Past Meets Present in Astronomy And Astrophysics

2005 PAST MEETS PRESENT IN Astronomy and Astrophysics PROCEEDINGS OF THE 15TH PORTUGUESE NATIONAL MEETING

EDITORS JOSE AFONSO NUNO SANTOS ANDRE MOITINHO RUI AGOSTINHO

2005 PAST MEETS PRESENT IN

Astronomy and Astrophysics PROCEEDINGS OF THE 15TH PORTUGUESE NATIONAL MEETING

EDITORS JOSE AFONSO NUNO SANTOS ANDRE MOITINHO RUI AGOSTINHO UNIVERSITY

OF L I S B O N ,

PORTUGAL

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PAST MEETS PRESENT IN

Astronomy and Astrophysics PROCEEDINGS OF THE 15TH PORTUGUESE NATIONAL MEETING

UNIVERSITY OF LISBON ft LISBON ASTRONOMICAL OBSERVATORY 28 - 30 JULY 2005

\fc World Scientific NEW JERSEY • LONDON • SINGAPORE • BEIJING • SHANGHAI • HONGKONG

• TAIPEI • CHENNAI

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

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2005: PAST MEETS PRESENT IN ASTRONOMY AND ASTROPHYSICS Proceedings of the 15th Portuguese National Meeting Copyright © 2006 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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FOREWORD With a first appearance in 1991, the Portuguese National Meetings of Astronomy and Astrophysics (ENAA) are annual events that gather the astrophysics community in order to share the results of its research work. More recently, the creation of the Portuguese Astronomical Society (SPA) has stimulated and brought new dynamics to the ENAA. This is also the result of a young and growing astronomical community, active at the forefront of current astrophysical research, as well as to the ever stronger connections with ESO and ESA. Between the 28th and the 30th of July, 2005, the XV ENAA gathered over 80 researchers in the Faculty of Sciences of the University of Lisbon and in the Astronomical Observatory of Lisbon (OAL), to discuss the most recent findings. For the second year running, and in collaboration with the Center for the History of Sciences of the University of Lisbon, attention was also devoted to the History of Astronomy, with contributions that stress the rich past of Portuguese Astronomy. This provided a particularly attractive merger between the old and the new, between the richness, diversity and frequently unknown past of Portuguese Astronomy and the current fast-moving research. The present ENAA was organised by the Center for Astronomy and Astrophysics of the University of Lisbon (CAAUL) and the Portuguese Astronomical Society (SPA). The programme was defined with the help of the Scientific and Local Organizing Committee: Jose Afonso (OAL/CAAUL), Nuno Santos (CAAUL), Andre Moitinho (CAAUL), Rui Agostinho (FCUL/CAAUL), Carlos Santos (CAAUL), Pedro Raposo (OAL) and Eugenia Carvalho (OAL). The meeting was made possible by the support of a number of entities: the Faculty of Sciences of the University of Lisbon, the Astronomical Observatory of Lisbon, the Foundation for Science and Technology (Portugal), Banco Espirito Santo and Delta Cafes.

v

VI

Finally, we thank the participants for their active contribution to the success of this venture. This book is made for them, and by them.

Jose Afonso, Nuno Santos, Andre Moitinho and Rui Agostinho

Organisation:

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ASTRONOMIA

Acknowledgements:

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FCT Fundai, ao para a Ciencia e a Tecnologia

BANCO ESPIRITO SANTO

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CONTENTS Foreword

v

Modern Astrophysics Evolution of the spin of Mercury and its capture into the 3/2 spin-orbit resonance A. CM. Correia andJ. Laskar 1 Trans-Neptunian Objects and Associated Families: confronting colors, correlations and evolution models N. Peixinho 5 The origin of the spins of Kuiper Belt objects P. Lacerda, C. Dominik, J. Luu and S. Kenyon

9

Magnetic Turbulence in the solar wind and the earth's plasma sheet I. Dorotovic and Z. Voros

13

The structure revealed by Spitzer in NGC 2264 P. S. Teixeira, C. J. Lada, M. Marengo, A. Muench, S. T. Megeath, G. Fazio, E. T. Young, J. Muzerolle, N. Siegler, G. Reike and L. Hartmann

