Dr. Serge Correia

Current Research Projects

S. Correia. Current research projects Frequency and properties of multiple T Tauri stars

High-order multiplicity in T Tauri stars : frequency and physical properties

We conducted a systematic search for high-order multiplicity among wide visual Pre-Main Se-quence (PMS) binaries using the Adaptive Optics system NACO at VLT. The sample, comprising58 PMS wide binaries from various star-forming regions, includes 52 T Tauri systems mostly withK- and M-type primaries. Of these 52 systems, 7 are found to be triple (2 new) and 7 quadruple(1 new). These results allow us to derive meaningful high-order multiplicity statistics amongyoung stars and ultimately place strong constraints on models of multiple star formation. Themain conclusions of this study are briefly summarized in the following, for details see Correia etal. (2006).

Considering a restricted sample composed of systems at a distance of 140-190 pc, the de-gree of high-order multiplicity (number of triples and quadruples divided by the number ofsystems) is 26.8±8.1% (30 binaries, 5 triples, 6 quadruples) in the separation range 10/14 AU- 1700/2300 AU (Fig. 1). This degree of multiplicity is twice as high in Taurus as in Ophiuchusand Chamaeleon, for which the same number of sources are present in our sample. The observedfrequency agrees with results from previous multiplicity surveys within the uncertainties, althougha significant overabundance of quadruple systems compared to triple systems is apparent in oursurvey with respect to all other surveys, the latter being however strongly hampered individu-ally by small number statistics. Tentatively including the spectroscopic pairs in our restrictedsample and comparing the multiplicity fractions to those measured for solar-type main-sequencestars in the solar neighborhood leads to the conclusion that both the ratio of triples to binariesand the ratio of quadruples to triples seems to be in excess among young stars in comparisonwith the Main Sequence. Finally, most of the current numerical simulations of multiple starformation, and especially smoothed particle hydrodynamic simulations, over-predict the fractionof high-order multiplicity when compared to our results.

Our study constitutes the most complete census of high-order young multiples to date and hasshown that there is a possible overabundance of these multiple systems compared to binariesat young ages. If it is confirmed that many known binaries are in fact hierachical multiples,the presence of additional components would have important implications for the dissipation ofdisks in multiple systems, including the premature exhaustion of disk material around the closestpairs, and therefore the probability of forming planets in those systems.

High-order multiples are particularly well-suited to constrain the models of star formation, e.g.through coevality of their components when placed on the HR diagram and compared with PMSevolutionary tracks. Theories of disk evolution and dissipation can also greatly benefit from thestudy of such systems, and ultimately also theories of planet formation in multiple systems. Tothis end, we have undertaken a systematic study of the physical properties of the single com-ponent of high-order multiple TTauri systems using spatially-resolved adaptive optics near-IRspectroscopy with VLT/NACO, VLT/ISAAC and GEMINI/NIFS. The spatially-resolved R∼1500K-band spectra of 4 pairs in triple/quadruple systems observed with VLT/NACO are presentedin Fig. 2 and Figure 3 shows the GEMINI/Altair/NIFS R∼5000 spectra in the J-, H-, and K-bandof 3 sub-arcsec pairs of hierarchical triple/quadruple systems.

S. Correia, H. Zinnecker, Th. Ratzka & M.F. Sterzik, A VLT/NACO Survey for Triple andQuadruple Systems among Visual Pre-Main Sequence Binaries, 2006, A&A 459, 909.

S. Correia, T. Beck, C., Aspin & H. Zinnecker, in preparation.

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S. Correia. Current research projects Frequency and properties of multiple T Tauri stars

Figure 1: Overview of confirmed and candidate triple (upper panel) and quadruple (lower panel)systems detected in our VLT/NACO survey (Correia et al. 2006). Most of the high-ordermultiple system candidates are probably physical systems, based on both statistical and color-color magnitude diagrams grounds. North is up, east is left.

