Optical and Infrared Photometric Study of Pre-Main Sequence in Young NGC 7419

A Dissertation Submitted in Partial Fulfillment of the Requirements for the Award of the Degree of

Master of Philosophy in Physics

by

Vrinda Mukundan (Reg. No. 1235209)

Under the Guidance of K T Paul Professor

Department of Physics

CHRIST UNIVERSITY BANGALORE, INDIA December 2013

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Approval of Dissertation

Dissertation entitled Optical and Infrared Photometric Study of Pre-main Sequence Stars in Young Open Cluster NGC 7419 by Vrinda Mukundan, Reg. No. 1235209 is approved for the award of the degree of Master of Philosophy in Physics.

Examiners:

1.

2.

3.

Supervisor(s):

Chairman:

Date: ______

Place: Bangalore

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Declaration

I Vrinda Mukundan hereby declare that the dissertation, titled Optical and Infrared Photometric study of Pre-main Sequence Stars in Young Open Cluster NGC 7419 is a record of original research work undertaken by me for the award of the degree of Master of Philosophy in Physics. I have completed this study under the supervision of Dr K T Paul, Associate Professor, Department of Physics.

I also declare that this dissertation has not been submitted for the award of any degree, diploma, associate ship, fellowship or other title. It has not been sent for any publication or presentation purpose. I hereby confirm the originality of the work and that there is no plagiarism in any part of the dissertation.

Place: Bangalore Date: Vrinda Mukundan Reg No. 1235209 Department of Physics Christ University,Bangalore

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Certificate

This is to certify that the dissertation submitted by Vrinda Mukundan (Reg. No.1235209) titled ‘Optical and Infrared Photometric study of Pre-main Sequence Stars in Young Open Cluster NGC 7419’ is a record of research work done by her during the academic year 2012-2013 under my supervision in partial fulfilment for the award of Master of Philosophy in Physics.

This dissertation has not been submitted for the award of any degree, diploma, associate ship, fellowship or other title. It has not been sent for any publication or presentation purpose. I hereby confirm the originality of the work and that there is no plagiarism in any part of the dissertation.

Place:Bangalore Date: Dr K T Paul Associate Professor Department of Physics Christ University, Bangalore - 29

Dr George Thomas C Professor & Head Department of Physics Christ University, Bangalore - 29

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Acknowledgements

It gives me great pleasure to express my sincere gratitude to my guide, Dr K T Paul, for the constant support and guidance that he has provided all along my work. This work would not have been possible without his active support.

I am deeply indebted to Dr Annapurni Subramaniam (Indian Institute of Astrophysics) for the fruitful discussions, valuable suggestions and support that shehas provided for my work. I have been benefited by her constructive suggestions and inspirations.

The support extended by Ms Aadara B (IISER,Trivandrum), for carrying out the study is gratefully acknowledged.

I thank to Dr George Thomas C (Associate Dean & Head of the Department of Physics), Dr Bubbly S G (M.Phil Co-ordinator) and all other faculty members of the Department of Physics for help and encouragement for carrying out this work.

I extend my heartfelt gratitude to my parents and my husband for being strong pillars of support all through my endeavours.

This study makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. iv

Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration

This research has made use of the WEBDA database, operated at the Department of Theoretical Physics and Astrophysics of the Masaryk University

Place: Bangalore Vrinda Mukundan Date:

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Abstract

The embryonic stage of development of stars has been a field of astrophysics which still pose many unresolved queries. The problem is largely due to the solitary nature of embryonic stars. Even the largest optical telescopes were not able to reveal many details about such stars as they are formed deep inside molecular cloud thorough which optical wavelength cannot pass through. Forbidding distances to such stars is an added problem. However, recently, this field is slowly turning into observational science. Even though optical radiations fail to reach the destination, longer wavelength infrared (IR) radiations can penetrate through such dusty region. These infrared radiations have been known for decades. But it is only during very recent times that infrared telescopes that have very high sensitivity and resolution came in to scenario. So now the astronomers are able to observe these stellar embryos. Studies about this stage of formation can help us to solve many questions like how the birth of a star takes place, how these stars accrete matter into it from surroundings, when will this accretion stops, why do some stars have planetary systems around them etc. Current study is an attempt to look for the presence of these embryonic stars in an open cluster using infrared data and to get more details on nature of those stellar embryos.