17

Recent Results on Interstellar Turbulence M. A. de Avillez and D. Breitschwerdt

19

Asteroseismology and Variability of Young Stars F. J. G. Pinheiro

23

On the problem of magnetic braking J. M. Ferreira, A. Aibeo and J. Lima

27

vn

Vlll

A first step for Automatic Stellar Parameter Determination S. G. Sousa Giant Transiting Planets Observations - GITPO C. Afonso

31

35

Probing the structure and atmospheres of extra-solar planets N. C. Santos

39

What's going on in Canis Major? A. Moitinho, G Cagarro, R. A. Vazquez, G. Baume and E. E. Giorgi

43

Study of three galaxy clusters at intermediate redshifts C. Lobo andM. S. Roos

47

Modelling the Warm Absorber in NGC 3783 with the TITAN code A. C. Gonqalves, A. Rozanska, S. Collin, A. M. Dumont, M. Mouchet, L. Chevallier and R. W. Goosmann

51

Astrophysical Tests of Fundamental Physics C. J. A. P. Martins

55

Gamma Ray Bursts as Cosmological Probes O. Bertolami and P. T. Silva

59

Braneworld cosmology: sneutrino inflation and leptogenesis N. M. C. Santos, M. C. Bento and R. G. Felipe

63

XCS - Current Status P.T.P. Viana

67

Deep radio observations in the CDFS/GOODS field: optical and X-ray identifications J. Afonso

71

The nature of the optical faint sub-millijansky radio sources: the VLT/VIMOS view D. Sobral and J. Afonso

75

IX

AMS - a magnetic spectrometer on the international space station L. Arruda, F. Barao, G. Barreira, J. Borges, F. Carmo, P. Gongalves, R. Pereira and M. Pimenta

77

History of Astronomy The legacy of Sacrobosco: Tractatus de Sphaera B.Almeida Astronomical and Geophysical Activities in Rio de Janeiro (Brazil) during 1781-88 by Bento Sanches Dorta J. M. Vaquero, R. M. Trigo and M. C. Gallego

79

83

Comparison between Monteiro da Rocha and Wilhelm Olbers' Methods for the determination of the orbits of comets F. B. Figueiredo and Joao Fernandes 85 The 1870 Portuguese solar eclipse expedition a preliminary report V. H. Bonifacio, I. Malaquias and J. M. Fernandes

89

The Science Palaces J.D.C.G. Jorge

93

The astronomer/instrument maker Campos Rodrigues and the contribution of the Observatory of Lisbon for the 1900-1901 solar parallax programme P.Raposo

97

The Astronomical Observatory of Lisbon P. M. deAbreu

101

Time Service and Legal Time in Portugal M. Silva and R. Agostinho

105

Documents of the OAL's architecture R. G. Batista andR. Agostinho

109

MODERN ASTROPHYSICS

EVOLUTION OP T H E S P I N OF M E R C U R Y A N D ITS CAPTURE INTO THE 3 / 2 SPIN-ORBIT RESONANCE

ALEXANDRE C. M. CORREIA Departamento de Fisica da Universidade de Aveiro, Campus Universitdrio de Santiago, 3810-193 Aveiro, Portugal JACQUES LASKAR Astronomie et Systemes Dynamiques, IMCCE-CNRS UMR8028 7 7 Av. Denfert-Rochereau, 75014 Paris, France

The present spin of Mercury is very peculiar and was only discovered in 1965: the planet spins three times around its axis exactly in the same time as it completes two orbital revolutions 1 . The way the planet evolved into this configuration remained a mystery until very recently 2 . In order to understand this phenomena we must take into account the planetary perturbations over Mercury's orbit, that continuously change its eccentricity. As a result, for any initial rotation rate it was found that the chances of capture in the present configuration rise to about 55.4%.