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S. Correia. Current research projects Frequency and properties of multiple T Tauri stars

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Figure 2: VLT/NACO R∼1500 K-band spectra of pairs in triple/quadruple systems (black line)together with spectral templates (red line). The derived spectral types, visual extinctions andK-band excess are reported. Accretion activity is readily detected in LkHα 263 A and CGHα 6 Bfrom Brγ emission with EW=-3.8 A and -0.8 A respectively (Correia et al., in prep.).

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Figure 3: GEMINI/Altair/NIFS R∼5000 J-, H- and K-band spectra of close pairs in hierarchicaltriple/quadruple T Tauri systems. These are UZTauBC, GG TauAaAb and LkHα 263 AB withseparations 0.′′37 (∼ 50 AU), 0.′′25 (∼ 35 AU) and 0.′′41 (∼ 113 AU), respectively. Also plottedis the best-fit spectral template (red line) and the derived spectral type, visual extinction andveiling in J, H and K (Correia et al., in prep.).

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S. Correia. Current research projects Disks in ONC binaries

Disks around young binaries in the Orion Nebula Cluster

In addition to the frequency of multiple systems (and its dependence on age, environment, andprimary mass) and their properties (i.e. separation and mass ratio distributions), the frequencyand properties of disks in young multiple systems are able to give insights in the multiple forma-tion process (e.g. Clarke 2001, IAU Symp. 200). In addition, the study of the evolution of disksin binary systems can tell us whether or not these systems are likely sites of planet formation incomparison to the case of single stars.

The occurrence of disks among T Tauri binaries has been the subject of several previous studies,mainly focused on Tau-Aur. Through the use of various disk and/or accretion diagnostics, thesestudies suggested that the presence of an optically thick dusty disk around each component wascorrelated, i.e. that CTTs (Classical T-Tauri stars) primaries had preferentially CTTs secondaries(Prato & Simon, 1997, ApJ, 474, 455., Duchene et al.1999, A&A, 351, 954, White & Ghez 2001,ApJ, 556, 265, Prato & Monin, 2001, IAU Symp. 200, Hartigan & Kenyon 2003, ApJ, 583,334). However, a statistically more robust investigation, based on a compilation of ∼60 systemsfrom several star-forming regions, has been recently conducted which contradicts this previousresult. It shows that mixed-pairs, i.e. pairs formed by a CTTs and a Weak-lined T-Tauri star(WTTs), are in fact common among both close and wide binaries (Monin et al., 2007, PPV). Afraction of mixed-pairs relative to the sum of CTTs pairs (CC) and WTTs pairs (WW) as highas (CW+WC):(CC+WW)∼ 40 % was found, in contrast with the 15-25 % found in the aboveearlier studies.

To explain this difference, it has been suggested that the incidence of mixed systems variesbetween different star forming regions, depending on, but probably not only, the mean age ofstars in that region, and that in particular a young cloud like Taurus presents a small fractionof mixed-pairs (Monin et al. 2007). Further confirmation of a regional dependence of diskproperties among multiple systems requires to obtain spatially resolved near-infrared spectroscopyand photometry of a statistically significant sample in other clouds than Taurus, and especiallyin OB associations where most of the stars in the Galaxy are thought to originate.

Reipurth et al. (2007, AJ, 134, 2272) have just completed the hitherto largest survey for youngbinaries in the Orion Nebula Cluster using the Hubble Space Telescope, and have identified 78young binary systems. Using this unique database, we are seeking to understand disk evolutionin newborn binary systems and conditions for planet formation in young binaries. The aim isto obtain an assessment of the fraction of primaries and secondaries with and without disksignatures in the Orion region bordering the Trapezium, and compare this fraction with that ofother star-forming environments like the Taurus-Auriga T association and the young embeddedcluster ρOph.

Specifically, we wish to measure the near-infrared excess from individual components of a largesample of multiple systems in the ONC with angular separations ranging from 0.′′1 to 1.′′5 usingVLT/NACO and GEMINI/NIFS-NIRI. We have demonstrated the feasibility of our approach witha pilot study including 6 binaries carried out with GEMINI/NIFS-NIRI (Fig. 4) and we now wishto perform a full statistical analysis. Preliminary results of our study suggest that the frequencyof mixed-pairs is relatively large in the ONC (see Fig. 4). Such a trend need however to beconfirmed with a larger sample size.

