"67-71","source":"Crossref","is-referenced-by-count":318,"title":["X-ray and Molecular Emission from the Nearest Region of Recent Star Formation"],"prefix":"10.1126","volume":"277","author":[{"given":"J. H.","family":"Kastner","sequence":"first","affiliation":[{"name":"J. H. Kastner, Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37-667a, Cambridge, MA 02139, USA."},{"name":"B. Zuckerman, Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA."},{"name":"D. A. Weintraub, Department of Physics and Astronomy, Vanderbilt University, Post Office Box 1807 Station B, Nashville, TN 37235, USA."},{"name":"T. Forveille, Groupe d'Astrophysique, Observatoire de Grenoble, 414 rue de la Piscine, F-38041 Grenoble Cedex, France."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"B.","family":"Zuckerman","sequence":"additional","affiliation":[{"name":"J. H. Kastner, Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37-667a, Cambridge, MA 02139, USA."},{"name":"B. Zuckerman, Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA."},{"name":"D. A. Weintraub, Department of Physics and Astronomy, Vanderbilt University, Post Office Box 1807 Station B, Nashville, TN 37235, USA."},{"name":"T. Forveille, Groupe d'Astrophysique, Observatoire de Grenoble, 414 rue de la Piscine, F-38041 Grenoble Cedex, France."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"D. A.","family":"Weintraub","sequence":"additional","affiliation":[{"name":"J. H. Kastner, Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37-667a, Cambridge, MA 02139, USA."},{"name":"B. Zuckerman, Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA."},{"name":"D. A. Weintraub, Department of Physics and Astronomy, Vanderbilt University, Post Office Box 1807 Station B, Nashville, TN 37235, USA."},{"name":"T. Forveille, Groupe d'Astrophysique, Observatoire de Grenoble, 414 rue de la Piscine, F-38041 Grenoble Cedex, France."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"T.","family":"Forveille","sequence":"additional","affiliation":[{"name":"J. H. Kastner, Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37-667a, Cambridge, MA 02139, USA."},{"name":"B. Zuckerman, Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA."},{"name":"D. A. Weintraub, Department of Physics and Astronomy, Vanderbilt University, Post Office Box 1807 Station B, Nashville, TN 37235, USA."},{"name":"T. Forveille, Groupe d'Astrophysique, Observatoire de Grenoble, 414 rue de la Piscine, F-38041 Grenoble Cedex, France."}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"221","reference":[{"key":"e_1_3_1_2_2","first-page":"217","volume":"121","author":"Rucinski S. M.","year":"1983","unstructured":"Rucinski S. M., Krautter J., Astron. Astrophys. 121, 217 (1983).","journal-title":"Astron. Astrophys."},{"key":"e_1_3_1_3_2","doi-asserted-by":"crossref","first-page":"L61","DOI":"10.1086\/185511","volume":"343","author":"de la Reza R.","year":"1989","unstructured":"de la Reza R., Torres C. A. O., Quast G., Castilho B. V., Vieira G. L., Astrophys. J. 343, L61 (1989).","journal-title":"Astrophys. J."},{"key":"e_1_3_1_4_2","unstructured":"The combination of H\u03b1 emission and strong Li absorption indicates stellar youth and is commonly used to identify T Tauri stars. The strength of the H\u03b1 emission serves to classify such stars either as classical (commonly defined as a T Tauri star with an H\u03b1 emission equivalent width of \u226510 \u00c5) or weak-lined."},{"key":"e_1_3_1_5_2","doi-asserted-by":"crossref","first-page":"L69","DOI":"10.1086\/185441","volume":"340","author":"Weintraub D. A.","year":"1989","unstructured":"Weintraub D. A., Sandell G., Duncan W., Astrophys. J. 340, L69 (1989).","journal-title":"Astrophys. J."