Laura VENUTI

214 Pages·2017·14.4 MB·English

by  Laura Venuti

Accretion regimes and variability in young stars:
imprints on UV photometry
Laura Venuti
To cite this version:
Laura Venuti. Accretion regimes and variability in young stars: imprints on UV photometry. As-
trophysics [astro-ph]. Université Grenoble Alpes; Università degli Studi di Palermo, 2015. English.
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THÈSE
Pour obtenir le grade de
DOCTEUR DE L’UNIVERSITÉ GRENOBLE ALPES
préparée dans le cadre d’une cotutelle entre
l’Université Grenoble Alpes et l’Università degli
Studi di Palermo

Spécialité : Astrophysique et Milieux Dilués
Arrêté ministériel : le 6 janvier 2005 - 7 août 2006

Présentée par
Laura VENUTI

Thèse dirigée par Jérôme BOUVIER
codirigée par Giovanni PERES

préparée au sein de l’Institut de Planétologie et
d’Astrophysique de Grenoble et de l’INAF – Osservatorio
Astronomico di Palermo ‘‘G. S. Vaiana’’

dans l’Ecole Doctorale de Physique

Accretion regimes and
variability in young stars:
imprints on UV photometry

Thèse soutenue publiquement le 23 Octobre 2015,
devant le jury composé de :
M. Juan Manuel ALCALA’
Astronome, INAF-OAC, Rapporteur
Mme Matilde FERNANDEZ
Chercheuse, IAA-CSIC, Rapporteur
M. Jonathan FERREIRA
Professeur, IPAG/UJF, Examinateur, Président
M. Fabio REALE
Professeur, UNIPA/INAF-OAPa, Examinateur
M. Jérôme BOUVIER
DR1, IPAG, Directeur de thèse
M. Giovanni PERES
Professeur, UNIPA/INAF-OAPa, co-Directeur de thèse

THÈSE
Pour obtenir le grade de
DOCTEUR DE L’UNIVERSITÉ GRENOBLE ALPES
préparée dans le cadre d’une cotutelle entre
l’Université Grenoble Alpes et l’Università degli
Studi di Palermo

Spécialité : Astrophysique et Milieux Dilués
Arrêté ministériel : le 6 janvier 2005 - 7 août 2006

Présentée par
Laura VENUTI

Thèse dirigée par Jérôme BOUVIER
codirigée par Giovanni PERES

préparée au sein de l’Institut de Planétologie et
d’Astrophysique de Grenoble et de l’INAF – Osservatorio
Astronomico di Palermo ‘‘G. S. Vaiana’’

dans l’Ecole Doctorale de Physique

Accretion regimes and
variability in young stars:
imprints on UV photometry

Thèse soutenue publiquement le 23 Octobre 2015,
devant le jury composé de :
M. Juan Manuel ALCALA’
Astronome, INAF-OAC, Rapporteur
Mme Matilde FERNANDEZ
Chercheuse, IAA-CSIC, Rapporteur
M. Jonathan FERREIRA
Professeur, IPAG/UJF, Examinateur, Président
M. Fabio REALE
Professeur, UNIPA/INAF-OAPa, Examinateur
M. Jérôme BOUVIER
DR1, IPAG, Directeur de thèse
M. Giovanni PERES
Professeur, UNIPA/INAF-OAPa, co-Directeur de thèse