We investigate the properties of young stars and their circumstellar disks in young open cluster NGC 7419. The pre-determined V magnitude and B magnitude of these stars are combined with infrared data from 2MASS J,H, K and 4 band WISE data. The color-color diagrams are made using these 9 bands to identify young stellar objects. The individual SED fitting is done for these identified stars and parameters like their age, mass, temperature, disk radius and disk mass are estimated. These are used to comment on the evolutionary stage of the pre-main sequence stars and their circumstellar disk.

Dissertation has been divided into five chapters. Chapter 1 gives a general introduction to the work done. Literatures referred are explained in Chapter 2. Data used in the current study and methodology adopted is described in chapter 3. Current work done on the young stellar objects in cluster NGC 7419 and results obtained from the study are explained in Chapter 4 and Chapter 5 respectively.

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Contents

Approval of Dissertation i Declaration ii Certificate iii Acknowledgements iv Abstract vi List of Tables ix List of Figures x List of Abbreviations xi

Chapter 1 : Introduction 1 1.2 Star formation …………………………………………………………...... 2 1.2 Circumstellar Disk ………………………………………………………...... 4 1.2.1 Transition Disk…………………………………………………………...... 5 1.3 Classification of Young Stellar Objects (YSOs)…………………………...... 6 1.4 Stellar Clusters……………………………………………………………...... 8 1.4.1 Globular Clusters………………………………………………………...... 8 1.4.2 Open Clusters…………………………………………………………...... 9 1.5 Photometry……………………………………………………………………...... 9 1.6 Thesis Objective……………………………………………………………...... 10

Chapter 2 : Literature Review 12 2.1 Previous Works………………………………………………………………...... 13 2.2 NGC 7419……………………………………………………………………...... 17

Chapter 3 : Data and Methodology 20 3.1 Infrared Data…………………………………………………………………...... 21 3.1.1 Two Micron All Sky Survey(2MASS)………………………………...... 21 3.1.2 Wide Field Infrared Survey Explorer (WISE)…………………………...... 21 3.2 Optical Data…………………………………………………………………...... 22

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3.3 Methodology…………………………………………………………………...... 22 3.3.1 Cross matching of Optical and Infrared Data………………………...... 22 3.3.2 Color-Color Diagrams (CCDs)……………………………………...... 24 3.3.3 Spectral Energy Distribution (SED)…………………………...... 24

Chapter 4 : Results and Discussion 27 4.1 Classification based on WISE color-color diagram…………………………...... 28 4.2 Classification based on Spectral Energy Distribution………………………...... 31 4.3 Parameter Estimation using SED ……………………………………………...... 34 4.4 Parameter Plots………………………………………………………………...... 39 4.4.1 Age Vs Disk mass...... 39 4.4.2 Age Vs Disk accretion rate...... 40 4.4.3 Age Vs Mass of central source...... 41 4.4.4 Disk mass Vs Disk accretion rate...... 41

Chapter 5 : Summary and Conclusion 43 5.1 Summary of work done………………………………………………………...... 44 5.2 Future Scope…………………………………………………………………...... 47

References 48

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List of Tables

Table Title Page no

3.1 Cross matched list………………………………………………...... 23 4.1 Estimated parameters of Class II sources………………………...... 35 4.2 Estimated parameters of Class II sources……………………...... 36 4.3 Estimated parameters of Transition Disk sources………………...... 37 4.4 Estimated parameters of Transition Disk sources………………...... 37 4.5 Estimated parameters of Class II / Transition Disk sources……...... 38 4.6 Estimated parameters of Class II / Transition Disk sources……...... 38 5.1 Classification of sources based on CCD and SED………………...... 46

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List of Figures

Figure Title Page No 1.1 Schematic representation of various processes involved in star 3 formation .…………………………………………………………….. 1.2 An artistic view of Transition Disk…………………………………… 5 1.3 SEDs of different classes of YSOs…………………………………… 7 2.1 NGC 7419…………………………………………………………….. 17 3.1 SEDs in presence and absence of circumstellar disk…………………. 25 4.1 WISE band 1, 2 and 3 color-color diagram showing the distributionof 29 Class I and Class II objects………………………………………… 4.2 WISE band 1, 2 and 4 color-color diagram showing the distribution of 30 Class I and Transition disk objects……………………………….. 4.3 SEDs of Class II objects……………………………………………… 32 4.4 SEDs of Transition Disk objects…………………………………...... 33 4.5 SEDs of sources classified as Class II/Transition Disk objects………. 33 4.6 Age Vs Disk Mass of Class II and Transition Disk sources………….. 39 4.7 Age Vs Disk Accretion Rate of Class II and Transition Disk 40 sources………………………………………………………………… 4.8 Age Vs Mass of Central Source of Class II and Transition Disk 41 sources………………………………………………………………… 4.9 Disk Mass Vs Disk Accretion rate of Class II and Transition Disk 42 sources ………………………………………………………………..

