An optical study of young stellar clusters [Elektronische Ressource] / put vorward by Da Rio Nicola

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An Optical StudyofYoung Stellar ClustersNicola Da RioMax-Planck-Institut fur AstronomieDissertation in Astronomysubmitted to theCombined Faculties of the Natural Sciences and Mathematicsof the Ruperto-Carola-University of Heidelberg, Germanyfor the degree ofDoctor of Natural SciencesPut forward byMaster Phys. Da Rio Nicolaborn in: Udine (Italy)thOral examination: December 13 , 2010An Optical StudyofYoung Stellar ClustersReferees: Prof. Dr. Thomas HenningProf. Dr. Ralf KlessenAbstractIn the first part of this thesis a new analysis of the Orion Nebula Cluster, one of themost studied star-forming regions in the Galaxy, is presented. Based on multi-bandoptical photometry and spectroscopy obtained with the Wide-Field Imager (WFI) at theESO/MPI 2.2-m telescope at La Silla Observatory, I study the systematic effects thatbias the derivation of stellar parameters of pre-main sequence (PMS) stars. I derive thenew H-R diagram of the entire region, and assign masses and ages to all the members.The age is found to be∼ 2−3 Myr, older than previously estimated. I also confirm thepresence of an age spread, and show how the found mass-age correlation canbe affected by the sample incompleteness and uncertainties in the evolutionary models.

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An Optical Study
of
Young Stellar Clusters
Nicola Da Rio
Max-Planck-Institut fur AstronomieDissertation in Astronomy
submitted to the
Combined Faculties of the Natural Sciences and Mathematics
of the Ruperto-Carola-University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
Put forward by
Master Phys. Da Rio Nicola
born in: Udine (Italy)
thOral examination: December 13 , 2010An Optical Study
of
Young Stellar Clusters
Referees: Prof. Dr. Thomas Henning
Prof. Dr. Ralf KlessenAbstract
In the first part of this thesis a new analysis of the Orion Nebula Cluster, one of the
most studied star-forming regions in the Galaxy, is presented. Based on multi-band
optical photometry and spectroscopy obtained with the Wide-Field Imager (WFI) at the
ESO/MPI 2.2-m telescope at La Silla Observatory, I study the systematic effects that
bias the derivation of stellar parameters of pre-main sequence (PMS) stars. I derive the
new H-R diagram of the entire region, and assign masses and ages to all the members.
The age is found to be∼ 2−3 Myr, older than previously estimated. I also confirm the
presence of an age spread, and show how the found mass-age correlation can
be affected by the sample incompleteness and uncertainties in the evolutionary models.
In the second part of this thesis, I carry out a research on the low-mass stellar pop-
ulation of the young stellar cluster LH 95 in the Large Magellanic Cloud, based on deep
optical photometry with the Advanced Camera for Surveys (ACS) onboard the Hubble
Space Telescope; the deepest ever obtained toward this galaxy, down to V ≃ 28 mag. I
isolatethePMSclusterpopulation,andderivethefirstextragalacticInitialMassFunction
(IMF) down to the subsolar regime. It shows a flattening below 1 M , in agreement with

theGalacticIMFonceresultsarecorrectedforunresolvedbinarity. Istudytheagedistri-
bution of LH 95, introducing a statistical method to derive average age and age-spreads
accounting simultaneously for unresolved binarity, differential extinction, variability, ac-
cretion and crowding of PMS stars. The best-fit solution for LH 95 suggests an age of
∼ 4 Myr with a gaussian age spread of σ∼ 1.2 Myr. Finally, I study the early-type high-
mass stellar population of the cluster, through ground based spectroscopy obtained with
the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the ESO/MPI 2.2-m
telescopeatLaSillaObservatory,andphotometryfromthe1-mtelescopeatSidingSpring
Observatory. The derived stellar masses are combined with my results on the low-mass
IMF of the cluster for the study of the most complete extragalactic IMF ever performed.
ivZusammenfassung
Im ersten Teil dieser Dissertation wird eine neue Analyse des Orion Nebula Cluster, eine
der am meisten studierten Sternentstehungregionen der Galaxis, pr¨asentiert. Basierend
auf optischer Photometrie und Spektroskopie in verschiedenen B¨andern, aufgenommen
mit dem Wide-Field Imager (WFI) am ESO/MPI 2.2-m Teleskop des La Silla Observa-
toriums, untersuche ich die systematischen Effekte, die die Ableitung stellarer Param-
eter von Vorhauptreihensternen (PMS) beeinflussen. Ich leite ein neues H-R Diagram
der gesamten Region her und ordne allen Mitgliedern Masse und Alter zu. Das Alter
wurde auf ∼> 2−3 Myr bestimmt, ¨alter als zuvor angenommen. Ich best¨atige die Ex-
istenz einer Altersstreuung und zeige, wie die vorherige Masse-Alter-Korrelation durch
Unvollst¨andigkeit des Samples und Unsicherheiten in den Entwicklungsmodellen beein-
flusst werden kann.
