Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~palla/webpage/Science_Beat2014.pdf Äàòà èçìåíåíèÿ: Wed Nov 4 17:18:29 2015 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 03:54:51 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 5

I N S IGHT S | PE RSPECTI VES

Node lines

Oscillations inside a young star. Adjacent layers contract and expand periodically. At the node lines, gas remains stationary.

A S T R O NO MY

The beat of young stars
Pulsations from young stars provide a new chronometer for stellar evolution
By Steven Stahler1 and Francesco Palla
2

REFER ENCE S

1

10.1126/science.1258477

Department of Astronomy, University of California, Berkeley, CA 94720, USA. 2Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy. E-mail: stahler@berkeley.edu

5 14

1 A UG U S T 2 0 1 4 · V OL 34 5 I SS UE 619 6

sciencemag .org SC IEN CE
Published by AAAS

ILLUSTRATION: FIGURE COURTESY OF M. MARCONI; ADAPTED BY P. HUEY/SCIENCE

1. Wall and Melzack's Textbook of Pain (Elsevier/Saunders, Philadelphia, ed. 6, 2013). 2. A. I. Basbaum, D. M. Bautista, G. Scherrer, D. Julius, Cell 139, 267 (2009). 3. J. J. Balcita-Pedicino, N. Omelchenko, R. Bell, S. R. Sesack, J. Comp. Neurol. 519, 1143 (2011). 4. N. Schwartz et al., Science 345, 535 (2014). 5. G. F. Koob, M. Le Moal, Neurobiology of Addiction (Academic Press, London, 2006). 6. J. Olds, P. Milner, J. Comp. Physiol. Psychol. 47, 419 (1954). 7. A. DahlstrÆm, K. Fuxe, L. Olson, U. Ungerstedt, Acta Physiol. Scand. 62, 485 (1964). 8. E. S. Bromberg-Martin, M. Matsumoto, O. Hikosaka, Neuron 68, 815 (2010). 9. F. Brischoux, S. Chakraborty, D. I. Brierley, M. A. Ungless, Proc. Natl. Acad. Sci. U.S.A. 106, 4894 (2009). 10. S. Lammel et al., Nature 491, 212 (2012). 1 1. J. Jensen et al., Neuron 40, 1251 (2003). 12. B. Knutson, C. M. Adams, G. W. Fong, D. Hommer, J. Neurosci. 21, RC159 (2001). 13. M. N. Baliki et al., Nat. Neurosci. 15, 1117 (2012). 14. A. V. Kravitz, L. D. Tye, A. C. Kreitzer, Nat. Neurosci. 15, 816 (2012). 15. A. Tripathi, L. Prensa, C. CebriÀn, E. Mengual, J. Comp. Neurol. 518, 4649 (2010).

A

star is a ball of gas held together by the compressive force of selfgravity and supported from within by thermal pressure. For most of a star's life, this dynamical balance is stable; perturbing the star does not lead to explosion or collapse, but to oscillation about its equilibrium configuration. Such perturbations arise constantly from small motions within the star itself. Consequently, stars of many evolutionary phases exhibit periodic fluctuations in their luminosity (see the first figure). On page 550 of this issue, Zwintz et al. (1) report on oscillations of stars so young that they are not yet fusing hydrogen into helium. Expanding on earlier studies of such pre­main-sequence stars (2), Zwintz et al. find that the observed frequencies of oscillation of a star vary with its age, and do so in the way that theory

predicts. Thus, the oscillations potentially provide a new chronometer--something greatly needed in the field of early stellar evolution. The standard way to assess the age of a star is to measure two quantities: its luminosity, L, and its effective (or surface) temperature, Teff . L is a measure of the total power emitted by the star, which is inferred from the measured flux on Earth, together with the distance to the star. Teff is determined from a high-resolution spectrum of the starlight. The pattern of absorption lines reflects specific wavelengths where the flux is diminished because of absorption by atoms in the stellar surface. Given L and Teff , the star can then be placed in the Hertzsprung-Russell (HR) diagram (see the second figure). The