S. Correia, B. Reipurth, C., Aspin, R. Kohler, Th. Ratzka & H. Zinnecker, in preparation.

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S. Correia. Current research projects Disks in ONC binaries

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Figure 4: Up : Six ONC binaries observed in Ks and L’ with GEMINI/NIRI in February 2008(2′′ × 2′′ FOV), and example of spatially-resolved GEMINI/NIFS R∼ 5000 K-band spectra, su-perimposed with best-fitted standard star spectra (M1/M3, Av=1.1/0). The absence of Brγemission suggests a pair of non-accreting T Tauri stars. Down : L’-band magnitude differenceas a function of K-band magnitude difference for each of the 6 pairs of the pilot sample. WhileONC 3, 58 and 69 likely have similar Ks-L’ color excess in each component, i.e. are CC or WWpairs, ONC 10, 15 and 31 are consistent with being mixed-pairs.

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S. Correia. Current research projects HH 212 protostellar outflow

Figure 5: Overview of the inner part of the protostellar outflow HH 212 in the 1-0 S(1) line ofH2 (left), and position of the slits on the first H2 knots NK1 (up) and SK1 (down).

H2 velocity structure of inner knots in the protostellar outflow HH 212

HH 212 is certainly one of the most remarkable protostellar outflows known to date (Zinnecker,McCaughrean & Rayner 1998, Nature, 394, 862). It is driven by a deeply embedded, low-mass protostar in its main accretion phase and is seen at infrared (H2 ro-vibrational lines) and(sub)millimeter wavelengths (e.g., CO rotational lines). The structure of its two lobes showspronounced symmetry about the driving source. Close to the source, the jet beams are markedby series of bright knots with sizes of about 1-2 arcseconds (400-800 AU) and bracketed furtherout by pairs of successively larger bow shocks.

High resolution (R∼50 000) long-slit echelle spectroscopy of the HH 212 jet bow shocks inthe 1-0 S(1) line of H2 at 2.12 µm was obtained at GEMINI with the near-infrared long-slitspectrograph PHOENIX. The velocity resolution of ∼6 km.s−1, spatial resolution of ∼0.′′45(∼200 AU at 450 pc) as well as several slit orientations allow us to obtain a more detailedpicture of the kinematics of this highly symmetrical outflow (Fig. 5). In particular, multiple slitorientated observations of the northern first knot NK1 clearly show a double-peaked velocityfield consistent with that of a radiative bow shock. In contrast, the velocity distribution of thesouthern first knot SK1 remains single-peaked suggesting a significantly smaller shock velocitycompared to NK1 (Fig. 6). Comparison with a semi-empirical model of bow shock emissionallows to constrain parameters such as the bow inclination to the line of sight, the bow shockand jet velocities for each flow. Although a few features are not reproduced by this model, itconfirms the presence of several dynamical and kinematical asymmetries between opposite side ofthe HH 212 bipolar jet. The position-velocity diagrams of both knots exhibit complex dynamicswhich does not exclude jet rotation although a clear signature of jet rotation is missing.

We were recently granted IRTF/TEXES observing time to investigate the kinematics of theseknots in the mid-IR H2 pure rotational 0-0 S(2) line at 12.3 µm.

S. Correia, H. Zinnecker, S. Ridgway & M. McCaughrean, A&A, in press (astro-ph/09080075).

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S. Correia. Current research projects HH 212 protostellar outflow

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Figure 6: Position-velocity diagrams of knots of the HH 212 outflow. First row : Northernknots NK4-NK2-NK1 with the slit parallel to the outflow axis. Second row : NK1 with theslit perpendicular to the outflow axis at four positions along the axis (see Fig. 5). Third row :Southern knots SK1-SK2-SK4 with the slit parallel to the outflow axis (left panel) and SK1 withthe slit perpendicular to the outflow axis (right panel). The values and (spatial-spectral) locusof the mean velocities are indicated. The spatial coordinates for the slits perpendicular to theoutflow are oriented positively towards East, i.e. opposite to that of Fig. 5.