Im zweiten Teil der Dissertation fuhre¨ ich eine Untersuchung der massearmen Ster-
nenpopulation des jungen Sternhaufen LH 95 in der Großen Magellanschen Wolke durch.
Diese basiert auf tiefer optischer Photometrie mit der Advanced Camera for Surveys
(ACS) an Bord des Hubble Space Telescope, die tiefste bisher erreichte innerhalb der
Großen Magellanschen Wolk, mit Helligkeiten bis zu V ≃ 28 mag. Ich isoliere die PMS-
PopulationdesSternhaufensundleitedieersteextragalaktischeAnfangsmassenverteilung
(IMF) ab, die bis in den subsolaren Bereich reicht. Diese zeigt eine Abflachung unterhalb
¨von 1 M , in Ubereinstimmung mit der galaktischen IMF sobald die Ergebnisse fur¨ nicht

aufgel¨oste Mehrk¨orpersysteme korrigiert wurden. Ich untersuche die Altersverteilung von
LH 95 und fuhre¨ dabei eine statistische Methode ein, um Durchschnittsalter und Al-
tersstreuungenabzuleiten, diesimultannichtaufgel¨osteMehrk¨orpersysteme, differentielle
Extinktion, Variabilit¨at, sowie Akkretion und Crowding der PMS-Sterne einbezieht. Die
am besten passende L¨osung fur¨ LH 95aßtl¨ auf ein Alter von ∼ 4 Myr schliessen mit
einen Gauß‘schen Altersstreuung von σ ∼ 1.2 Myr. Abschließend untersuche ich die
massereicheSternpopulation desSternhaufen mit HilfevonerdgebundenerSpectroskopie,
die mit dem Fiber-fed Extended Range Optical Spectrograph (FEROS) am ESO/MPI
2.2-mTeleskopsdesLa SillaObservatoriumsaufgenommen wurde, sowiePhotometriedes
1-m Teleskops des Siding Spring Observatoriums. Die abgeleiteten Massen der Sterne
sind, kombiniert mit den Ergebnissen der Untersuchung der IMF der massearmen
des Sternhaufens, die kompletteste Untersuchung einer extragalaktischen IMF, die bisher
durchgefuhrt¨ wurde.
vAi miei genitori e alla nonna Olga
viTable of Contents
Table of Contents vii
Introduction 1
I The Orion Nebula Cluster 6
1 A Multi-Color Optical Survey of the ONC - The New HR diagram 7
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 The observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.1 Images and source catalogue . . . . . . . . . . . . . . . . . . . . . . 13
1.3.2 Absolute Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4 The Source Catalog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.5 The TiO spectrophotometric index . . . . . . . . . . . . . . . . . . . . . . 26
1.5.1 The Hα photometry . . . . . . . . . . . . . . . . . . . . . . . . . . 37
1.6 Analysis of the photometric colors of the ONC . . . . . . . . . . . . . . . . 45
1.6.1 Synthetic photometry . . . . . . . . . . . . . . . . . . . . . . . . . . 46
1.6.2 Constraining the intrinsic colors for ONC stars . . . . . . . . . . . . 51
1.6.3 Bolometric corrections . . . . . . . . . . . . . . . . . . . . . . . . . 55
1.6.4 Extinction law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
1.6.5 Mass accretion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
1.7 Analysis of stars with known spectral type . . . . . . . . . . . . . . . . . . 61
1.7.1 Spectral types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
1.7.2 Analysis of completeness . . . . . . . . . . . . . . . . . . . . . . . . 65
1.7.3 Spectral type-temperature relations . . . . . . . . . . . . . . . . . . 66
1.7.4 Derivation of reddening and accretion . . . . . . . . . . . . . . . . . 69
1.7.5 Systematic effects in deriving A . . . . . . . . . . . . . . . . . . . 73V
1.8 The H-R diagram of the ONC . . . . . . . . . . . . . . . . . . . . . . . . . 75
1.8.1 Completeness of the HRD . . . . . . . . . . . . . . . . . . . . . . . 80
1.8.2 Distribution in the mass-age plane . . . . . . . . . . . . . . . . . . 82
1.8.3 The age distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 86
1.8.4 The mass function . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
viiII The Young Cluster LH 95 in the Large Magellanic Cloud 93
2 The Complete IMF Down to the Subsolar Regime in the Large Magel-
lanic Cloud with HST/ACS Observations 94
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
2.2 Photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
2.2.1 Completeness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
2.3 The Pre-Main Sequence Population of LH 95 . . . . . . . . . . . . . . . . . 101
2.3.1 Field subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
2.3.2 Topography of the PMS Stars . . . . . . . . . . . . . . . . . . . . . 103
2.3.3 LH 95 Central region . . . . . . . . . . . . . . . . . . . . . . . . . . 106
2.3.4 Interstellar Extinction . . . . . . . . . . . . . . . . . . . . . . . . . 108
2.4 The PMS models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
2.5 The Initial Mass Function . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
