Reports

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The earliest phases in the lives of stars determine their future fate. For example, the production of chemical elements, which are used to trace the history of Galactic evolution, depends on the initial mass and metal-licity of young stars. Also, the angular momentum obtained during the birth process is vital for the further stellar life. Therefore, understanding the physical processes that occur during the earliest stages of stellar evo-lution is essential.

During the early evolutionary phases, following the star’s appear-ance on the birthline (1–3), the central temperatures and densities are not yet high enough to initiate significant nuclear burning, hence, the early star derives all of its energy (heat and radiative) from the release of grav-itational potential energy. The interior structure undergoes several changes as the temperature rises and the opacities, in general, decrease (1). Initially, the higher opacities and the outward transportation of the released gravitational energy force the star to be nearly fully convective. But, as the star heats up and the opacities drop, a radiative core develops. At this point the star leaves the Hayashi track (4) and follows the Heny-ey track (5), moving leftward to hotter surface temperatures in the evolu-tionary diagram. Eventually, the star’s central regions become hot enough for nuclear burning to occur. Models predict that, for intermedi-ate-mass stars (i.e., with 1.5 to 8 solar masses, M◉), just before the star reaches this nuclear burning stage, called the main-sequence (MS) phase,

there is a “bump” or brief period of nuclear burning where carbon is “burned” by the CNO cycle (a chain of hydrogen burning reactions catalyzed by the presence of C, N, and O) until C and N reach nuclear burning equilibri-um. As this happens, the star’s core briefly becomes convective and the star’s luminosity increases. Shortly thereafter, the star resumes its contrac-tion onto the zero-age main sequence (ZAMS), when nuclear burning of hy-drogen re-ignites and remains the dom-inant source of energy throughout most of the rest of the star’s life. The precise definition of the ZAMS is critical when comparing ages of young stars. Here we define the ZAMS as the time following the CN equilibrium burning bump, when the nuclear burning accounts for at least one percent of the total luminos-ity of the star.

For pre-MS stars with 1.5 to 4 M◉, the interior structure will become un-stable to acoustic oscillation modes at some point during their evolution along the Henyey track. These can be ob-served at the surface as periodic varia-tions in luminosity and temperature or as Doppler shifts of spectral lines. So, for at least some pre-MS stars, it is possible at some evolutionary stages to use asteroseismology to explore their structure and evolution. The pre-MS stars addressed in this study show co-herent self-excited low-radial-order pressure (p) modes (6) with periods from ~18 min up to 6 hours. Until 2008, only 36 of them were known (7); through 2011, the number increased significantly to more than 60 (8) using dedicated ground-based observations

and high-precision time-series photometry obtained with the Microvari-ability and Oscillations of STars (MOST) (9) and COnvection, ROtation et Transits planétaires (CoRoT) (10) satellites. Although the pulsations of pre-MS stars could be studied in relative detail, little information was available about the stars’ fundamental astrophysical properties, such as effective temperature (Teff), surface gravity (log g), luminosity, mass (M), projected rotational velocity (vsini) and metallicity ([Fe/H]), due to the lack of spectroscopic measurements for this type of objects.

Our sample comprises 34 stars with confirmed pre-MS evolutionary status, pulsational variability and accurate fundamental parameters de-termined from detailed spectroscopic analyses. We assessed the pre-MS status of the stars in our sample in two different ways: through their membership in open clusters known to be younger than 10 million years, which ensures that 1.5-4 M◉ stars have not reached the ZAMS yet; or through their identification as Herbig Ae stars (11), which are associated with star-forming regions.

The pulsational properties of the sample stars were determined from ground and space-based observations. For nineteen stars, high-precision space photometry from the MOST (9) and CoRoT (10) satellites was used to investigate the pulsations. For eight of those (i.e., HD 261737, HD 262014, HD 262209, HD 37357, NGC 2244 45, NGC 2244 183,