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S. Correia. Current research projects R CrA circumstellar disk

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Figure 7: Simultaneous fit of the MIDI visibilities (left) and SED (right) of R CrA. The bestfit model shown is composed of the star (R=1.8 R⊙, Teff=7500 K), an accretion disk (power-law temperature profile index 0.55, T(1AU)=650 K, minimum radius 0.16 AU, maximum radius30 AU, inclination 30 deg.) and a ring representing the puffed-up inner rim (radius 0.12 AU,width 0.05 AU, temperature 1500 K, same inclination). We assumed Av=3mag and a distanceof 130 pc. Squares are measurements, while the continuous line is the best fit model. In theSED plot, the ISO spectrum is also shown (red line).

The disk around the Herbig Ae star R Corona Australis unveiled by VLTI/MIDI

R CrA is a young Herbig Ae star located at the center of the small ”Coronet” cluster at a distanceof 130 pc which exhibits several of the characteristics indicating the presence of a circumstellardisk around a young star, in particular a large IR-excess, a high degree (8%) of optical linearpolarization, and optical brightness variations of the UX Ori type.

We observed R CrA with MIDI at the VLTI and obtained 6 sets of spectrally dispersed visibilitiesin the spectral range 8-13 µm. A first comparison of the data with a simple geometrical modelof an inclined ring shows that the emitting region has a typical size of 6-10 AU, consistent withprevious MIDI observations of Herbig Ae stars with disks. The inclination is constrained to44 +8

−17 degrees, roughly perpendicular to the symmetry axis of a bipolar reflection nebula derivedfrom NIR imaging polarimetry. Using additional baselines with a larger range of position angles,we are able to derive a disk semi-major axis at position angle 59 +9

−8 degrees E of N, which is alsoperpendicular to the symmetry axis of the nebula.

Figure 7 shows our first attempt to model both spectral energy distribution (SED) and mid-IRvisibilities with a simple model composed of a star (L=10 L⊙), a bright ring at 25 R⊙ (∼0.1 AU)and a disk emitting locally as a blackbody, self-shadowed from 25 to 35 R⊙ and emitting from35 R⊙ to 30 AU. The model reproduce fairly well both SED and visibilities and in particular itpredicts a relatively flat temperature profile slope of ∼ 0.5 (i.e. T∝R−0.5), characteristic of apassive disk and reminiscent of flat spectrum T Tauri stars (e.g. HLTau). A detailed radiative-transfer modelling of the disk will be undertaken which will include the contribution from aninfalling envelope.

S. Correia, R. Kohler, G. Meeus, H. Zinnecker 2008, ESO Astrophysics Symposia, p. 175(astro-ph/0507064).

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S. Correia. Current research projects Nature of Infrared Companions

Figure 8: VLTI/MIDI spectrally-dispersed visibilities of Haro 6-10 N with 3 baselines (up) andGlass I with 2 baselines (down) overplotted with several best-fit models.

Constraining the geometry of warm circumstellar environment ofInfrared Companions with VLTI/MIDI

Infrared Companions (IRCs) are unusual, optically obscured, young stellar companions gravita-tionally bound to more or less typical T Tauri stars. As such, they promise to be the source ofinformation on either a particular phase in the development of young binary and multiple starsor on a particular mode of development. Despite more than a decade of investigations, thenature of IRCs is still a matter of debate. While the hypothesis that IRCs could be in an earlierevolutionary stage than their primaries implies that they are embedded in an optically thick(spherical) envelope, recent high spectral resolution near-infrared spectroscopy rather favors thescenario of IRCs being normal T Tauri stars seen through an almost edge-on disk.

We spatially resolved the inner warm (∼ 300 K) circumstellar regions around two IRCs in themid-infrared with VLTI/MIDI, permitting an additional observational test of the geometry ofmaterial closely surrounding IRCs (Figure 8). While additional baselines are required for Glass I,the data for Haro 6-10 N suggests that an elongated structure might be present within a fewAU around this IRC, which would favor the scenario of IRCs being young stars obscured behinda close to edge-on dusty disk. We were recently granted GEMINI/PHOENIX observing time toinvestigate the nature of Glass I in parallel through high-resolution NIR spectroscopy.