Tidal dissipation and core-mantle friction will drive the obliquity of Mercury close to zero. For zero degree obliquity, the averaged equation for the rotational motion near the p resonance (where p is a half-integer) writes 2 ' 3 : X

3n

„C

C

22

= —6- f

iff (p, e) sin 2(/ - PM) - 3 ^ ( f )

Q

[11(e)* - N(e)] ,

where x = i/n is the ratio of the rotation rate to the mean motion n, M the mean anomaly, e the eccentricity, £ a structure constant and H(p, e) Hansen coefficients 3 . Q(e) = (1 + 3e 2 + 3e 4 /8)/(l - e 2 ) 9 / 2 , JV(e) = (1 + 15e 2 /2 + 45e 4 /8 4- 5e 6 /16)/(l - e 2 ) 6 , fc2 and Q are the second Love number and quality factor, while a, m, ma are the semi major axis, the mass of the planet, and the solar mass. The equilibrium is achieved when x = 0, that is, for a constant eccentricity e, when x = xi(e) = N(e)/Ci(e). In a circular orbit (e = 0) this equilibrium coincides with synchronization (x — 1), while the equilibrium rotation rate x = 3/2 is achieved for 63/2 = 0.284927.

1

2

For the present value of Mercury's eccentricity e « 0.206 the capture probability in the 3/2 spin-orbit resonance 3 is estimated to be about 7.73%. However, using the present value of the eccentricity of Mercury is questionable, as the eccentricity suffers strong chaotic variations in time, due to planetary secular perturbations 4 ' 5 . Indeed, the eccentricity of Mercury can vary from nearly zero to more than 0.45, and thus reach values higher than the critical value e 3 / 2 = 0.284927 (Fig.l). Additional capture into resonance can then occur, at any time during the planet's history.

Figure 1. Examples of the possible variations of the eccentricity some 4 Gyr ago. All these solutions converge to the known present evolution of the planet's orbit. Traced horizontal line corresponds to the critical eccentricity e 3 / 2 = 0.2844927.

3 In order to check this scenario, it is not possible to use a single orbital solution, as due to its chaotic behavior, the motion cannot be predicted precisely beyond a few tens of millions of years. A statistical study of the past evolutions of Mercury's orbit is then performed, with the integration of 1000 orbits over 4 Gyr in the past, starting with very close initial conditions, within the uncertainty of the present ones (Fig.l). This statistical study was made possible by the use of the averaged equations for the motion of the Solar System 4 ' 5 . For each of these 1000 orbital motion of Mercury, the rotational motion (Eq.l) was integrated numerically with planetary perturbations, for p = k/2;k = 1,...,10. Simulations were started at io = —4 Gyr, with a rotation period of 20 days (x « 4.4), using £ = 0.3333, k-z = 0.4 and Q = 50. As e is not constant, x(t) will tend towards a limit value x(t) that is similar to an averaged value of xi(t) and capture into resonance can now occur more often (Fig.2).

1.8 1.7 1.6 X 1.5 1.4 1.3 1.2 1.1

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0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16

-3.98

J

-4

1

-3.98

-3.96

I

I

-3.96

-3.94

I

I

-3.94

-3.92

I

I

-3.92

-3.9

L

-3.9

time (Gyr)

Figure 2. Rotation rate for a non constant eccentricity (b). The limit solution of equation (1) is no longer x\ = N(e)/fl(e) [(a) dotted line], but now given by: x(t) = (x(0) + K f* N(e(r))g(r)dr) /g(t), where g(t) = exp(K /„' « ( e ( r ) ) dr) [(a), filled line]. In this example, there is no capture at the first crossing of the 3/2 resonance (at t ss —3.9974 Gyr). About 100 Myr later, as the mean eccentricity increases, additional crossing of the 3/2 resonance occurs, leading to capture with damping of the libration.

