aa r X i v : . [ a s t r o - ph ] D ec Astron. Nachr. / AN , No. 88, 789 – 793 (2007) /
DOI please set DOI!
Results and Perspectives of Young Stellar Object long look programs
S. Sciortino ,⋆ INAF-Osservatorio Astronomico di Palermo Giuseppe S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, ItalyReceived 3 Sept 2007, accepted ?? ?? 2007Published online later
Key words
X-rays: stars – Stars: pre-main-sequence – Stars: flare – Stars: formationBoth
Chandra and
XMM-Newton have performed long look programs for studying the YSO physics. I will discuss re-cent results on the controversial issue of Class 0 YSO X-ray emission, the observational evidence of magnetic funnelsinterconnecting the YSO with its circumstellar disk and the Fe 6.4 keV fluorescent line emission and its origin. Whilerecent results of the XMM-Newton
DROXO program challenge the ”standard” interpretation of the Fe 6.4 line origin asdue to photoionized fluorescing disk material, the discovery of X-ray excited Ne 12.81 µ m line is a clear evidence of theinteraction between X-rays and disk material. Future long look observations with XMM-Newton are required to clarifythe X-ray effects on YSO disk. c (cid:13) X-ray emission likely traces, and is related to, magneticfields at work in the interaction region between the centralYoung Stellar Object (YSO) and its surrounding disk. Be-cause of their role, X-rays have started to be recognized asan important element for understanding star formation andearly evolution. Since the launch of
Chandra and
XMM-Newton
X-ray emission from YSOs has been the subjectof many studies focussed on nearby Star Forming Regions(SRFs), among those are notable few selected long look pro-grams. So far two of these programs have been performedwith
Chandra : i)
COUP ( C handra O rion U ltradeep P roject,PI: E. Feigelson, cf. Getman et al. 2005), a 850 ks longcontinuous observation of the Orion Nebula Cluster regionwhich has allowed us to study the X-ray properties of knownOrion YSOs as well as to discover and characterize the OrionYSO embedded population, and ii) a 450 ks long observa-tion of the young cluster N 1893 in the external side of theGalaxy. This latter program, led by G. Micela, aims to studythe IMF in the external region of the Galaxy where the en-vironmental conditions are different than in the vicinity ofthe Sun. I will not discuss anymore this latter program sinceI will concentrate on the role of X-rays on YSO physics andevolution. In this specific realm only one long look program,led by myself, has been performed with XMM-Newton, itis nicknamed DROXO ( D eep R ho O phiuchi X MM-Newton O bser-vation). It consists of a 500 ks long continuous ob-servation of the ρ Oph core F region (Sciortino et al. 2006).Thanks to XMM-Newton high throughput the
DROXO timeresolved spectroscopy is allowing us to study the X-ray emis- ⋆ Corresponding author: e-mail: [email protected] sion of the 1 Myr old ρ Oph YSOs and its impact on YSOphysics.Another extensive program devoted to the study of YSOphysics is
XEST ( X MM-Newton E xtended S urvey of T aurus,G¨udel et al. 2007) that has adopted an observational strategydifferent from a long look one. The program and its resultsare presented in this volume by M. G¨udel.In the current scenario of Class I-II YSOs (Hartmann1998) magnetically funneled accretion streams connect thecentral star with its circumstellar disk. In such a system X-rays could be emitted by the PMS star corona, by the funnelplasma that is shocked as it accretes on the star, by the fluo-rescing disk matter or by gas shocked in a jet. Some of therecent observations, with crucial contributions from COUP and
XEST , have shown evidence that all the above contribu-tions can indeed be present. However from an observationalpoint of view, we need a stronger and more compelling ev-idence, i.e. to find a way to distinguish, recover and studythe properties of those distinct contributions. On more gen-eral ground X-rays are likely to be crucial to understand thechemistry and evolution of proto-planetary disks. In the fol-lowing I will briefly discuss some of the current open issueson YSO physics.