S. Correia, Th. Ratzka, G. Duchene, H. Zinnecker 2007, IAU Symposium 240, p. 213.

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S. Correia. Current research projects YSOs in Chamaeleon and Gum Nebula

Figure 9: Upper panel : L-band contours overimposed on K-band images of Chamaeleon YSOCed 110 IRS4 and Gum Nebulae YSOs CG30, Re4 and Re5. Lower panel : Preliminary resultsof a 3D radiative transfer modelling of the near-IR scattered-light emission showing 2000 AU ×

2000 AU (∼ 4.′′5 × 4.′′5) J,H,K composite of Re5 and two models with inclinations to the plane ofthe sky of 45◦ and 75◦, respectively. The model is a r=1000 AU spherical envelope of 1.5.10−4

M⊙ of typical interstellar dust grains mixture, with 2 parabolic-shape evacuated cavities anda central star of Teff=4000K. The density distribution has a power-law index of -2.0 and nocircumstellar disk was employed in a first stage, i.e. the central pixel is just masked out.

YSOs in Chamaeleon and Gum Nebula : new observations and first modeling

As a result of a survey of selected young stellar objects (YSOs) at high (0.′′4) spatial resolutionwith VLT/ISAAC, four YSOs from the Gum Nebula (HH 46/47, CG 30, Re 4, Re 5) and onefrom the Chamaeleon I dark cloud (Cederblad 110 IRS4) were studied in the near-IR : they allshow a complex structure, including in most cases bipolar blue and red scattering lobes likelydue to the illumination of outflow cavities by the central star(s), which is (are) completely orpartially hidden by a flattened circumstellar disk or envelope (see Zinnecker et al. 1999, A&A352, L73). For these five objects, we additionally obtained near-IR polarimetry, narrow-bandimaging at 2.12 µm and thermal-IR imaging (L- and M-band) with ISAAC, as well as 3mm maps(2.′′4 resolution) with ATCA, allowing a detailed disk/envelope modeling (Fig. 9).

S. Correia, B. Stecklum & H. Zinnecker, in preparation.

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S. Correia. Current research projects Extrasolar planets around white dwarfs

Search for extrasolar planets around white dwarfs : angular differential imaging withNICMOS/HST and NACO/VLT

I’m actively involved in a search for massive giant planets (6-12 Jupiter masses) in the nearinfrared around the known seven single white dwarfs (WDs) in the Hyades cluster at subarcsecseparations with NICMOS/HST and around young white dwarfs not associated with a clus-ter with VLT/NACO. The WDs have progenitor masses of about 3/4 solar masses, and massivegaseous giant planets should have formed in the massive circumstellar disks around these A stars,probably at orbital separations similar or slightly larger than that of Jupiter. Such planets wouldhave survived the post-Main-Sequence mass loss of the parent star and would have migratedoutward adiabatically to a distance of about 25 AU.

The observing strategy includes the subtraction of images taken at two different roll angles, i.e.angular differential imaging (Fig. 10). The analysis of the NICMOS data that I performed did notreveal any evidence for either brown dwarfs nor planetary mass companions with masses downto about 10 Jupiter masses around any of the seven white dwarfs of the Hyades for separationslarger than 0.′′5 (∼ 25 AU at the distance of 45 pc).

In 2004 two field white dwarfs were observed with NACO in the H-band using the same an-gular differential imaging technique. Close to one of our targets a second object was foundat 0.′′5 separation (Zinnecker et al. 2006, IAU Coll. 200). If both objects were physically re-lated, this would correspond to a separation of 25 AU at a distance of 50 pc. The measuredabsolute H magnitude of 13.7 mag at an estimated age of 0.5 Gyr yields a mass of 33 MJUP.However, a second epoch observation performed in August 2006 with SINFONI in order to ob-tain information about common proper motion and the near-infrared spectrum of the putativecompanion in the H- and K-band have shown that this source is a background giant star (Fig. 11).