4

All the 1000 solutions were followed, starting from —4 Gyr, until they reached the present date or get captured into the 2/1, 3/2, or 1/1 resonances. Contrarily to previous studies, it was found that capture into the 1/1 resonance is possible, as the eccentricity of Mercury may decrease to very low values, where the capture can occur, and the resonance remains then stable. The 3/2 remains stable, except for extremely small values of the eccentricity 2 . Indeed, over 554 solutions that were captured into the 3/2 resonance, a single solution, initially captured at —3.995 Gyr, escaped from resonance at about —2.396 Gyr. The solution then got trapped into the 1/1 resonance at —2.290 Gyr, capture that was favored by the low eccentricity required to destabilize the 3/2 resonance. Out of the 56 solutions initially trapped into the 2/1 resonance, 10 were destabilized and only 2 of them were further captured, one into 3/2 resonance, and one into 1/1 resonance. Globally, only 38.8% of the solutions did not end into resonance, and the final capture probability distribution was 2 : Pi/i = 2.2%,

P 3 / 2 = 55.4%,

P 2 / 1 = 3.6% .

With the consideration of the chaotic evolution of the eccentricity of Mercury, it is then shown that with a realistic tidal dissipative model that properly accounts for the damping of the libration of the planet, the present 3/2 resonant state is the most probable outcome for this planet. The largest unknown remains the dissipation factor fo/Q in (Eq.l). A stronger dissipation would increase the probability of capture into the 3/2 resonance, as x{t) would follow more closely xi(e(t)) (Fig.2), while lower dissipation will slightly decrease the capture probability. Acknowledgements This work was supported by PNP-CNRS, Paris Observatory CS, and project POCTI/FNU/43656/2001, Portugal. The numerical computations were made at IDRIS-CNRS, and Paris Observatory. References 1. 2. 3. 4. 5.

Pettengill, G. H., and Dyce R. B., Nature 206, 1240 (1965) Correia, A.C.M., and Laskar, J., Nature 429, 848 (2004) Goldreich, P., and Peale, S.J., AJ 71, 425 (1966) Laskar, J., Icarus 88, 266 (1990) Laskar, J., A&A 287, L9 (1994)

T R A N S - N E P T U N I A N OBJECTS A N D ASSOCIATED FAMILIES: C O N F R O N T I N G COLORS, CORRELATIONS A N D EVOLUTION MODELS.

N. PEIXINHO CAAUL,

Observatorio Astrnomico de Tapada da Ajuda, 1349-018, Portugal E-mail: [email protected]

Lisboa,

LESIA, Observatoire de Paris, 5, Place Julles Janssen, 92195 Meudon Cedex, France

With the last update of the "Meudon Multicolor Survey" we now possess a data sample of visible colors for 122 objects. Through this large data set we have analyzed: a) the interrelations between the colors and orbital parameters of T r a n s Neptunian Objects and associated populations; b) the "genetic" links between them; and c) the compatibility between our statistical results and the surface and dynamical evolution models for these objects.

1. Introduction Discovered in 19921, Trans-Neptunian objects (TNOs) are a population of small icy bodies beyond the orbit of Neptune, forming the EdgeworthKuiper belt (EKB). Presently more than 1000 objects have been identified. TNOs are expected to be well-preserved fossil remnants of the formation of the solar system, assumed to be icy-conglomerates composed by water ice, complex molecules formed out of H, C, N, O, and dust. TNOs are usually subdivided in the dynamical families: Classical objects, Scattered Disk Objects (SDOs), Plutinos and other resonants. TNOs are also the probable precursors of Centaurs - objects with chaotic orbits between Jupiter and Neptune - short period comets (SPCs) and irregular satellites of giant planets. The long-term irradiation of the icy surfaces of TNOs should generate a solid crust of carbon-complex compounds with reddish colors2. Impact

5

6 collisions, however, may excavate buried unirradiated material, with bluish colors, from beneath such crust 3 . Moreover, sublimation of volatiles that may refreeze on the surface is also expected. The competition between these effects should generate the wide color variety currently observed among TNOs 4 . The knowledge of their physical and chemical properties is crucial to constrain the formation and evolution models of our solar system. 2. Observational data TNOs are inherently faint (my ~ 22 — 23) and difficult to detect. Consequently, with today's instrumentation, only multicolor photometry — which provides a first-order indication of their surface composition — provides a representative analysis of TNO's surfaces. Prom two large observation programs, the "ESO Large Program on Centaurs and TNOs", using NTT and VLT, and the "Meudon Multicolor Survey", using CFHT, we have obtained visible photometry for a total of 122 objects — the largest data sample analyzed for far. 3. Discussion In order to understand the physical processes responsible for their color distribution we have carried out a statistical analysis of the interconnections between the colors and orbital parameters of each family. Major results are summarized below. 3.1. Color-orbital