},{"key":"e_1_3_1_6_2","first-page":"306","volume":"468","author":"Feigelson E. D.","year":"1996","unstructured":"Feigelson E. D., ibid. 468, 306 (1996).","journal-title":"ibid."},{"key":"e_1_3_1_7_2","doi-asserted-by":"crossref","first-page":"549","DOI":"10.1086\/116082","volume":"103","author":"Gregorio-Hetem J.","year":"1992","unstructured":"Gregorio-Hetem J., Lepine J. R. D., Torres C. A. O., de la Reza R., Astron. J. 103, 549 (1992).","journal-title":"Astron. J."},{"key":"e_1_3_1_8_2","doi-asserted-by":"crossref","first-page":"494","DOI":"10.1038\/373494a0","volume":"373","author":"Zuckerman B.","year":"1995","unstructured":"Zuckerman B., Forveille T., Kastner J. H., Nature 373, 494 (1995).","journal-title":"Nature"},{"key":"e_1_3_1_9_2","unstructured":"Hen(3) 600 and CoD \u221233\u00b0 7795 might be considered classical T Tauri stars on the basis of H\u03b1 emission alone. However some authors also use the presence of excess near-IR emission to discriminate between classical and weak-lined T Tauri stars. Extrapolating from existing photometry we anticipate that CoD \u221233\u00b0 7795 does not have a near-IR excess and therefore is likely to be better classified as weak-lined. The near-IR flux of Hen 600 is more difficult to predict and its status (weak-lined versus classical) remains uncertain."},{"key":"e_1_3_1_10_2","first-page":"391","volume":"297","author":"Neuhaueser R.","year":"1995","unstructured":"Neuhaueser R., Sterzik M. F., Schmitt J. H. M. M., Wichmann R., Krautter J., Astron. Astrophys. 297, 391 (1995).","journal-title":"Astron. Astrophys."},{"key":"e_1_3_1_11_2","first-page":"1071","volume":"280","author":"Lawson W. A.","year":"1996","unstructured":"Lawson W. A., Feigelson E. D., Huenemoerder D. P., Mon. Not. R. Astron. Soc. 280, 1071 (1996).","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"e_1_3_1_12_2","doi-asserted-by":"crossref","first-page":"752","DOI":"10.1086\/175214","volume":"439","author":"Casanova S.","year":"1995","unstructured":"Casanova S., Montmerle T., Feigelson E. D., Andr\u00e9 P., Astrophys. J. 439, 752 (1995).","journal-title":"Astrophys. J."},{"key":"e_1_3_1_13_2","first-page":"623","volume":"416","author":"Feigelson E. D.","year":"1993","unstructured":"Feigelson E. D., Casanova S., Montmerle T., Guibert J., ibid. 416, 623 (1993).","journal-title":"ibid."},{"key":"e_1_3_1_14_2","unstructured":"W. Voges et al. Astron. Astrophys. in press."},{"key":"e_1_3_1_15_2","doi-asserted-by":"crossref","first-page":"625","DOI":"10.1086\/191951","volume":"91","author":"Stauffer J. R.","year":"1994","unstructured":"Stauffer J. R., Caillault J.-P., Gagn\u00e9 M., Prosser C. F., Hartmann L. W., Astrophys. J. Suppl. Ser. 91, 625 (1994).","journal-title":"Astrophys. J. Suppl. Ser."},{"key":"e_1_3_1_16_2","first-page":"134","volume":"300","author":"Randich S.","year":"1995","unstructured":"Randich S., Schmitt J. H. M. M., Prosser C. F., Stauffer J. R., Astron. Astrophys. 300, 134 (1995).","journal-title":"Astron. Astrophys."},{"key":"e_1_3_1_17_2","first-page":"785","volume":"305","author":"___","year":"1996","unstructured":"___, ibid. 305, 785 (1996).","journal-title":"ibid."},{"key":"e_1_3_1_18_2","doi-asserted-by":"crossref","first-page":"489","DOI":"10.1086\/192346","volume":"106","author":"Patten B. M.","year":"1996","unstructured":"Patten B. M., Simon T. S., Astrophys. J. Suppl. Ser. 106, 489 (1996).","journal-title":"Astrophys. J. Suppl. Ser."