H. Zinnecker, S. Correia, W. Brandner, S. Friedrich, M. McCaughrean, Search for extraso-lar planets around white dwarfs : direct imaging with NICMOS/HST and NACO/VLT, ”DirectImaging of Exoplanets : Science & Techniques”, IAU Colloquium No. 200, Claude Aime &Farrokh Vakili eds., Cambridge University Press, pp.19-24, 2006.

S. Friedrich, H. Zinnecker, W. Brandner, S. Correia, M. McCaughrean, A NICMOS DirectImaging Search for Giant Planets around the Single White Dwarfs in the Hyades, ”14th EuropeanWorkshop on White Dwarfs”, D. Koester, S. Moehler eds., ASP Conference Series, Vol. 334, p.431, 2005. Astro-ph/0501148.

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S. Correia. Current research projects Extrasolar planets around white dwarfs

Figure 10: Two upper rows : The NICMOS/HST observations of one of the Hyades white dwarfs(HZ7) at two different roll angles and the result of their subtractions in the F110W filter (firstrow) and the F160W filter (second row). The field of view is 2.′′75 × 2.′′75. Maximum cuts areat 2% of the peak intensity for the observations, and ± 0.5% for the subtracted frames (linearstretch). Two bottom rows : same as above but with an artificial planetary mass companionadded at 0.′′4 (∼ 20 AU) separation and with 6mag brightness difference.

Figure 11: Left panel : First-epoch (2004.4) H-band VLT/NACO image of the field white dwarfWD1847-223J with its candidate companion at ∼ 0.′′5 SW and a faint source at ∼ 0.′′9 NW. Rightpanel : Second-epoch (2006.7) H+K-band VLT/SINFONI image showing an apparent motionof the white dwarf relative to the two sources which is consistent with its known proper-motion(110 mas yr−1 at PA=117 ◦, east of north, R.D. Scholz, private communication).

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S. Correia. Current research projects The proto-star cluster S255-IR

Figure 12: Left : DSS1 E red plate of the field around the S255-IR embedded cluster with thetwo almost diametrically opposite HII regions S255 (east) and S257 (west), powered by the B0stars ALS 19 and HD 253327 respectively. Right : H-band VLT/NACO image of HD 253327resolved as a visual ∼ 0.′′13 binary.

The embedded cluster S255-IR : massive star ejections and stellar content

We have started to carry out a comprehensive high-angular resolution study of the embeddedhigh-mass star cluster S255-IR (diameter ≈ 2 arcmin, i.e. 1.5 pc at a distance of 2.5 kpc) usingVLT/ISAAC and VLT/NACO, to characterise its high- to low-mass stellar content, the frequencyof infrared excess (i.e. disks) around stars of various masses, the occurrence of protostellar jets,and its stellar density in the core.

In addition, this very young (1 Myr) star cluster in the making may have ejected from its coretwo or even three B0 stars beyond the cluster periphery where they are now observed as isolatedvisible massive stars exciting low-density HII regions (S254, S255, and S257, see Fig. 12, leftpanel). If the ejection hypothesis, suggested by dynamical models of young star clusters, iscorrect, at least one of the massive stars must be a close (sub-arcsec) visual binary system, ahypothesis that can be tested with NACO observations. Our observations revealed that oneof these stars (HD 253327) is indeed a visual ∼ 0.′′13 binary (i.e. ∼ 300 AU at the distance of2.5 kpc) which therefore gives support to the ejection hypothesis (Fig. 12, right panel).

Fig. 13 shows the cluster in unprecedented details and it will allow us to conduct the analysisaforementioned.

S. Correia, H. Zinnecker, M. McCaughrean, G. Meeus & B. Stecklum, in preparation.

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S. Correia. Current research projects The proto-star cluster S255-IR

Figure 13: Up : VLT/ISAAC JHK color-composite image of the embedded cluster S255-IR andits immediate surrounding (∼ 2.6×2.6 arcmin). Down : VLT/NACO JKL’ color-composite imageof the central region of the cluster (∼ 27×27 arcsec, FOV delineated above).