parameters'

correlations

The most recent models for the solar system show that the giant planets migrated while scattering a disk of planetesimals 5 . As a consequence, the EKB should consist in a superposition of a low inclined ("cold") and a high inclined ("hot") population. Our sample shows a red cluster of Classical TNOs at orbital inclinations i < 4.5°, in contrast with a large color

dispersion at higher inclinations, supporting such dynamical models6. See Figure 1. We also detect a color-perihelion (q) correlation among Classical TNOs, that is much stronger for objects larger than ~ 220 km6. This points to some surface alteration by cometary activity (outgassing), which is more effective for larger objects 4 . See Figure 2. Other families do not evidence for physically relevant trends between colors and orbital parameters. Note, however, that due to the unstable dy-

7 namical history of these families such results cannot immediately be taken as surprising. This issue needs further studies.

1

1

Classical TNOs

—• _p_

— 1 —

•— 1

•—

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14

15

16 17 IB IB log mass (kg)

20

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1 14

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16 17 16 19 log mass (kg)

20

*=10-« (s)=0.40 21

14

15

16 17 18 19 log mass (kg)

20

21

Figure 2. Evolution of target's spin period, as it grows, for six combinations of (z) and k. Different lines correspond to target's initial spin period P RS 3.3 hr (solid), P = 9.23 hr (dotted), and no initial spin (dashed).

ing K B O for nearly isotropic accretion, i.e., if (z) « 0. Ratios k > 0.01 also explain the measured spin periods, even under completely isotropic growth. This corresponds to a ratio of projectile to target radius of /c 1 / 3 « 0.2. 5.

Summary

Our main conclusions are: (1) Collisions have not changed the spins of the largest K B O s (r > 200 km) in the last ~ 4 G y r : their present spins must have been set by the end of the accretion phase; (2) A 10% anisotropy in the accretion process can produce the observed spins of KBOs; (3) If the (last) planetesimals accreted onto the large K B O s were at least 20% of the size of the growing bodies then isotropic accretion can explain the observations.

References 1. 2. 3. 4. 5.

J. X. Luu and D. C. Jewitt, ARA&A 40, 63 (2002). S. J. Kenyon and J. X. Luu, AJ118, 1101 (1999). S. S. Sheppard and D. C. Jewitt, Earth Moon and Planets 92, 207 (2003) D. C. Jewitt and S. S. Sheppard, AJ 123, 2110 (2002) J. L. Ortiz, P. J. Gutierrez, A. Sota, V. Casanova and V. R. Teixeira, A&A 409, L13 (2003) 6. P. Lacerda and J. Luu, AJ in press (2006) 7. D. Rabinowitz, M. Brown and C. Trujillo, AJ in press (2006)

MAGNETIC TURBULENCE IN THE SOLAR WIND AND THE EARTH'S PLASMA SHEET I. DOROTOVIC Observatorio Astronomico da Universidade de Coimbra and Grupo de Astrofisica da Universidade de Coimbra, Portugal; UNINOVA-CRI/CA3, Caparica, Portugal, Email: [email protected]; Slovak Central Observatory, Hurbanovo, Slovak Republic, Email: dorotovic@suh. sk Z. VOROS Space Research Institute AAS, Austria, Email: [email protected] In general, if turbulence is present in MHD plasmas, it cannot be ignored. There is also evidence that the Sun is the main driver of space weather. It has already been demonstrated in our recent study that the non-Gaussian characteristics of magnetic turbulence in the solar wind and the occurrence of intermittent magnetic turbulence in the Earth's plasma sheet can be interconnected. In this respect a comparative analysis of the solar wind magnetic and plasma parameters with the time evolution of the geomagnetic indices is insufficient. Therefore, a wider statistical study which includes the consideration of intermittency parameters during several coupling events during the period of 1996 - 2002 was performed as well.

