We present new photometric and spectroscopic data for the M-type members of
the TW Hya association with the aim of a comprehensive study of accretion,
disks and magnetic activity at the critical age of ~10 Myr where circumstellar
matter disappears.Comment: 4 pages, to appear in Proceedings IAU Symposium No. 314, Young Stars
  & Planets Near the Sun, 201

Alcala, J. M.

Frasca, A.

Stelzer, B.

English

arXiv

AbstractWe present new photometric and spectroscopic data for the M-type members of the TW Hya association with the aim of a comprehensive study of accretion, disks and magnetic activity at the critical age of ~ 10 Myr where circumstellar matter disappears.</jats:p

B. Stelzer

A. Frasca

J.M. Alcalà

Crossref

Proceedings of the International Astronomical Union

Accretion, Disks, and Magnetic Activity in the TW Hya Association

We present new photometric and spectroscopic data for the M-type members of the TW Hya association with the aim of a comprehensive study of accretion, disks and magnetic activity at the critical age of ~ 10 Myr where circumstellar matter disappears

STELZER, BEATE

FRASCA, Antonio

ALCALA', JUAN MANUEL

OA@INAF - Istituto Nazionale di Astrofisica

2016Publication Year2020-04-29T14:45:06ZAcceptance in OA@INAFAccretion, Disks, and Magnetic Activity in the TW Hya AssociationTitleSTELZER, BEATE; FRASCA, Antonio; ALCALA', JUAN MANUELAuthors10.1017/S174392131500633XDOIhttp://hdl.handle.net/20.500.12386/24335HandlePROCEEDINGS OF THE INTERNATIONAL ASTRONOMICAL UNIONSeriesvol. 10, S314NumberarXiv:1509.00170v1  [astro-ph.SR]  1 Sep 2015Young stars & Planets Near the SunProceedings IAU Symposium No. 314, 2015J.H. Kastner, B.Stelzer S.A. Metchev, eds.c© 2015 International Astronomical UnionDOI: 00.0000/X000000000000000XAccretion, disks, and magnetic activity inthe TWHya associationB. Stelzer1, A. Frasca2 and J.M. Alcalà31 INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italyemail: stelzer@astropa.inaf.it2 INAF - Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy3 INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello, 16, 80131 Napoli, ItalyAbstract. We present new photometric and spectroscopic data for the M-type members of theTWHya association with the aim of a comprehensive study of accretion, disks and magneticactivity at the critical age of ∼ 10Myr where circumstellar matter disappears.Keywords. stars: low-mass, brown dwarfs, stars: chromospheres, accretion, accretion disks1. IntroductionAccretion, outflows and magnetic activity are key features driving young stellar evolu-tion. All these phenomena are linked in a complex feedback mechanism where the activityfrom the stellar chromosphere and corona influences the evolution of disks and planet for-mation while mass accretion from the disk to the star in turn may act as a heating agenton the structure of the outer stellar atmosphere. The details of this mechanism, how itdepends on stellar mass, and how it evolves with time is, thus, of utmost importance foran understanding of pre-main sequence evolution.At a distance of ∼ 50pc the TW Hya association (TWA) represents one of the mosteasily accessible laboratories for low-mass star formation. Moreover, its age of ∼ 8Myrplaces the TWA at a crucial evolutionary phase where disks dissipate and the accre-tion/outflow process comes to a halt.We are performing a broad-band (350-2500nm) mid-resolution spectroscopic surveywith X-Shooter@VLT. These observations give access to a rich database of emission lineswhich probe accretion, outflows and magnetic activity. In addition, we have carried out aoptical/near-infrared (NIR) monitoring campaign of both stars in the wide binary systemTWA30 to study photometric variability related to their circumstellar environments.2. Chromospheric properties of TWA starsWe have analyzed X-Shooter/VLT spectra of 24 non-accreting pre-main sequence stars,so-called Class III sources, from three nearby star-forming regions (σOrionis, Lupus III,and the TWA). Of particular interest here are flux-flux relations between activity di-agnostics that probe different atmospheric layers (from the lower chromosphere to thecorona). We examine such relations for the first time for pre-main sequence stars. Formain-sequence late-type stars the fluxes