},{"key":"e_1_3_1_19_2","doi-asserted-by":"crossref","first-page":"798","DOI":"10.1093\/mnras\/266.4.798","volume":"266","author":"Pye J. P.","year":"1994","unstructured":"Pye J. P., Hodgkin S. T., Stern R. A., Stauffer J. R., Mon. Not. R. Astron. Soc. 266, 798 (1994).","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"e_1_3_1_20_2","unstructured":"Low-mass stars possess deep convective envelopes. Such deep convective zones in combination with fast rotation have been shown empirically to be conducive to chromospheric activity and coronal x-ray emission [D. S. Hall in The Sun and Cool Stars: Activity Magnetism and Dynamos I. Tuominen D. Moss A. Rudiger Eds. (IAU Colloquium No. 130 Springer-Verlag Berlin 1991) pp. 353\u2013369]. Over much of its pre\u2013main sequence and early main sequence evolution (that is up to an age of \u223c100 My) a low-mass star is continually contracting. Contraction has two observable consequences: a steady decrease in bolometric luminosity and a steady increase in rotation rate (once a circumstellar disk dissipates) attributable to conservation of angular momentum. Its increasing rate of rotation combined with ongoing deep convection ensures that a low-mass star's magnetic activity is maintained for the first \u223c100 My of its life. This inference is supported by the observation that the mean x-ray luminosity of low-mass stars remains approximately constant from late pre\u2013main sequence through early main sequence stages ["},{"key":"e_1_3_1_20_3","doi-asserted-by":"crossref","first-page":"740","DOI":"10.1086\/118293","volume":"113","author":"Briceno C.","year":"1997","unstructured":"Briceno C., et al., Astron. J. 113, 740 (1997);","journal-title":"Astron. J."},{"key":"e_1_3_1_20_4","unstructured":"] and by the correlation between the rotation rate and log ( L x \/ L bol ) for Pleiades members (14). Thus with L x remaining roughly constant and L bol steadily declining for low-mass (\u22720.7 M \u2299 ) stars over their first \u223c100 My of evolution (21 22) the net result is the observed steady increase in L x \/ L bol over the same period (Fig. 1). By the time a low-mass star nears the main sequence (that is by Pleiades age) its relative x-ray luminosity is near the empirical \u201csaturation\u201d level of log ( L x \/ L bol ) \u223c\u22123 (14). This would appear to represent the maximum efficiency with which dynamo activity (resulting from the combination of fast rotation and deep convection) can convert a star's luminosity to coronal x-ray emission. Subsequently single low-mass stars gradually spin down and become less active; by the time such stars are as old as the sun they are relatively meager x-ray emitters."},{"key":"e_1_3_1_21_2","unstructured":"The presence and strength of Li absorption in the spectrum of a young star are useful but controversial age indicators [see discussion in (32)]. Models predict that Li should be fully depleted in stars of the mass of TW Hya by \u223c10 My (22); however substantial abundances of Li are present in low-mass stars that are as old as 30 to 100 My ["},{"key":"e_1_3_1_21_3","doi-asserted-by":"crossref","first-page":"776","DOI":"10.1086\/303930","volume":"479","author":"Stauffer J. R.","year":"1997","unstructured":"Stauffer J. R., et al., Astrophys. J. 479, 776 (1997);","journal-title":"Astrophys. J."},{"key":"e_1_3_1_21_4","doi-asserted-by":"crossref","first-page":"685","DOI":"10.1093\/mnras\/284.3.685","volume":"284","author":"Zboril M.","year":"1997","unstructured":"Zboril M., et al., Mon. Not. R. Astron. Soc. 284, 685 (1997)].","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"e_1_3_1_22_2","unstructured":"S. Stahler personal communication."},{"key":"e_1_3_1_23_2","unstructured":"F. D'Antona and"},{"key":"e_1_3_1_23_3","doi-asserted-by":"crossref","first-page":"467","DOI":"10.1086\/191867","volume":"90","author":"Mazzitelli I.","year":"1994","unstructured":"Mazzitelli I., Astrophys. J. Suppl. Ser. 90, 467 (1994).","journal-title":"Astrophys. J. Suppl. Ser."