1. Introduction It is generally accepted that nonlinear couplings and turbulence play a key role in the study of solar wind - magnetosphere interaction processes. There is also evidence that if turbulence is present in MHD plasmas, it cannot be ignored. Recent approaches to address the problem of intermittency in solar wind turbulence have been discussed e.g. in [1]. In our preliminary study ([2]), we investigated the non-Gaussian characteristics of intermittent magnetic field fluctuations available from simultaneous observations in the solar wind (SW) and in the Earth's plasma sheet (PS), and we demonstrated that the non-Gaussian characteristics of magnetic turbulence in the solar wind and the occurrence of intermittent magnetic turbulence in the Earth's plasma sheet (PS) can be reliably interconnected. In the next step ([3]) we performed a wider statistical study which includes the consideration of intermittency parameters (skewness and kurtosis) during several coupling events during the period of 1996 - 2002. We intend to publish here only a brief overview of the main results of this investigation, while details can be found in the last two cited papers.

13

14 2.

Solar wind and plasma sheet data

In these studies we used velocity (Vx) and magnetic field (Bz) measurements in the SW available from the WIND satellite ([4], [5]) and ACE spacecraft ([6], [7]). The SW measurements are compared to simultaneous measurements of velocity (Vx) and magnentic field (Bx) in the Earth's PS available from the GEOTAIL mission. We selected only those events when the satellite was in GSM positions X e (-15 •*• -25) RE, Ye (-10 -*- 10) RE, and, moreover, when the I Bx\ < 15 nT, to ensure that the GEOTAIL was in the PS during the selected events. Based on these selection criteria, we identified 38 suitable events (with a different duration from 6 to -18 hours). All the data from GEOTAIL, Wind and ACE for the selected events during the period of 1996-2002 were obtained from the web site http://rumba.gsfc.nasa.gov/cdaweb/. Further details on input data can be found in [2] and [3]. 3. Non-Gaussian intermittent fluctuations in SW and PS interaction processes To be able to evaluate the statistics of magnetic fluctuations at different time scales, we usually consider two-point differences defined by Equation 2.1 in the paper [1]:

KB=B{t+c)-B{c)

(1)

Then we constructed the so-called probability distribution functions (PDFs) for two ranges of time scales: x = 15, 30,..., 120 s (taul), and x = 540, 555,..., 645 s (tau2), respectively. The scale-dependent changes in the shape of PDFs represent a measure of intermittent character of turbulent plasma flows. The flows are more intermittent when the peakedness of PDFs grows towards small scales [3]). The main aim of our investigation referred in [3] was to perform statistical study of skewness (s) and kurtosis (k) estimated for individual events. In order to study the effect of solar wind turbulence on plasma sheet fluctuations, scatterplots ofs and k were constructed in both regions [3]. 4. Conclusions Based on these studies ([2], [3]), we can conclude that: stronger interconnection of SW and PS occurs at smaller time scales (taul), larger values of the kurtosis occur in the magnetotail when the corresponding kurtosis is also larger in the SW,

15 All these facts indicate that coupling mechanisms between the solar wind and Earth's magnetosphere might be stronger during the intervals when turbulence is present in the SW. Acknowledgments The authors are grateful to N. Ness and D.J. McComas for providing ACE data, R. Lepping and K.W. Ogilvie for providing Wind, and S. Kokubun and L. Frank for the GEOTAIL data. This work has been supported by FCT (MCES, Lisbon, Portugal) grants SFRH/BPD/14628/2003, and partially POCTI-SFA-2-675 (I.D.). References 1.

2.

3.

4. 5. 6. 7.