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S. Correia. Current research projects Determination of dynamical masses of PMS stars

Determination of dynamical masses of pre-Main Sequence stars

The mass of a star is one of the most fundamental parameters that drives its evolution. Althoughthis quantity can be accurately estimated for main sequence stars by placing them in a HRdiagram, this is much more complicated in the case of low-mass pre-main sequence (PMS)stars, largely because of the significant uncertainties related to the current pre-main sequenceevolutionary models (see Hillenbrand & White 2004, Mathieu et al. 2007, Zinnecker, 2007), andespecially at the lowest masses and for the youngest ages (Baraffe et al. 2002). It is thereforeespecially important to obtain model-independent mass estimates of a relatively large sample ofT Tauri stars, so as to discriminate among different theoretical models as well as to provide anaccurate mass calibration of these models for masses M

∼< 2.0 M⊙.

Most if not all star formation studies use the masses and ages inferred by a particular set of PMSevolutionary tracks and isochrones which differ primarily in their choice of atmosphere/opacities,the equation of state, and their treatment of convection. The validity of such models is thereforecritical in order e.g. to derive the stellar mass distribution and the star formation history in youngclusters or associations. Theoretical PMS mass and age estimates have thus to be comparedwith those that can be derived empirically (e.g. Palla & Stahler 2001). Unfortunately, veryfew PMS stars have provided detailed observational tests of these models until now, in largepart because of the lack of accurate dynamical mass determinations. The number of PMS starshaving measured masses to date amounts to 19, of which only 10 have uncertainties < 10%(Hillenbrand & White 2004, Mathieu et al. 2007). In particular, there are no PMS dynamicalmass constraints at masses of less than 0.3 M⊙.

A powerful method to derive accurate dynamical masses of PMS stars is to combine the radialvelocity orbital solution with the visual (astrometric) orbital solution of double-lined spectroscopicbinaries. With such observations, near-infrared long baseline interferometry at facilities like VLTIand Keck hold great promise to improve the mass determinations of young low-mass stars to< 10 % accuracy for a wide range of masses and ages, allowing a proper calibration of PMSevolutionary models (Correia et al. 2003, Zinnecker & Correia 2004, Boden et al. 2005, Schaeferet al. 2008).

As a starting point of our project, I proposed to spatially resolve the low-mass pre-main sequencetriple system Cru-3 composed of an optical double-lined spectroscopic pair and a third unseencomponent and to obtain the first points of their orbits 1. This will allow us, by combination withtheir spectroscopic orbital solutions and additional AMBER time, to derive the dynamical massesof the individual components with an accuracy of a few percent and eventually to constrain thecurrent pre-main sequence evolutionary models. Additionally, the determination of the relativeorientation of inner and outer orbits (coplanarity or not) will place constraints on the physicalprocesses involved at the earliest stages of multiple star formation.

Our test run on March 24, 2008 was successful despite non-ideal weather conditions (seeing∼> 1′′)

and the current calibration/data reduction issues of AMBER in low-resolution mode. The anal-ysis of the data shows that the derived astrometric measurements are consistent with expectedvalues, and that the third compoment is detected (Fig. 14).

S. Correia, H. Zinnecker, C. Melo, T. Ratzka, M. Scholler & M. Sterzik, in preparation.

1Cru - 3, associated with the lower Centaurus-Crux group at ∼ 110 pc, is a hierarchical triple PMS systemcomposed of a double-lined weak-line T Tauri spectroscopic binary (Cru-3 AB), with an orbital period of 58.28 dand almost identical component spectral types of K4 and K5 (hence solar masses), and a third unseen component(Cru-3 C) on a ∼ 4.5 yr orbit and a mass of ∼ 0.3-0.4 M⊙(Alcala et al. 2002).

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S. Correia. Current research projects Determination of dynamical masses of PMS stars

Figure 14: VLTI/AMBER low resolution observation of the PMS triple system Cru-3. Squaredvisibility amplitudes and closure phases for a baseline triplet together with the besfit binary (upperpanel) and triple model (lower panel). The triple model yields a better fit of both visibilities andclosure phases.

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