},{"key":"e_1_3_1_24_2","unstructured":"After this paper was accepted we became aware of the Hipparcos satellite parallax measurements of the distances to TW Hya 56 \u00b1 7 pc (R. Wichmann U. Bastian J. Krautter I. Jankovics S. M. Rucinski paper presented at the Hipparcos Venice '97 Symposium Venice Italy 13 to 16 May 1997) and HD98800 47 \u00b1 6 pc [from data available in the Hipparcos catalog () as of 17 June 1997]. These distances agree with our estimates (Table 1) and confirm our earlier conclusions."},{"key":"e_1_3_1_25_2","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1086\/192116","volume":"96","author":"Magnani L.","year":"1995","unstructured":"Magnani L., Caillault J.-P., Buchalter A., Beichmann C. A., Astrophys. J. Suppl. Ser. 96, 1 (1995).","journal-title":"Astrophys. J. Suppl. Ser."},{"key":"e_1_3_1_26_2","unstructured":"A. Ghez personal communication."},{"key":"e_1_3_1_27_2","unstructured":"We used receivers A2 B3i and C2 for measurements in the 230- 345- and 460-GHz atmospheric windows. Beam sizes at these frequencies were 20\" 14\" and 11\" respectively. The back end was the JCMT Digital Autocorrelation Spectrometer (DAS); we used a DAS bandwidth of 250 MHz which yields a spectral resolution of 189 kHz. We obtained data by switching the secondary mirror at 1 Hz between the position of TW Hya and a nearby reference position located 60\" away (40\" away for observations at 460 GHz). Typical total integration times were on the order of 1 hour. To convert from antenna temperatures to brightness temperatures we applied aperture efficiencies of 0.57 0.49 and 0.31 for observations in the 230- 345- and 460-GHz windows respectively."},{"key":"e_1_3_1_28_2","unstructured":"For the calculation of circumstellar molecular mass we assumed a gas phase CO abundance typical of interstellar molecular clouds (CO\/H 2 \u223c 10 \u22124 by number). If CO were severely depleted then we would underestimate the circumstellar H 2 mass. Temperatures \u227350 K are too high for substantial CO ice mantles to form on dust grains however so any missing CO likely is trapped in small planetesimals or in planets rather than frozen out into grains. The measured line ratios likewise preclude a large mass of \u201cmissing\u201d gas at low temperature. The inferred H 2 mass and emitting region radius suggest an average H 2 density \u223c5 \u00d7 10 7 cm \u22123 (approximating the emitting region as a cylinder of radius R CO and height 0.2 R CO ). Such a density is sufficient to excite the (3\u20132) and (4\u20133) transitions of HCN [R. Genzel in Millimeter and Submillimeter Astronomy R. D. Wolstencroft and W. B. Burton Eds. (Kluwer Norwell MA 1987) p. 223]."},{"key":"e_1_3_1_29_2","doi-asserted-by":"crossref","first-page":"280","DOI":"10.1086\/172131","volume":"402","author":"Beckwith S. V. W.","year":"1993","unstructured":"Beckwith S. V. W., Sargent A. I., Astrophys. J. 402, 280 (1993);","journal-title":"Astrophys. J."},{"key":"e_1_3_1_29_3","first-page":"L87","volume":"396","author":"Omodaka T.","year":"1992","unstructured":"Omodaka T., Kitamura Y., Kawazoe E., ibid. 396, L87 (1992).","journal-title":"ibid."},{"key":"e_1_3_1_30_2","doi-asserted-by":"crossref","first-page":"2321","DOI":"10.1086\/113937","volume":"90","author":"Rucinski S.","year":"1985","unstructured":"Rucinski S., Astron. J. 90, 2321 (1985).","journal-title":"Astron. J."},{"key":"e_1_3_1_31_2","first-page":"420","volume":"266","author":"Cox P.","year":"1992","unstructured":"Cox P., Omont A., Huggins P. J., Bachiller R., Forveille T., Astron. Astrophys. 266, 420 (1992);","journal-title":"Astron. Astrophys."},{"key":"e_1_3_1_31_3","unstructured":"; R. Bachiller T. Forveille P. J. Huggins P. Cox ibid. in press."