of chromospheric emission lines follow power-law relations, with the exception of a subgroup of M-type stars which define a separate“active” branch (Martinez-Arnáiz et al. 2010). Fig. 1 (left) shows that all the Class IIIstars from our sample lie on this “active” branch, and – spanning a range of massesthroughout the full M spectral sequence – they extend it to lower fluxes. We conclude12 B.Stelzer, A.Frasca & J.M. Alcalà0 5 10 15 20n0.010.101.0010.00log (fHn/f Hβ)TWA accretorsTWA30 ATWA27 (P84)LupusLupus sub−luminousFigure 1. (Left) Flux-flux relation between chromospheric Hα and Ca IIK emission for Class IIIstars; upper/lower black lines denote the “active”/“inactive” branches defined by the main-se-quence sample of Mart́ınez-Arnáiz et al. (2010) and the blue line is the best-fit to the pre-mainsequence sample. (Right) Balmer decrements for accretor candidates in TWA (colored curves)compared to those for accretors in Lupus (black curves; Antoniucci et al. 2015, in prep).that chromospheric flux-flux relations can serve as a tool for confirming the youth of low-mass stars. More detailed investigations covering ages between the ∼ 10Myr of TWA andthe > 1Gyr of main-sequence field stars are required to explore this new age diagnostic.Flux ratios between individual emission lines of the Class III sources show a smoothdependence on the effective temperature. Meunier & Delfosse (2009) found that in thesolar chromosphere the different behavior of Hα and Ca IIH&K is due to their link todifferent structures, so-called filaments and plages. However, the observed spread in Fig. 1and in the Hα/Ca IIK flux ratio for given Teff suggests that a third parameter, in additionto age and Teff , determines the chromospheric structure of late-type stars.Hα and Ca IIH&K are the best-studied emission lines in chromospheres of late-typestars. Our knowledge on the whole emission line spectrum is more elusive as a result oflimited spectral range and sensitivity of most available spectrographs. This gap can beclosed with wide-band spectroscopic studies, such as our comprehensive X-Shooter studyof Class III sources (see Stelzer et al. 2013).3. Accretion measurements for TWA membersWe have obtained X-Shooter spectra for 15 “potentially accreting” TWA members.Here we concentrate on measurements of mass accretion rates (Ṁacc). We consider as“potential accretors” all TWA stars previously known to be accreting and further TWAmembers with evidence of IR (disk) excess in published photometry or a Balmer jumpin the X-Shooter spectra. This sample excludes the Class III stars discussed in Sect. 2.Two methods are used to measure mass accretion rates from the X-Shooter spectra.First, we determine the Balmer jump – if any – by modelling the UVB spectrum with acombination of a non-accreting Class III template and a hydrogen slab model representingAccretion, disks, and magnetic activity in the TWHya association 3emission from the accretion column as described by Manara et al. (2013). This yields theaccretion luminosity, Lacc. Secondly, we use the empirical calibrations between fluxes ofindividual emission lines and the Lacc values measured with the slab modelling derivedby Alcalá et al. (2014) to obtain Ṁacc from observed line fluxes.We compare our preliminary results for Ṁacc as a function of stellar mass (M∗) to theanalogous study for the Lupus star forming region by Alcalá et al. (2014), and find thatthe TWA stars display smaller Ṁacc at given mass. However, the slope in the M∗− Ṁaccdiagram is the same as for Lupus. This indicates that the TWA disks represent an evolvedversion of the same type of disks that are present in Lupus.In Fig. 1 (right) we compare the Balmer decrements of the TWA accretor sample tothose of the accretors in Lupus (see Antoniucci et al. 2015, in prep.). The TWA starstend to have slightly steeper decrements than the younger stars in Lupus. The physicalconditions determining the shape of the decrements will be investigated by comparisonto decrements predicted from the radiative transfer models by Kwan & Fischer (2011).4. Multiband photometric monitoring of the TWA30 binaryTWA30A and 30B are nearly equal-mass components of a wide binary (spectral typesM5 and M4; 80′′ separation) discovered and confirmed