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List of Abbreviations

2MASS Two Micron All Sky Survey

χ2 Chi Square Value

Mʘ Solar Mass

CCD Color-Color Diagram IR Infrared

NASA National Aeronautics and Space Administration

PMS Pre-main Sequence

SED Spectral Energy Distribution

WISE Wide Field Infrared Survey Explorer

YSO Young Stellar Object

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Chapter 1 Introduction

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1.1 Star formation

Star formation starts from interstellar clouds of gas and dust which is mainly composed of molecular hydrogen. These clouds will not be having a uniform density throughout. There will be regions having density as high as 10,000 molecules per cubic centimetres. At certain point of time, such high density regions of molecular clouds will become gravitationally unstable and will begin to fall under its own gravity. During this tightening process, they shrink into lesser and lesser volume and this causes the pressure, density and temperature inside the clump to rise to extremely high values. As the temperature increases, the clump begin to radiate light and then a protostar is said to be born.

A protostar can be defined as a prestellar object that is hot enough to radiate infrared radiation but not hot enough to generate energy by nuclear fusion. Protostars thus mark the first stage of stardom. A feature of protostars is that, during its formation, it accumulates mass into its core from the surrounding region of gas and dust. This process is known as accretion. Along with this accretion process, there occurs powerful outflow of material from either pole of stars which is called as bipolar jets. Thus two characteristic features of protostars are accretion and bipolar jets.

Density of protostars will be gradually increasing during the shrinking procedure. During this time, the material which is getting accreted rotates slowly around the core. As the radius of the protostar is decreasing due to the shrinking process, the velocity of rotation of material will gradually increase. This occurs for obeying the law of conservation of angular momentum, which is the product of rotational velocity and radius. The material rotates faster as it comes closer to the core of star. Some of the material in the rotating region will be rotating at very high velocities so that they fall into an orbit rather than falling into the star. Matter gets into orbits of varying sizes depending on its velocity and finally forms a disk. This is called as circumstellar disk.

Material will be accreted into the star from this surrounding. As time passes this disk gradually disappears and the accretion stops. In this stage star is called as a pre-main sequence star (PMS). Protostars and PMS stars are together known as young stellar objects Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 2

(YSO).

The force of gravity continues to pull material towards the centre region of the star and this result in an increase in temperature in core region. Finally when the core attains a temperature as high as millions of degree Kelvin, nuclear fusion reactions will be initiated in the core. This marks the beginning of main sequence phase of stellar life time.

Figure 1.1 Schematic representation of various processes involved in star formation. (a) Dense region of molecular clouds. (b) Denser region undergoing gravitational collapse under its own gravity (c) Protostar formation (d) T Tauri stage characterised by bipolar outflows and protoplanetary disk (e) Disk gradually dissipates and PMS stars forms (f) In some cases a left over debris disk with planets formed within them will be retained around the star (Picture Courtesy: Spitzer Science Centre)

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1.2 Circumstellar disks

A simple contraction of the molecular cloud cannot directly result in the formation of protostars. These clouds will be having some net rotation. During the contraction process, rotation gradually grows and causes conservation of angular momentum. This hurriedly spinning core of the cloud then flattens into a spinning disk called circumstellar disk.

During the first few million years of the PMS stage of star, star is called as T-Tauri stars. This marks the first stage of a star where it becomes optically detectable. There will still be a disk of dust and gas around it, and star will be showing bipolar jets also. Disk in this stage is called as protoplanetary disk. Gradually this protoplanetary disc disappears leaving behind a totally bear PMS star. In certain situations, it is observed that some large sized materials continue to rotate around the core of star and the disk that they constitute is termed as debris disk. Many bodies of our solar system, like planets and asteroids are believed to be originated from such disks.