Acknowledgements
« E quindi uscimmo a riveder le stelle » (D. Alighieri, Divina Commedia, Inferno XXXIV)
Let’s start from the end. Because the end is just as the beginning. A little girl gazing in
wonderment at the infinity of the night sky. Just like that, a glimpse between two piles of
bricks; and it never looks the same, and it always feels like the first time. And even when the
stars become points on a diagram, after a whole day spent chasing them on the screen of a
computer, you go out in the night and instinctively look up, and the sky is always there, ready
to catch you in the enchantment of its poetry.
Shortly before starting my PhD, I heard someone saying that the three most important in-
gredients for an astronomer are fascination with observing the stars, doubts, and encounters.
The first element is what brought me to approach the world of Astronomy; the third is what
brough me here, writing these words; discovering the second and embracing it as a most im-
portant part of scientific growth is what led me to completing these three years of research and
being eager to do more. For all of this, and much more, I would like to express my gratitude to
Jérôme. Thanks for always trusting me, even when I did not; thanks for letting me being wrong
and figuring that out by myself; thanks for always being willing to listen, and for your thorough
responses; thanks for the long hours of discussions and exchanges; thanks for your cordiality,
affability, and good humor. I would also like to thank Giovanni, whom I first met as a Bachelor
student, atatimewhen“astronomy” waslittlemorethanawordforme. Thanksforintroducing
me to Astrophysics, for fostering my interest in the study of the stars and for encouraging me
to pursue it; thanks for your mentoring and your valuable advice. Thanks to both of you for
allowing me to do a joint PhD between Grenoble and Palermo, and thus benefit from assiduous
interaction with diverse expertise and viewpoints during my work.
I should mention here the CSI 2264 Collaboration, within which the study related in this
thesis has been developed. I would like to thank in particular Silvia Alencar, Ann Marie Cody,
KevinCovey, EttoreFlaccomio, LynneHillenbrand, JonathanIrwin, GiusiMicela, LuisaRebull,
John Stauffer, for numerous discussions and exchanges of information and ideas. Thanks to the
ODYSSey team at IPAG and to the YSOs experts in Palermo, for encouraging me to present
regularlytheadvancesinmythesisworkandfortheirattentivecommentsandsuggestions. Ialso
thankJuan-ManuelAlcalàandMatildeFernandezforacceptingtoreviewmythesismanuscript,
and Jonathan Ferreira and Fabio Reale for integrating my defense committee in the quality of
examiners.
Essential to the good functioning of life at the lab are the administration and IT groups. I
would like to thank them for helping me with solving the various little technical or bureaucratic
problems that unavoidably arose during my stay. A special mention goes to Richard: he may
not remember it, but he was the first who showed me around upon my arrival at IPAG and
made me feel at home.
During these three years, spent in good part abroad, I had the opportunity to travel a little
bit. I discovered France, of which I had limited knowledge before starting my PhD, and set
foot in Switzerland, Germany, and even the US. However, the most amazing trips that I will
remember from my time in Grenoble are those simple, everyday discussions around lunch or
coffee break with students, postdocs and researchers from all over the world. Adriano, Alana,
Ana,Colin,Dipan,Francesco,Francisco,Helio,Jaqueline,Javier,Julia,Kike,Melinda,Nathalia,
Nicole, Raquel, Sudeep, Susmita, Thiago, Victor: thanks for all you have taught me in these
intercultural exchanges. I have crossed paths with some of you only for a short while, with
others for a longer interval; but each of you has left something to me. I now know a bit more
of geography, I learnt some Portuguese and even some Bengali; but especially, it feels like I had
v
never really perceived the vastness of the world before. I hope that the bonds that we have
developed will endure in the years to come.
Special thanks to my office-mates of “bureau 013”, Susmita, Victor, and Sudeep (even if for
a short time). Thanks for sharing the everyday madness (I will never forget the “jump out
of teenage”), the joys and frustrations of life at the lab. And speaking of Susmita, I cannot
omit mentioning Ritam: getting close to him, watching him as he grew up, started to walk and
to speak was the most unexpected and incredible experience associated with my PhD period.
Thanks to both Susmita and Ritam for their help in “releasing the pressure” during the toughest
phases of thesis-writing, and to them and Nirupam for “the best end of thesis-writing ever”.
I should not forget here the “friends of a lifetime”; in particular, thanks to Anna for being
therewithoutfailsincethetimesofhighschool. Wehavechangedinsomanywayssincetheday
we first met, and we hardly get to see each other more than a few times per year, but nothing
of that really matters, and time after time we find each other just as we used to do before. Also
thanks to Giuseppe, with whom I shared the undergraduate university studies; as we embarked
on analogous tracks, we have always been a constant reference and support to each other.
Last but not least, my thought goes to my family, without whom many things would have
been much harder. Thanks to my parents for their unconditional support, and to my sisters
for keeping my feet on the groud; thanks for your affection and esteem. And I owe a special
acknowledgement to my aunt for the numerous exchanges on the Grenoble-Milano line during
my PhD.
The last paragraph is for my grandparents, who will never read these lines. I was still little
whenNonnaMariucciapassedaway,andnotmuchmoregrown-upwhenNonnoPeppinofollowed
her; their memories are stored in the drawers of my childhood. Nonna Sara accompanied me for
alongerwhile,tillIfirstsetfootinGrenoble. Shewastheonewhotaughtmehowtoread,before
I started to go to school. She knew before me that I would eventually go on a PhD, but she
was not there to see it happen; I wish I were able to make that phone call to tell her “You were
right”. Nonno Ciccio was there till almost the end, till the first words of this manuscript were
written. I will always remember our discussions around dinner about telescopes and observing
sites, and his surprised expression when I once told him that I would stay at the Observatory
until late for work: “What can you possibly observe? It’s so cloudy!”
vi
Contents
Acknowledgements v
List of Tables xi
List of Figures xiii
1 Introduction 1