Downloaded from www.sciencemag.org on November 4, 2015

neurons. Furthermore, long-term depression (weakening of specific glutamate synapses) was lost in DADR2 neurons. Schwartz et al. identified the neuropeptide galanin as the link between the selective changes in glutamate receptor function in the nucleus accumbens and the reduced motivation to work for a palatable taste reward. Decreasing galanin 1 receptor expression in the nucleus accumbens with RNA interference prevented both the behavioral and glutamate receptor changes induced by persistent pain. Furthermore, preventing NMDA-dependent long-term depression both blocked the pain-induced reduction in AMPA receptor function in DADR2 neurons and blocked the reduction in motivation to work for food. This supports a causal link between the reduction in the excitability of DADR2 neurons and the reduced motivation to work for a reward. By identifying a critical circuit element, Schwartz et al. have taken a vital step toward solving the fundamental neurobiological problem of action selection in the presence of conflicting motivations. It will be informative to relate the activity of DADR2 neurons to the effort expended to obtain food in awake behaving animals and to determine how the presence of ongoing pain changes this activity. Further work is also needed to define the behavior-relevant circuit, first by identifying the input pathway to the galanin-releasing neurons in the nucleus accumbens that mediate the change in glutamate receptor function in DADR2 neurons. To understand why reduced excitability in DADR2 neurons reduces the motivation to work for palatable food, it will be essential to determine how they act on their downstream targets.


star's position in the diagram is the indicator interior is dammed up, causing the star to inof both its age and evolutionary phase. flate. The opacity then falls, and the release As stars evolve, both L and Teff change. of excess energy causes the star to shrink, Thus, the point representing a star will move thus restarting the cycle. Whether or not the within the diagram. These paths are premechanism operates depends sensitively on dicted by theory and are called pre­mainthe star's effective temperature. If Teff is too sequence tracks (3). Physically, the young low, the ionization region is so deep that it star is contracting, and its diminishing surcannot move overlying gas. Conversely, in a face area changes its energy output, which star with too high a Teff , the layer is at such comes from the contraction itself. All tracks a shallow depth that only a small amount of eventually land on a curve known as the the outgoing luminosity can be absorbed. main sequence, where hydrogen ignites Pursuing this line of reasoning, Marconi and halts contraction. Note that stars of and Palla showed that there is an "instabilhigher mass contract to the main sequence ity strip" in the HR diagram (see the figure). relatively quickly because of their stronger Pre­main-sequence stars that pass through self-gravity. this area are susceptible to the oscillations. In principle, assigning an age to a pre­ Following their prediction, several dozen main-sequence star should be relatively young pulsating stars have been found. straightforward. After using observations to Gratifyingly, the stars indeed all lie within place the object in the HR diagram, the evoluthe instability strip. The actual pulsation tionary track is identified that passes through amplitudes are small, and satellite observathat point. The star's age--that is, how long it tions have greatly increased the precision of has been contracting--and its mass can then the measurements in recent years. be read off. Applying this procedure not just Any star lying within the instability strip to one star but to a whole cluster, the resultcan oscillate not just at a single frequency, ing collection of ages and masses yields the but at many, just as a taut string can vibrate star formation history of that group. in many normal modes. There is an upper In reality, however, the situation is more limit to the oscillation frequency, called the complicated. Young stars are surrounded by acoustic cutoff. At higher frequencies, incopious amounts of dusty cloud gas, which ternal pressure disturbances (that is, sound both dims and reddens their light before waves) do not reflect at the star's surface, it reaches us. Discerning the star's true inas is necessary to sustain the oscillation. trinsic luminosity and surface temperature Instead, these high-frequency waves pass can thus be difficult. In addition, many of these stars have circumstellar disks that may periodically 2 dump matter onto the stellar surface. This accretion 3.0 process releases energy, 2.8 which could be misinter2.6 peted as part of the underlying stellar luminosity (4). 1.5 2.4 Both problems are ameliorated with stars that either 2.2 have quickly dissipated their disks or have expelled 2.0 much of the nearby cloud gas. 1 1.8 In 1998, Marconi and Palla discovered that a sub1.6 set of pre­main-sequence stars, with masses between 1 and 4 times that of the Sun, can sustain self-excited 0.5 pulsations (5). These pulsa4.1 4 3.9 3.8 3.7 tions are driven by variaEfective temperature log Tef (K) tions in the opacity of the star's outer layers, where hydrogen and helium are Pulsating young stars in the Hertzsprung-Russell diagram. The solid partially ionized. If the star curves are pre­main-sequence tracks for stars of different masses, here contracts, the opacity inindicated in solar units. The shaded area is the instability strip, as found by creases. As a result, energy Marconi and Palla (5). Solid circles represent some of the known pulsators. emanating from the deep From top to bottom, the stars range in age from about 1 to 10 million years.
Luminosity log L/L
S CI E N CE sciencemag .org