Voros, Z., Leubner, M. P., and Baumjohann, W.: 2005, Cross-scale coupling induced intermittency near interplanetary shocks, J. Geophys. Res., in press. Dorotovid, I., and Voros, Z.: 2004, in Multi-Wavelength Investigations of Solar Activity, Proceedings IAU Symposium No. 223, eds. A.V. Stepanov, E.E. Benevolenskaya and A.G.Kosovichev, 537, DOI: 10.1017/S1743921304006763. Dorotovic, I. and Voros Z.: On the Earth's plasma sheet response to the magnetic turbulence in the solar wind, in Proceedings of the 11th European Solar Physics Meeting: The Dynamic Sun: Challenges for Theory and Observations, Leuven, Belgium, September 11-16, 2005, ESA SP-600, ed. D. Danesy, in press. Ogilvie K. W., et al.: 1995, Space Sci. Rev., 71, 55. Lepping, R. P., et al.: 1995, in The Global Geospace Mission, ed. by C. T. Russell, Kluwer, 207. McComas, D. J., Bame, S. J., Barker, P., Feldman, W. C , Phillips, J. L., Riley, P., Griffee, J. W.: 1998, Space Sci. Rev., 86, 563. Smith, C. W., L'Heureux, J. L., Ness, N.F., Acuna, M. H., Burlaga, L. F., and Scheifele, J.: 1998, Space Sci. Rev., 86, 613.

T H E S T R U C T U R E R E V E A L E D B Y SPITZER

I N N G C 2264

P. S. TEIXEIRA** C. J. LADA, M. MARENGO, A. MUENCH, S. T. MEGEATH, AND G. FAZIO Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Mail Stop 72, Cambridge, MA 02138, USA Email: [email protected] E. T. YOUNG, J. MUZEROLLE, N. SIEGLER, AND G. REIKE Steward Observatory, University of Arizona 933 North Cherry Avenue, Tucson, AZ 85721, USA L. HARTMANN Dept. Astronomy, University of Michigan 500 Church St., 830 Dennison Building, Ann Arbor, MI 48109, USA

We present initial results on a very young and highly embedded region of NGC 2264. The 24fj,m sources detected are found to be mostly protostellar and spatially located in linear patterns that coincide with dense filaments of molecular material. Furthermore, their quasi-regular separations is consistent with the value for the Jeans length in the region, indicating that the filaments have thernally fragmented and we are observing the primordial substructure of the cluster.

1. I n t r o d u c t i o n N G C 2264 is cluster t h a t has been very well studied 5 and known as an active star-forming region 1,6 > 2 . We present results on one of the youngest regions within N G C 2264, IRS-2, observed in t h e submillimeter 6 and in the millimeter 3 . This paper presents results obtained by observing N G C 2264 with t h e Spitzer Space Telescope in the following wavelengths: 3.6, 4.5, 5.8, 8, 24, 70, and 160/xm.

* University of Lisbon, Portugal tWork sponsored by the graduate fellowship SFRH/BD/13984/2003 of the Fundagao para a Ciencia e Tecnologia, Portugal

17

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n g u r e JL. (jrey scaie image 01 one 01 ine youngesi regions m i\HjUis;«>4, 1H.B-Z. l n e bright saturated source corresponds to IRAS 06382+0930, the wispy nebulosity corresponds to polycyclic aromatic hydrocarbon (PAH) emission. The image shows particular linear alignments of bright sources.

2. Analysis a n d Discussion The 24/i4m sources are found to be mostly protostars (~60% are Class I sources). These sources are additionally found to be aligned in linear patterns that seem to point back at IRAS 06832+0930, and these chains of protostars are coincident with dense dusty filaments of molecular material 6 3 ' . The main results obtained are that the bright 24 /an sources are tracing the primordial substructure of the cluster, and that the molecular filaments have thermally fragmented since we observe that the spacing between the protostars is regular and consistent with the Jeans length for that region 4 . References 1. M. Margulis, C. J. Lada, and R. L. Snell, ApJ, 333, 316 (1988). 2. B. Reipurth, K. Yu, G. Moriarty-Schieven, J. Bally, C. Aspin, and S. Heathcote, A J, 127, 1069 (2004). 3. N. Peretto, P. Andre, and A. Belloche, A&A, in press (2005). 4. P. S. Teixeira, C. J. Lada, E. T. Young, M. Marengo, A. Muench, J. Muzerolle, N. Siegler, G. Rieke, L. Hartmann, S. T. Megeath, and G. Fazio, ApJ, in press (2005). 5. M. F. Walker, A J, 59, 333 (1954). 6. G. Wolf-Chase, G. Moriarty, Schieven, M. Fich, and M. Barsony, MNRAS, 344, 809 (2003).