After many years of search, the occurrence of X-ray emis-sion from Class 0 YSOs is still controversial. Either thisemission is weak or rare or it is hidden due to the conspicu-ous amount of intervening absorbing material. In fact, whileX-rays are quite penetrating – indeed the absorption at 2keV and at 2 µ m are similar (Reyter 1996)– Class 0 sourcescan be subject to extinction up to hundreds of magnitudespreventing the escape of any X-rays. One of the most (if c (cid:13)
90 S. Sciortino: Young Stellar Object long look programs
Fig. 1
Stacked ACIS image from events collected in theenergy interval ∆ E = 0.5 − ×
200 pixels centered on the position of the 6 knownClass 0 sources in the Serpens. The circular region in thecenter is 5”, corresponding to positional uncertainties ofthe known mm/submm sources. No X-ray excess has beenfound within this area indicating that the Serpens Class 0YSOs are unlikely to be X-ray sources with intensities justbelow the detection threshold (adapted from Giardino et al.2007a).not the most) stringent upper limit to the intrinsic X-rayluminosity of Class 0 has been obtained thanks to a 100ksec
Chandra observation toward the Serpens SFR (Gia-rdino et al. 2007a). By staking data taken at 6 known Class 0positions, i.e. by constructing a virtual ∼
600 ks long obser-vation, the Class 0 intrinsic X-ray luminosity has resulted tobe lower than 4 10 erg/s (assuming emission from an opti-cally thin isothermal plasma with kT = 2.3 keV seen throughan absorbing column with N H = 4 10 cm − ). However thebest upper limit so far obtained is still a dex higher than theX-ray luminosity of active Sun. Future deep observationsare needed to really advance our knowledge on this subjectthat could affect our understanding of star formation pro-cess. In fact with COUP we have discovered a deeply em-bedded population in Orion (Grosso et al. 2005), that hasbeen shown to locally dominate the ionization level withinthe given molecular cloud core (cf. Lorenzani et al. 2007,and Fig. 2). Still, as of today, we do not know when intenseX-ray emission from YSOs really develops likely affecting–for example by determining the effectiveness of ambipolardiffusion – the further evolution of star formation process.We do not know yet if this effect is just a small adjustment ofthe current interpretational scenario(s) or a major change isrequired if X-rays start acting at very early (Class 0) times. X-ray flares are a classic tool to derive physical parame-ters of an emitting region (cf. Reale 2007). In fact the use
Fig. 2
Two dimensional projection of a model computa-tion of the (color coded) ionization rate as function of posi-tion for the BN cloud core. Across the entire core the ion-ization rate is higher than predicted by cosmic rays (2 cdot − s − ). Around each of the embedded X-ray emittingYSOs develops a R¨ontgen sphere where the X-ray inducedionization rate is several orders of magnitude higher than the”background” level (courtesy of A. Lorenzani and F. Palla.).of dynamical information (decay time, etc.) allows derivingphysical characteristics of the flaring region. This is pos-sible because in order to have a flare with the typical de-cay phase the plasma must be confined (Reale, Bocchino& Peres 2002). As a results the behavior of flare light curve(and the related time resolved spectra) allows measuring thesize of flaring magnetic structure. In normal stars the ob-served flares are similar to solar ones, but sometimes muchstronger (up to 10 ), both in absolute terms and with respectto the star bolometric luminosity. In most cases the observedYSO flares fall in the same category, but there are a few no-table exceptions: in about 10 COUP (Favata et al. 2005a)and 2
DROXO (Flaccomio et al. 2007a) flares, the analysisresults in a size of the flaring region a least 3 times largerthan stellar radius and in few cases as long as 0.1 AU, i.e.the size of the star-disk separation. These long structureshave never been seen in more evolved normal stars. Suchlong structures, if anchored on the stellar surface, will suf-fer severe stability problem due to the centrifugal force –1-2 Myr YOSs are fast rotators (P rot = 1-8 days) with a diskcorotation radius of about 1-10 stellar radius – hence theywould be ripped open. A possible alternative scenario is onein which the loop connects the star with the disk at the coro-tation radius. This is compatible with the currently avail-able observational evidence. Such magnetic funnels havebeen predicted by magnetospheric accretion models (e.g.,Shu et al. 1997) and have been shown to occur in up-to-date MHD simulations of disk-star system (eg., Long et al.2007), but it is only thanks to the
COUP and