2.5.1 Construction of the IMF . . . . . . . . . . . . . . . . . . . . . . . . 113
2.5.2 IMF model fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
2.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
2.6.1 Stellar Binarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
2.6.2 Metallicity effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
2.6.3 Variability of the IMF within LH 95 . . . . . . . . . . . . . . . . . 127
3 A New Method for the Assessment of Age and Age Spread of Pre-Main
Sequence Stars in Young Stellar Associations of the Magellanic Clouds130
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
3.2 Description of the evolutionary models . . . . . . . . . . . . . . . . . . . . 132
3.2.1 Synthetic Photometry . . . . . . . . . . . . . . . . . . . . . . . . . 133
3.3 Field subtraction refinement . . . . . . . . . . . . . . . . . . . . . . . . . . 136
3.4 Age determination of PMS stars . . . . . . . . . . . . . . . . . . . . . . . . 142
3.4.1 From isochrones to density distributions in the CMD . . . . . . . . 143
3.4.2 Derivation of stellar ages. The single star case. . . . . . . . . . . . . 149
3.4.3 Spatial variability of stellar ages . . . . . . . . . . . . . . . . . . . . 153
3.4.4 Age of the entire cluster . . . . . . . . . . . . . . . . . . . . . . . . 156
3.5 Age spread of PMS stars in LH 95 . . . . . . . . . . . . . . . . . . . . . . . 159
3.5.1 Verification of an age spread . . . . . . . . . . . . . . . . . . . . . . 159
3.5.2 Evaluation of the age spread . . . . . . . . . . . . . . . . . . . . . . 162
3.5.3 Luminosity-dependence of the age and age spread . . . . . . . . . . 165
3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
4 The high mass population of LH 95 169
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
4.2 The Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.3 Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
4.4 Spectral classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
viii4.5 The H-R diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
4.6 Discussion on the derived ages and masses . . . . . . . . . . . . . . . . . . 183
4.6.1 Stellar multiplicity . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
4.6.2 Evidence for an Age Spread . . . . . . . . . . . . . . . . . . . . . . 186
4.7 The high-mass IMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
4.7.1 The relation between the most massive star and the cluster mass. . 191
Conclusions 193
Acknowledgements 199
ixIntroduction
The star formation process determines the conversion of gas to stars, and the outcome
are stars with a range of masses. The distribution of the stellar masses in a given volume
of space in a stellar system at the time of their formation is known as the Initial Mass
Function(IMF)ofthesystem(Salpeter,1955;Scalo,1986;Kroupa,2002;Chabrier,2003).
Together with the time modulation of the star formation process, known as the star for-
mationrate, itdictatestheevolutionandfateofstellarsystemsuptoentiregalaxies. The
vast majority of stars are not formed in isolation, but as part of stellar associations and
clusters (Lada & Lada, 2003), where hundreds or even thousands of stars along the entire
mass spectrum are found, from the most massive OB-type members, down to low-mass
<stars (0.08 < M/M 2) and Brown Dwarfs (M ≤ 0.08 M ). One of the important
⊙ ∼ ⊙
debates concerning the star formation within a cluster is whether the process takes place
rapidly, on dynamical time-scales, or is a quasi-static process in which protostellar cores
take many free-fall time-scales to contract (e.g., Tan et al., 2006; Elmegreen, 2007). The
observational study of young stars in a star forming region is fundamental to improve our
understanding of such problem, constraining the rapidity with which stars are formed. In
fact, whereas the high-mass population of these regions is the direct sign of the youthful-
ness of these systems, the low-mass pre-main sequence (PMS) members preserve a record
of the complete recent star formation history of the star-forming region over long periods,
since their evolution towards the main–sequence is extremely slow and can last for many
tens of Myr. Moreover, young (few Myr old) clusters are usually not dynamically evolved
nor have had time to disperse; thus the mass distribution of their members reflects that
at the time of formation, i.e., their IMF. As a consequence, the study of the low-mass
PMS population in young clusters is fundamental to improve our understanding of the
star formation processes and the IMF.
In the Galaxy, observational studies of young stars have been carried out throughout
the last decades in several stellar systems, from nearby, loose stellar associations, such as
1