Echography of young stars reveals their evolution K. Zwintz,1* L. Fossati,2 T. Ryabchikova,3 D. Guenther,4 C. Aerts,1,5 T. G. Barnes,6 N. Themeßl,7 D. Lorenz,7 C. Cameron,8 R. Kuschnig,7 S. Pollack-Drs,7 E. Moravveji,1 A. Baglin,9 J. M. Matthews,10 A. F. J. Moffat,11 E. Poretti,12 M. Rainer,12 S. M. Rucinski,13 D. Sasselov,14 W. W. Weiss7 1Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium. 2Argelander Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany. 3Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia. 4Department of Astronomy and Physics, St. Mary’s University, Halifax, NS B3H 3C3, Canada. 5Department of Astrophysics, IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, Netherlands. 6The University of Texas at Austin, McDonald Observatory, 82 Mt. Locke Rd., McDonald Observatory, Texas 79734, USA. 7Universität Wien, Institut für Astrophysik, Türkenschanzstraße 17, 1180 Vienna, Austria. 8Department of Mathematics, Physics and Geology, Cape Breton University, 1250 Grand Lake Road, Sydney, Nova Scotia, Canada, B1P 6L2. 9LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195, Meudon, France. 10Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada. 11Départment de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada. 12INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy. 13Department of Astronomy and Astrophysics, University of Toronto, 50 St. George St., Toronto, ON M5S 3H4, Canada. 14Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.

*Corresponding author. E-mail: konstanze.zwintz@ster.kuleuven.be.

We demonstrate that a seismic analysis of stars in their earliest evolutionary phases is a powerful method to identify young stars and distinguish their evolutionary states. The early star that is born from the gravitational collapse of a molecular cloud reaches at some point sufficient temperature, mass and luminosity to be detected. Accretion stops and the pre-main sequence star that emerges is nearly fully convective and chemically homogeneous. It will continue to contract gravitationally until the density and temperature in the core are high enough to start nuclear burning of hydrogen. We show that there is a relationship for a sample of young stars between detected pulsation properties and their evolutionary status, illustrating the potential of asteroseismology for the early evolutionary phases.

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NGC 2244 271 and NGC 2244 399) the pulsational variability was dis-covered in the course of this study, while the seismic properties for the remaining eleven objects observed from space have been described in previous work (see Supplementary Material for all references). Addi-tionally, the pulsational properties for 15 stars were taken from ground-based observing campaigns reported in the literature (see Supplementary Material for all references).

For the determination of the fundamental parameters, we obtained high-resolution, high signal-to-noise ratio spectra for 27 objects of our sample to complement the variability analysis; for the other seven stars, fundamental parameters were available in the literature (Supplementary Material).

To put the stars’ positions in the Teff – log g plane in context to their relative evolutionary stage, we computed pre-MS evolutionary tracks with the Yale Stellar Evolution Code (YREC, 12) for solar metallicity (i.e., Z = 0.02) (Figs. 1 and 2). We find that our sample includes stars that gain their energy solely from gravitational contraction (i.e., stars that are on the Henyey track just before arriving on the ZAMS), stars in which incomplete nuclear burnings have started (i.e., stars that are brief-ly burning CN producing the short rise in the luminosity before the star settles onto the ZAMS as discussed above), and stars close to the ZAMS where hydrogen-burning in the cores has already started.

In general, the stellar lifetime strongly depends on the birth mass. In the case of pre-MS stars, this dependence is manifested by the Kelvin-Helmholtz contraction time scale (13) rather than the nuclear time scale that rules the evolution of MS stars. The 34 pre-MS stars of our sample have masses between 1.5 and 3.0 M◉. Although this mass range is rather narrow, the differences in stellar ages at the ZAMS are significant: a star with 3.0 M◉ needs only ~8 million years from the birthline (i.e., the point where the star emerges from the molecular cloud and mass accretion stops) until hydrogen is ignited in its core (i.e., the arrival on the ZAMS), while a star with 1.5 M◉ has a ZAMS age of ~33 million years. The duration of the star formation process is thus strongly dependent on the mass of the pre-MS star.

The evolution of stellar rotation rates during the pre-MS phases is poorly understood. As young stars contract during their transition from the birthline to the ZAMS, stellar evolution theory predicts an increase in their rotational velocities if angular momentum is conserved (14). The fraction of the projected rotational velocity (vsini) over the critical breakup velocity (vcrit) for the 34 pre-MS stars (Fig. 1) indeed indicates the occurrence of higher rotation rates at the times of the first nuclear burning compared to the one of stars in the earlier stages. We find that the majority of the pre-MS stars in our sample rotates at a low to moder-ate fraction of their critical breakup velocity (fig. S3), which points to angular momentum loss by, e.g., winds or star-disk coupling. Our results agree with recent results reported for Herbig Ae/Be stars in a similar mass range (15).