Presence of circumstellar disk around PMS was first determined from the infrared IR observations which showed excess in the near-IR region when compared with stellar photoshperic flux (Mendoza 1968). Light emitted by the star will be absorbed by the material/dust present around it. This absorbed light will be reprocessed and will be re-emitted by the circumstellar dust in longer wavelengths. It is this re-emission which is causing IR excess.

Circumstellar disk plays a very important role in the process of star formation. They act as a source of material which will be accreted into the star during its formation stage. Also for understanding the phenomenon of planet formation, a study of circumstellar disk has to be done as they are believed to be the spot of planet formation (Lada and Wilking 1984; Wilking et al. 1989)

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1.2.1 Transition Disk

Transition disks are circumstellar disks with large inner holes. Strom et al. (1989) defined transition disks as “disks whose inner regions are relatively devoid of distributed matter although the outer region still contains substantial amount of dust”. They observed some stars showing very small excess in the near-IR region but large excess in mid and far–IR regions. His team proposed that these objects may be in a “transition phase from very massive optically thick structures to very low mass, tenuous, perhaps post-planet-building” structures. Weak emission in the near-IR region indicates the absence of optically thick material in the inner region and excess emission shown in mid and far-IR region points to the presence of optically thick disk in the outer region. Studies about these transition disk have improved to a great extent after National Aeronautics and Space Administration's (NASA) which was launched in 2003 (Werner et al. 2004). Nomenclature “transition disk” was first adopted by Calvet et al. (2004) for his studies based on the data obtained from Spitzer. How the material in the inner region of the disk gets cleared is still not a completely solved problem. Importance of various proposed mechanisms like photoevapouration, planet formation etc still remains to be established.

Figure 1.2 An artistic view of transition disk (Picture Courtesy: Institute of Astronomy, University of Hawaii)

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1.3 Classification of Young stellar Objects (YSOs)

The different stages of evolution of YSOs are characterized by the distribution of energy across electromagnetic spectrum, known as the spectral energy distribution (SED) (Lada and Wilking 1984; Adams et al. 1987; Andre et al. 1993). Lada & Wilking (1984) classification

was based on the slope, 'a', of the constructed energy distribution [i.e. log (λFλ) Vs log (λ)]. The slope was defined as (1.1) Classification of YSOs based on the different stages of evolution is as follows:

 Class I: For these sources, 0 < a < 3. They have a positive slope for SED over near to mid-IR region. These class I sources will be deeply embedded in the molecular clouds and are also characterised by bipolar outflows. Here the mid-IR emission which comes from the dust wrap is leading over the stellar black body emission. During this stage core of the star will be more stable and bipolar outflows are observed. Class I phase exist for about 100,000 years.

 Class II: For class II source -2 < a <0. These stars are believed to have circumstellar disks as the width of SED of the disk component is much wider than that expected from a single temperature black body. The circumstellar disk of these sources usually emits in 3-12 μm region (or longer) due to the heating of the dust by the central star (Wilking et al 1989). This causes a near and mid-IR excess. Cloud around the star starts to clear out during this phase and accretion slows down. Class II stage last for approximately 1 million years.

 Class III: These sources are characterised by a slope -3 < a < -2. For such stars circumstellar disk would have almost totally degenerated and the SED for the stars will be similar to that of a black body spectrum. Star would have completely stopped accretion. Duration of this phase varies between 1 to 10 million years

Andre et al. (1987) later added one more class of YSOs named as Class 0. Their defining feature is that they are very young and are deeply embedded in their parent molecular cloud. Property of Christ University. They are extremely faint in optical and near IR. Most of their energy is emitted in the Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 6

submillimeter range. Terebay et al. (1993) interpreted this submillimeter excess as that their circumstellar disk is approximately 20 times more massive than the central core for a solar mass star. Figure 3 shows SEDs of different classes of YSOs.

Figure 1.3 SEDs of different classes of YSOs

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1.4 Stellar clusters

Stellar clusters comprise group of stars which are held together by the force of gravity. They are formed when the molecular cloud contracts and fragments, and form an association of stars. All of the stars in a cluster are formed at about the same time and from same molecular cloud which implies that they will be having same age and same initial composition. Still differences can be seen among the stars in a cluster and this must be due to the difference in mass. Thus cluster involves stars in wide range of masses and thus acts as wonderful laboratories for studying stellar evolution. Clusters are also vital tracers of Milky Way and other parent . Contemporary structure and theories related to the origin and evolution of the galaxies can be understood in a better way by analysing the clusters.