R E C E N T RESULTS O N INTERSTELLAR T U R B U L E N C E

M. A. D E A V I L L E Z 1 ' 2 A N D D . B R E I T S C H W E R D T 2 Department of Mathematics, University of Evora, Portugal Institut fur Astronomie, Universitat Wien, Austria E-mail: mavillez,[email protected]

The statistical properties of interstellar turbulence are studied by means of threedimensional high-resolution HD and MHD simulations of a SN-driven ISM. It is found that the longitudinal and transverse turbulent length scales have time averaged (over a period of 50 Myr) ratios of 0.5-0.6, almost similar to the one expected for isotropic homogeneous turbulence. The mean characteristic size of t h e larger eddies is found to be ~ 75 pc. Furthermore, the scalings of the structure functions measured in the simulated disk show unambiguous departure from the Kolmogorv (1941) model being consistent with the latest intermittency studies of supersonic turbulence (Politano & Pouquet 1995; Boldyrev 2002). Our results are independent of resolution, indicating that convergence has been reached, and that the unresolved smaller dissipative scales do not feed back on the larger ones.

1. Introduction Interstellar turbulence is mainly driven by the energy injected into the ISM by supernovae, with the driving scale still being uncertain. It is also unclear what the statistical properties of the turbulent interstellar gas are, if the full available range of energies is taken into account. In this paper we discuss the statistical properties of the interstellar turbulence in the Galactic disk based on three-dimensional adaptive mesh refinement (AMR) simulations of the ISM, which include the disk-halo-disk circulation f1]. In particular we explore the injection scales (section 2) and the scalings of the velocity structure functions (section 3) of the interstellar turbulent gas and discuss (section 4) their implications. 2. The injection scale of interstellar turbulence The outer scale of the turbulent flow in the ISM is related to the scale at which the energy in blast waves is transferred to the interstellar gas. Such a scale can be determined by means of the longitudinal and transversal correlation lengths, L\\ and Lkk, respectively. Here, k = 2,3 refer to

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20 the directions perpendicular to the 1-direction along which the correlation lengths are calculated. For isotropic turbulence Lkk = 0.5Ln. Figure 1 shows the history of i n (left panel) and L22/L11 (right panel) during 50 Myr of evolution of the unmagnetized and magnetized ISM. For details on how i n and L22 are calculated see [6]. Although (£11 )t ~ 75 pc, the scatter of L\\ around its mean results from the oscillations in the local star formation rate, where the formation and merging of superbubbles, is responsible for the peaks observed in the two plots. The similarity between the average HD and MHD injection scales is due to the fact that magnetic pressure and tension forces cannot prevent break-up as long as L < /3pA, where L and A are the scale lengths of thermal and magnetic pressures (including tension forces) and ftp = A-KP/B2 is the plasma beta.

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Figure 1. History of the characteristic size (given by L\\) of the larger eddies (left panel) and of the ratio L22/LW (right panel) for the HD (dashed line) and MHD (solid line) runs.

Despite the large scatter seen in L22/L11, the time average ( Z ^ / i n ) * over the 50 Myr period is 0.51 and 0.6 for the HD and MHD runs, respectively. The discrepancy from 0.5 by about 20% in the MHD case is a consequence of the anisotropy introduced by the field into the flow. The (L22/Lu)t ~ 0.5 in the HD run indicates that in a statistical sense the interstellar unmagnetized turbulence is roughly isotropic. 3. Scalings of the Structure Functions The statistics of turbulent flows in physical space is commonly characterized by the velocity structure functions of order p defined as Sp(l) = (|AV/| P ), with AVi = v(x + l) —v(x), where v(x + l) and v(x) are the velocities along the a;—axis at two points separated by a distance I, such 77