The acoustic cut-off frequency is the highest frequency of a pressure mode that can still be trapped inside the star and is therefore an im-portant seismic characteristic. Theoretical computations for pre-MS stars predict the highest values to occur close to the ZAMS (i.e., around 1000μHz or 86.4d−1) and the lowest values (i.e., around 100μHz or 8.6d−1) close to the birthline (16). For high-order pressure modes, the acoustic cut-off frequency scales with the frequency of maximum power, νmax (17). For comparison with our observational sample, we used the highest significant pressure-mode frequency, fmax, which is the measura-ble quantity in our data coming closest to the acoustic cut-off frequency computed from seismic models. Our observations indeed confirm the theoretical predictions: the coolest and least evolved stars have the low-est fmax values, and therefore pulsate with the longest periods (fig. S4). The hottest and most evolved stars have the highest fmax values and pul-sate with the shortest periods. A similar relation is found when we use νmax derived from our data instead of fmax. In the present sample fmax

ranges from 76.47 μHz (for the star HR 5999) to 970.37 μHz (for the star HD 261711).

Our sample consists of a mixture of different types of pre-MS ob-jects (cluster members and Herbig Ae field stars) that also have different formation histories. By taking a subsample of pulsating stars that formed from the same molecular cloud and had the same initial conditions, we reduced possible ambiguities in the interpretation. In the young open cluster NGC 2264 (18, 19) nine pulsating pre-MS members were discov-ered by multi-epoch observations conducted with the MOST and the CoRoT space telescopes. The pulsational variability of HD 261737, HD 262014 and HD 262209 is reported for the first time in this work (Sup-plementary Material), while the seismic properties for V 588 Mon, V 589 Mon, HD 261230, HD 261387, NGC 2264 104 and HD 261711 have been described in previous work (see Supplementary Material for all references). The nine pre-MS pulsators in NGC 2264 (marked as “a” through “i” in Figs. 2 and 3) have different masses and are found to be in different relative evolutionary phases. Their evolutionary tracks in con-junction with their pulsational properties (Figs. 2 and 3) illustrate that the least-evolved stars pulsate slower (V 589 Mon) than the objects that are already located closer to the ZAMS (HD 261711). This is evidence of sequential star formation, making the assignment of a single age to this and other young open clusters unjustified. In particular, we find that the duration of the star formation process in NGC 2264 has been ongo-ing for much longer than the cluster age of 1 to 5 Myr reported in the literature (e.g., 18, 19).

A clear relation between the seismic observables and evolutionary status for the pre-MS stars emerges, even though our sample is relatively heterogeneous due to both the mixture of field and cluster stars of vary-ing metallicity, as well as different precision for the available data sets. This relation is even clearer when we consider stars formed from the same birth environment, such as members of the young cluster NGC 2264. We have shown with this study that asteroseismology can discern different stages of evolution not only for dying intermediate-mass stars, as was recently found (20). Such observations can be similarly applied to unravel the star formation history of young stars and even deliver the relative ages of the various mass populations of pre-MS stars of young open clusters.

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Acknowledgments: Data obtained with the MOST satellite are available through