1.1 On the origin of the T Tauri case . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 A new class of young, variable stars . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 The T Tauri phenomenon across the spectrum . . . . . . . . . . . . . . . 3
1.1.3 T Tauri phase in the paradigm of low-mass early stellar evolution . . . . . 4
1.2 Disks around young stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Star formation in a nutshell . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Structure and physics of circumstellar disks . . . . . . . . . . . . . . . . . 9
1.3 Disk accretion in T Tauri stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.1 Magnetospheric accretion . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.2 Disk–locking? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.4 The manifold variability of T Tauri stars . . . . . . . . . . . . . . . . . . . . . . . 21
1.4.1 Variability on mid-term (days to weeks) timescales . . . . . . . . . . . . . 21
1.4.2 Variability on shorter and longer timescales . . . . . . . . . . . . . . . . . 23
1.4.3 The space-borne revolution in YSO variability studies . . . . . . . . . . . 23
1.5 Open issues in disk accretion from an observational perspective . . . . . . . . . . 25
1.5.1 Aim and outline of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . 27
2 The Coordinated Synoptic Investigation of NGC 2264 29
2.1 The young open cluster NGC 2264 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2 The CSI 2264 project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.1 Overview of the observing campaign . . . . . . . . . . . . . . . . . . . . . 31
2.2.2 CFHT dataset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3 CSI 2264: a synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4 Specific contribution from this thesis . . . . . . . . . . . . . . . . . . . . . . . . . 45
3 Mapping the different accretion regimes in NGC 2264 47
3.1 The colors of young stars at short wavelengths. . . . . . . . . . . . . . . . . . . . 48
3.1.1 The color loci of field stars in the SDSS system . . . . . . . . . . . . . . . 48
3.1.2 Colors and UV excess of young stars . . . . . . . . . . . . . . . . . . . . . 50
3.1.3 UV excess vs. different accretion diagnostics . . . . . . . . . . . . . . . . . 52
3.2 A UV census of the NGC 2264 young stellar population . . . . . . . . . . . . . . 54
3.2.1 New CTTS candidates in NGC 2264 . . . . . . . . . . . . . . . . . . . . . 54
3.2.2 Field contaminants in the NGC 2264 sample. . . . . . . . . . . . . . . . . 56
vii
CONTENTS
3.3 Derivation of individual stellar parameters . . . . . . . . . . . . . . . . . . . . . . 56
3.3.1 Individual A estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
V
3.3.2 Spectral types and effective temperatures . . . . . . . . . . . . . . . . . . 59
3.3.3 Bolometric luminosities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.4 Stellar masses and radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4 UV excess and mass accretion rates . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.1 Measuring the UV flux excess . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.2 From u-band excess luminosity to total accretion luminosity . . . . . . . . 68
3.4.3 Mass accretion rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.5 Accretion regimes in NGC 2264 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.5.1 The M˙ M relationship . . . . . . . . . . . . . . . . . . . . . . . . . . 72
acc ⋆
−
3.5.2 Accretion variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.5.3 Different accretion regimes/mechanisms . . . . . . . . . . . . . . . . . . . 82
3.5.4 Evolutionary spread across the cluster . . . . . . . . . . . . . . . . . . . . 84
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4 The UV variability of young stars in NGC 2264 89
4.1 A closer look at photometric variability in the CFHT sample . . . . . . . . . . . 90
4.1.1 Measuring the variability of CTTS and WTTS: light curve rms . . . . . . 92
4.1.2 Measuring the variability of CTTS and WTTS: Stetson’s index J . . . . . 95
4.2 The imprints of disk accretion in UV variability . . . . . . . . . . . . . . . . . . . 98
4.2.1 A comparison between UV excess and u-band variability . . . . . . . . . . 98
4.2.2 Time evolution on the r vs. u r diagram of the cluster . . . . . . . . . . 99
−
4.2.3 Exploring the color signatures of different physical scenarios . . . . . . . . 100
4.2.4 A global picture of color variability for different YSO types . . . . . . . . 103
4.3 A spot model description of YSO variability . . . . . . . . . . . . . . . . . . . . . 103
4.3.1 Formulation of the spot model . . . . . . . . . . . . . . . . . . . . . . . . 105
4.3.2 Implementation of the model . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.3.3 A global picture of spot properties for TTS in NGC 2264 . . . . . . . . . 109
4.3.4 The different nature of modulated variability for CTTS vs. WTTS . . . . 111
4.4 Timescales of variability for the accretion process . . . . . . . . . . . . . . . . . . 113
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5 The accretion–rotation connection in young stars 117
5.1 Photometric period determination . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.1.1 Period-search methods used in this study . . . . . . . . . . . . . . . . . . 118
5.1.2 Implementation of the period-search routine . . . . . . . . . . . . . . . . . 121
5.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.2.1 Period distribution for NGC 2264: CTTS vs. WTTS . . . . . . . . . . . . 124
5.2.2 Mass dependence? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.2.3 Are CTTS periods similar in nature to WTTS periods? . . . . . . . . . . 126
5.3 The accretion–rotation connection . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6 Conclusions and perspectives 133
6.1 Case of study: a brief recap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.2 Main points of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.2.1 Different accretion regimes coexist within the cluster . . . . . . . . . . . . 134
6.2.2 M˙ reflect a diversity in accretion mechanisms and cluster evolution . . 134
acc
6.2.3 Variability in young stars has a broadly assorted nature . . . . . . . . . . 135
viii
CONTENTS
6.2.4 Timescales of days dominate the variability of WTTS and CTTS . . . . . 135
6.2.5 Disks have an impact on the rotation properties of young stars . . . . . . 136
6.3 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Bibliography 139
Appendices 149
A List of referred publications 151
B Mapping accretion and its variability in the young open cluster NGC 2264:
a study based on u-band photometry 153
C UV variability and accretion dynamics in the young open cluster NGC2264179
Abstract 195
Résumé 197
Sommario 199
ix