as kinematic TWA members byLooper et al. (2010a) and Looper et al. (2010b). They are among the nearest (∼ 50 pc)stars with signatures of disks, mass accretion and outflow activity. The disks of both starsare likely seen (nearly) edge-on: (i) The forbidden emission lines (FELs) have only smallradial velocity shifts indicating that their jets are oriented in the plane of the sky; (ii)the FELs are much stronger than the accretion signatures suggesting that the accretioncolumns are partially hidden by parts of the circumstellar disk.In the discovery papers, Looper et al. (2010a) and Looper et al. (2010b) showed withmulti-epoch low-cadence spectroscopy that both stars have strongly variable NIR colors.To constrain the time-scales of the variability over its full optical/NIR spectral energydistribution we have carried out a photometric monitoring campaign with the robotic60 cm telescope REM on La Silla operated/owned by the Italian Istituto Nazionale diAstrofisica (INAF). The REM is equipped with a NIR camera, REMIR, and an opticalcamera, ROSS 2, fed by the same telescope and providing simultaneous photometry inseven filters, g’r’i’z’JHK’. The REM lightcurves of TWA30A and 30B are shown in Fig. 2.TWA30A shows quasi-periodic dimmings throughout the monitoring. The lightcurvesobserved in the seven filters are highly correlated and the depth increases systematicallyfor bluer bands. The NIR colors show that the data points are roughly aligned alongthe reddening vector. However, during the whole 3-month monitoring campaign the ex-tinction did not reach the high (∼ 10mag) values inferred from previous spectroscopy.Similar lightcurves have been observed from 9 stars in the NGC2264 star forming regionbased on a long CoRoT and Spitzer monitoring campaign (Stauffer et al. 2015). The oc-currence of such periodic dimmings can be interpreted as dust structures levitated abovethe disk and temporarily occulting the star light as they pass in front of the line of sight.TWA30B was for most of the time during our REMmonitoring observed in a faint statewith occasional “bursts” of several magnitudes. Contrary to TWA30A, the brightnessvariations of TWA30B are much stronger in the NIR than at optical bands. The “bursts”observed in the REM lightcurve are likely responsible for the NIR excess seen in theprevious spectroscopic data, which was modelled by Looper et al. (2010b) as cool (∼700K) blackbody emission. A physical interpretation for this excess NIR emission couldbe starlight reprocessed from a small portion of the disk that rotates in and out of theline-of-sight. For TWA30B the NIR photometry has a much flatter slope in the J−H vs4 B.Stelzer, A.Frasca & J.M. AlcalàFigure 2. REM multi-band optical/NIR lightcurves of TWA30A (left) and TWA30B (right).H −Ks diagram than TWA30A, and NIR colors measured for the “bursts” seen duringour REM monitoring are different from those inferred from previous spectrophotometryby Looper et al. (2010b).ReferencesAlcalá, J.M., Natta, A., Manara, C.F., et al., 2014, A&A, 561A, 2Kwan, J. & Fischer, W., 2011, MNRAS, 411, 2383Looper, D.L., Mohanty, S., Bochanski, J.J., Burgasser, A.J., et al., 2010, ApJL, 714, 45Looper, D.L., Bochanski, J.J., Burgasser, A.J., Mohanty, S., et al., 2010, ApJS, 140, 1486Manara, C.F., Beccari, G., Da Rio, N., et al., A&A, 558A, 114Mart́ınez-Arnáiz, R., López-Santiago, J., Crespo-Chacón, I. & Montes, D., 2010, MNRAS, 414,2629Meunier, N. & Delfosse, X., 2009, A&A 501, 1103Stauffer, J., Cody, A.M., McGinnis, P., et al., 2015, AJ, 149, 130Stelzer, B., Frasca, A., Alcalà, J.M., et al., 2013, A&A, 558A, 141DiscussionRodriguez: Did you find periodicities in the REM lightcurves of TWA30 ?Stelzer: Yes, indeed. The dips of TWA30A are quasi-periodic with P ∼ 8 d. Assumingcorotation, the observed period implies that the dust is located at a distance of ∼ 0.2AUfrom the star (i.e. in the inner disk) and that its azimuthal position is roughly consistentover time. The variable depth of the dips indicates that the amount or optical depth ofthe occulting material is changing in time.

Accretion, disks, and magnetic activity in the TW Hya association

Accretion, disks, and magnetic activity in the TW Hya association

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