DROXO longlook observations that we have gained some observationalevidences of their existence. c (cid:13) stron. Nachr. / AN (2007) 791 Fig. 3 (Upper panels) Observed spectra (pluses) and best-fit models (solid steps) of the 7 COUP YSOs showing the Fefluorescent 6.4 keV line. The Fe 6.4 keV line Gaussian com-ponent is shown by dashed steps. The 6.4 and 6.7 keV lineposition are indicated by solid and broken arrows, respec-tively. Photon energy in keV is on the abscissa, while the or-dinate is the spectral intensity as counts s − keV − . (Lowerpanels) Residual to the fit in unit of χ values (adapted fromTusijmoto et al. 2005). The first detection of the Fe 6.4 keV fluorescent line in aYSO has been obtained with Chandra during an intense flareon YLW16A, a Class I YSO in the ρ Oph SFR (Imanishiet al. 2001). Thanks to COUP we have collected 134 OrionYSO good quality spectra that allows investigating the pres-ence of the ∼ α line. In 7 COUP sources the 6.4keV line (cf. Fig. 3) has been found (Tusijmoto et al. 2005)and the emission has been interpreted, following originalsuggestion of Imanishi et al. (2001), as due to the circum-stellar neutral disk matter illuminated by the X-rays emit-ted from the PMS star during the intense flares observed inall those seven sources. A Fe 6.4 keV fluorescent line hasalso been seen during a relatively short XMM-Newton ob-servation of the Class II YSO Elias 29 without any evidenceof concurrent flare emission (Favata et al. 2005b). In allthe above reports none or very limited time resolved spec-troscopy has been possible due either to the XMM-Newtontoo short observation or the Chandra limited collecting area.Very recently Czesla & Schimtt (2007) have reported the re-sults of time-resolved spectroscopy of V1489 Ori, one of the7 COUP sources with the Fe 6.4 eV line, showing that the K α line appears predominantly during the 20 ks rise phaseof a flare. Their initial calculation suggests that the photo-ionization alone cannot account for the observed intensityof the Fe K α lineThanks to DROXO it has been possible to perform, forthe first time, a detailed time-resolved study of the Fe 6.4keV fluorescent line emission of Elias 29 (Giardino et al.2007b). The line intensity is highly variable. It is absent atthe beginning of observation, then after a quite typical flare(a factor 8 in intensity with a 6 ksec decay time) it appearswith a conspicuous equivalent width, EW ∼
250 eV (cf.Fig. 4). Subsequently it continues to be present with EW ∼
150 eV for the remaining 300 ksec (i.e. for 4 days!) ofthe observation. Apart for the flare, the relatively soft X-rayspectra of Elias 29 remains essentially unchanged across theentire observation, with no obvious hardening of the spec-trum during the last 300 ksec of observation. This behaviorclearly challenges the ”standard” interpretation of the fluo-rescent emission being due to photo-ionizing X-ray photons(requiring an adequate flux of photons with E > DROXO results as well as on a recent time-resolved spectral analysisof the COUP data (Flaccomio 2007) I have not the space todiscuss here, I am convinced that the YSO emission of the6.4 keV fluorescent line is a more complex phenomena thatoriginally though and I am expecting soon further develop-ments on this matter. On a somewhat longer time scale thenext generation of imaging X-ray observatories (Simbol-X,XEUS, etc.) covering the bandpass up to ∼
60 keV will al-low us to directly probe the existence of a population ofnon-thermal electrons that is a key ingredient of the inter-pretational scenario proposed by Giardino and collaborators(2007b).Let me conclude by adding another piece of (contro-versial ?) evidence. The existence of an X-ray excited Ne12.81 µ m IR line has been predicted (Glassgold et al. 2007)and its detection has been reported in 4 YSOs (Pascucci etal. 2007). Using Spitzer archive spectra this line has beendetected also in 4 ρ Oph YSOs observed in X-rays, for 3of which we have
DROXO
EPIC spectra (Flaccomio et al.2007b). The X-ray brightest of them shows also the 6.4 keVFe fluorescent line. Fig. 5 shows a summary of the Pascucciet al. (2007) and of the new ρ Oph data together with themodel prediction. The ρ Oph Ne IR luminosities are morethan one dex higher than those of the somewhat older YSOsstudied by Pascucci et al (2007). As of today we have noexplanation to offer for this fact except to note the differentage between the two groups of YSOs. c (cid:13)
92 S. Sciortino: Young Stellar Object long look programs
Fig. 4
Spectra and spectral fit to the
DROXO data of Elias 29 before the flare (left) and after the flare (right). The spectraare very similar in overall shape, intensity, and resulting best fit model parameters. After the flare, however, a significantexcess of emission at 6.4 keV is present which is not visible in the data before the flare (adapted from Giardino et al.2007b).
Fig. 5
Scatter plot of 12.81 µ n IR line vs. X-ray lumi-nosity of ρ Oph YSOs. The data point from Pascucci et al.(2007) and the model prediction of Glassgold et al. (2007)are also shown.
There is growing evidence of interactions between X-raysand YSO circumstellar disks: big flares and the inferredlong magnetic structures, the Fe 6.4 keV fluorescent line,the X-ray excited Ne 12.81 µ m line, etc..Our understanding of the formation mechanism(s) of Fe6.4 keV line is still limited and controversial. DROXO time-resolved spectroscopy of the Fe line challenges the standardFe 6.4 keV formation scenario involving a direct interactionbetween the X-rays and disk material, but at the same timethe detection of Ne (IR) line requires such an interaction.More long look observations and time-resolved spec-troscopy are needed; 2-3 such XMM-Newton programs onnearby SFRs of, at least, 0.5 Msec each will serve this scope.
Acknowledgements.
DROXO is a XMM-Newton Large Program(PI: S. Sciortino) supported by ASI-INAF contract I/023/05/0. S. Sciortino acknowledges enlightening discussions with many col-leagues involved in the COUP and DROXO projects and their kindshare of results and figures in advance of final publication.
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