},{"key":"e_1_3_1_32_2","doi-asserted-by":"publisher","DOI":"10.1086\/115385"},{"key":"e_1_3_1_33_2","doi-asserted-by":"crossref","first-page":"984","DOI":"10.1086\/177026","volume":"460","author":"Soderblom D. R.","year":"1996","unstructured":"Soderblom D. R., Henry T. J., Shetrone M. D., Jones B. F., Saar S. H., Astrophys. J. 460, 984 (1996).","journal-title":"Astrophys. J."},{"key":"e_1_3_1_34_2","first-page":"870","volume":"452","author":"Torres G.","year":"1995","unstructured":"Torres G., Stefanik R. P., Latham D. W., Mazeh T., ibid. 452, 870 (1995).","journal-title":"ibid."},{"key":"e_1_3_1_35_2","unstructured":"J. H. Kastner unpublished data."},{"key":"e_1_3_1_36_2","unstructured":"Our criteria for selecting stars in the range of spectral types K5 to M3 were published spectral type or stellar effective temperature or B\u2013V and V\u2013I colors (or both) (where we adopted ranges of B\u2013V between 1.1 and 1.6 and V\u2013I between 1.6 and 2.9)."},{"key":"e_1_3_1_37_2","unstructured":"The circumstellar molecular abundances for TW Hya were calculated assuming local thermodynamic equilibrium (LTE) optically thin emission and a uniform excitation temperature of T ex \u223c50 K. The last approximation can be valid only in the outermost portion of the emitting region. However because abundances were calculated relative to 13 CO and because the partition functions for all of these species (including 13 CO) scale more or less linearly with T ex over a wide range of gas temperatures (from \u223c10 K to over 300 K) the relative abundance estimates do not depend on the detailed radial distribution of T ex ."},{"key":"e_1_3_1_38_2","unstructured":"A. Dutrey S. Guilloteau M. Guelin Astron. Astrophys. Lett. 317 L55 (1997)."},{"key":"e_1_3_1_39_2","doi-asserted-by":"crossref","first-page":"401","DOI":"10.1086\/176150","volume":"450","author":"Fleming T. A.","year":"1995","unstructured":"Fleming T. A., Schmitt J. H. M. M., Giampapa M. A., Astrophys. J. 450, 401 (1995).","journal-title":"Astrophys. J."},{"key":"e_1_3_1_40_2","unstructured":"J.H.K.'s research at the Massachusetts Institute of Technology was supported in part by the Advanced X-ray Astrophysics Facility Science Center as part of Smithsonian Astrophysical Observatory contract SVI-61010 under the NASA Marshall Space Flight Center. Additional support was provided by NASA Origins of Solar Systems program grants (NAGW 5020 and 5145) to Vanderbilt University (D.A.W.) and by an NSF grant (AST-9417158) to the University of California Los Angeles (B.Z.). We thank E. J. Gaidos (M.I.T.) and H. Matthews (Joint Astronomy Centre) who assisted in the molecular line observations and T. S. Simon who provided data before publication. Our molecular abundance calculations made use of the Jet Propulsion Laboratory's online Molecular Spectroscopy database."}],"container-title":["Science"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.science.org\/doi\/pdf\/10.1126\/science.277.5322.67","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2024,1,13]],"date-time":"2024-01-13T04:33:16Z","timestamp":1705120396000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.science.org\/doi\/10.1126\/science.277.5322.67"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[1997,7,4]]},"references-count":46,"journal-issue":{"issue":"5322","published-print":{"date-parts":[[1997,7,4]]}},"alternative-id":["10.1126\/science.277.5322.67"],"URL":"https:\/\/doi.org\/10.1126\/science.277.5322.67","relation":{},"ISSN":["0036-8075","1095-9203"],"issn-type":[{"value":"0036-8075","type":"print"},{"value":"1095-9203","type":"electronic"}],"subject":[],"published":{"date-parts":[[1997,7,4]]}}}