the MOST Public Data Archive (http://most.astro.ubc.ca//archive.html) and data from CoRoT in the CoRoT Archive (http://idoc-corot.ias.u-psud.fr/index.jsp). Spectroscopic data can be obtained at the ESO archive (http://archive.eso.org) and the CFHT archive (http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/cfht/). Data are also available from the public website http://www.ster.kuleuven.be/~konstanze/science/. We are grateful to the MOST and CoRoT teams for making this work possible. We would like to thank N. Piskunov for his advice on the best usage of the SME software. The research leading to these results has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007 – 2013) / ERC grant agreement No 227224 (PROSPERITY), from the Research Council of the KU Leuven under grant agreement GOA/2013/012, and from the Fund for Scientific Research of Flanders (FWO), Belgium, under grant agreement G.0B69.13. TR acknowledges partial financial support from the Presidium RAS Program “Nonstationary Phenomena in Objects of the Universe”. DBG, JM, AFJM and SMR acknowledge the funding support of the Natural Sciences and Engineering Research Council (NSERC) of Canada. RK and WWW are supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (P22691-N16) and by the Austrian Research Promotion Agency-ALR. EP and MR acknowledge financial support from the FP7 project “SPACEINN: Exploitation of Space Data for Innovative Helio- and Asteroseismology”. EM is beneficiary of a postdoctoral grant from the Belgian Federal Science Policy Office co-funded by the Marie Curie Actions FP7-PEOPLE-COFUND2008 n246540 MOBILITY GRANT from the European Commission. CC was supported by the Canadian Space Agency. EM is the beneficiary of a postdoctoral grant from the Belgian Federal Science Policy Office co-funded by the Marie Curie Actions FP7-PEOPLE-COFUND2008 n246540 MOBILITY GRANT from the European Commission. Spectroscopic data were obtained with the 2.7-m telescope at Mc Donald Observatory, Texas, US, the 3.6m-telescope at La Silla Observatory under the ESO Large Programme LP185.D-0056, the Canada France Hawaii Telescope (proposal number CFHT2012AC003), the Cerro Tololo Inter-american Observatory 1.5-m telescope and the Mercator Telescope (operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias) with the HERMES spectrograph, which is supported by the Funds for Scientific Research of Flanders (FWO), Belgium, the Research Council of K.U. Leuven, Belgium, the Fonds National Recherches Scientific (FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Genève, Switzerland and the Thuringer Landessternwarte Tautenburg, Germany.

Supplementary Materials www.sciencemag.org/content/science.1253645/DC1 Materials and Methods Figs. S1 to S6 Table S1 References (21-68)

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19 March 2014; accepted 11 June 2014 Published online 3 July 2014 10.1126/science.1253645

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Fig. 1. 34 pulsating pre-MS stars in the evolutionary diagram. Evolutionary diagram of surface gravity vs.

effective temperature for the 34 pulsating pre-MS stars. Symbol colors correspond to the percentage of critical breakup velocity (vsini / vcrit) according to the scale at top-left. Pre-MS evolutionary tracks (12) are shown from 1.5 to 3.0 M◉ in steps of 0.05 M◉ and are color-coded according to the contribution of gravitational potential energy to the total energy output: when gravitational contraction is the only energy source, the pre-MS tracks are drawn in black; the track brightness increases as the ratio of gravitational potential energy to the total energy output drops, with the onset of first non-equilibrium nuclear burnings closer to the ZAMS. The symbols marked with a white smaller square in their centers are members of the young cluster NGC 2264 labeled “a” to “i” in Figs. 2 and 3.

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Fig. 2. NGC 2264 evolutionary diagram. Evolutionary diagram of surface gravity against effective temperature showing a subset of nine pulsating pre-MS stars in the young cluster NGC 2264: V 589 Mon, V 588 Mon, HD 261230, HD 262209, HD 261737, HD 261387, HD 262014, NGC 2264 104 and HD 261711 labeled “a” to “i. Symbol colors correspond to the highest observed p-mode frequency, fmax, according to the scale at top-left. Pre-MS evolutionary tracks (12) are shown from 1.5 to 3.0 M◉ in steps of 0.1 M◉ and are color-coded according to the contribution of gravitational potential energy to the total energy output (as in Fig. 1). The amplitude spectra of these nine objects are shown in Fig. 3 using the same labels.

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Fig. 3. NGC 2264 amplitude spectra. Sequence of amplitude spectra for a subset of nine pulsating pre-MS stars in the young cluster NGC 2264 from least evolved (V 589 Mon), to most evolved (HD 261711). In each panel the X-axis shows frequency in μHz and the amplitudes on the Y-axis are given in millimagnitudes. The original amplitude spectra are shown in light grey, where all identified pulsation frequencies are marked with black solid lines. The location of the maximum pulsation frequency, fmax, for each star is marked with the dashed red line. The small insets show the region immediately around fmax to illustrate its statistical significance.