through the surface to the sparser external medium (6). In their sample of 34 stars, Zwintz et al. focused on the acoustic cutoff frequency, and found that it increases with stellar age, where the latter was gauged from a modified form of the HR diagram. This observed trend is just what theory predicts. For any star in balance between self-gravity and pressure, a sound wave crosses it in about the same time it would take the object to freely collapse in the absence of pressure. This hypothetical free-fall time is inversely proportional to the square root of the star's mean density. It follows that the frequency of any oscillation mode is directly proportional to the square root of the density. The frequency thus should increase as the star contracts and its density rises, and this is exactly what Zwintz et al. found. Although a reliable measurement of these stellar frequencies requires the precision and sensitivity of a space-based telescope, the resulting age is not fraught with the uncertainties that have plagued the traditional determination of L and Teff . Thus, the path lies open to a new determination of stellar age. Of course, the new chronometer must first be calibrated, through observations of stars that both oscillate and are relatively free of circumstellar matter. The chronometer will be most useful for young clusters populous enough to contain several stars in the proper mass range. Zwintz et al. have already started down this road, observing nine members of the NGC 2264 cluster and finding evidence for a substantial age spread in this group. The finding of Zwintz et al. already suggests the utility of the new chronometer. If there is indeed an age spread in NGC 2264, then the parent cloud forming the cluster did so over an extended time. Other clouds presumably spawn stars in a similarly protracted manner. Over the past 15 years, it has been argued that the clouds actually collapse quickly (in a free-fall time) and form their stars in a single burst. Any apparent spread in stellar ages is illusory, an artifact of the various uncertainties mentioned earlier (7). A new and more precise chronometer should settle the issue, and thus advance our knowledge of early stellar evolution.
R EF ER E N C E S

1. K. Zwintz et al., Science 345, 550 (2014). 2. K. Zwintz, W. W. Weiss, Astron. Astrophys. 457, 237 (2006). 3. S. Stahler, F. Palla, The Formation of Stars (Wiley, Weinheim, Germany, 2004), chap. 16. 4. I. Baraffe, G. Chabrier, J. Gallardo, Astrophys. J. 702, L27 (2009). 5. M. Marconi, F. Palla, Astrophys. J. 507, L141 (1998). 6. C. Aerts, J. Christensen-Dalsgaard, D. W. Kurtz, Asteroseismology (Springer, New York, 2010), chap. 3. 7. R. D. Jeffries, S. P. Littlefair, T. Naylor, N. J. Mayne, Mon. Not. R. Astron. Soc. 418, 1948 (2011). 10.1126/science.1257301
1 A U G U ST 2 0 1 4 · V O L 3 4 5 I S SU E 6 1 9 6

51 5

Published by AAAS


The beat of young stars Steven Stahler and Francesco Palla Science 345, 514 (2014); DOI: 10.1126/science.1257301

This copy is for your personal, non-commercial use only.

If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. Downloaded from www.sciencemag.org on November 4, 2015 The following resources related to this article are available online at www.sciencemag.org (this information is current as of November 4, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6196/514.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/345/6196/514.full.html#related This article cites 5 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/345/6196/514.full.html#ref-list-1 This article has been cited by 1 articles hosted by HighWire Press; see: http://www.sciencemag.org/content/345/6196/514.full.html#related-urls This article appears in the following subject collections: Astronomy http://www.sciencemag.org/cgi/collection/astronomy

Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.