Clusters are mainly of two types; open clusters (also known as galactic clusters) and globular clusters.

1.4.1 Globular Clusters

Globular clusters consist of mostly aged stars and have a spherical appearance. Number of stars in such clusters can vary from 105 to 106. Here stars are held tightly together to each other and it is this strong gravitational force that exists between the constituent stars helps the globular clusters to survive for billions of years. Stars are tightly packed at the core of the cluster and this density gradually decreases outwards.As they are very old clusters (studies suggests that their age may be around 11 billion years) star formation process in them would have almost completely stopped.So only old stars are supposed to be found here. These clusters are thought to be one one the oldest objects in the universe. Globular clusters are concentrated on the halo of the . NASA’s Hubble Space Telescope has discovered about 150 globular clusters in the Milky Way Galaxy. Messier 13, a cluster in the of Herules, is an example of galactic clusters.

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1.4.2 Open Clusters

Open clusters generally involves 100 to 1000 stars that are formed from the same molecular cloud and loosely held together by gravitational force, unlike the globular clusters in which stars are tightly held together by force of gravity.These clusters are much younger in age, usually less than 1 gigayear. . Some open clusters are quite small and some are large. As the stars are not crowded together in such clusters, they usually have an open, transparent appearance.

Also these clusters are found in spiral and irregular galaxies where active star formation takes place. They are also referred to as galactic clusters as they are found exclusively in the Milky Way galaxy. A well known example for open clusters is the cluster Pleiades, found in the constellation of Taurus. Due to the loose nature of satars in the cluster, open clusters are very much vulnerable to disruption, which can occur due to the close encounters with other clusters and molecular clouds during their orbiting period around the galactic centre.

Stars in the open cluster are formed from the same molecular cloud. They all are more or less at same distance from Earth and roughly of same age. As they have the same source of origin, their chemical composition also should be the same. But still there are differences in the apparent magnitudes of stars in the same cluster and this should only due to difference in their masses. This property makes open clusters very useful for studying stellar evolution process as when one star is compared to another in the same cluster; many of the variable parameters are fixed. Unlike globular cluster, they are at much closer distance to solar system and so are ideal for study.

1.5 Photometry

Only information that we can obtain about a star is the light reaching us from them. This star light has to be carefully analysed to determine the features and properties of a star. Photometry deals with measuring the flux emitted by celestial objects. This gives us information about the amount of energy emitted by a body over different wavelength of electromagnetic spectrum. Property of Christ University.

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Radiation from distant celestial objects falls on the telescope which will be filtered with optical filters. These filters will be allowing only certain range of frequencies to pass through them and the rest of the frequencies will be rejected. Thus amount of energy emitted by the body at that particular range of wavelength can be determined.In astronomy, different kinds of filters are used to accentuate light in a particular wavelength region. Most commonly used one are called the ultraviolet (U), blue (B), and photovisual (V) filters. This is known is broadband UBV or Johnson photometry. The measured flux helps to determine many features of the observed body like its total energy output, temperature, size and similar physical properties.

As described earlier, photometry is concerned with the measurement of fluxes. The brightness of a star is usually expressed in terms of quantity called magnitude. If F is the measured flux for a particular wavelength of a star, then magnitude m is define as

(1.2)

So for brighter objects flux emitted will be more and hence magnitude will be less, vice versa for fainter objects. The difference in magnitude measured at two different wavelengths for a star is known as color of the star. If m1 and m2are magnitudes corresponding to fluxes F1 and F2 then color is calculated as (1.3)

Usually magnitude measured at longer wavelengths will be subtracted from the value measured at shorter wavelength. So a high value of color indicates a larger amount of flux emitted at longer wavelength. Blue coloured stars emits strongly in the shorter wavelengths so they have negative values for color index and red coloured stars have positive color index.

1.6 Thesis Objective

The aim is to investigate the properties of young stars and their circumstellar disks in open cluster NGC 7419. Optical data for the cluster is available. Study will make an attempt to combine these optical data withProperty near and of mid Christ infrared University. data. Mid infrared data is provided by Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 10

NASA’a Wide Field Infrared Survey Explorer mission. Many studies have not been done using these WISE data, as it was very recently released. Inclusion of infrared will help in the reliable detection of circumstellar disks around the stars. Parameters of the stars and circumstellar disks will be estimated and these will be used to comment on the evolutionary stage of stars.

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Chapter 2 Literature Review

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2.1 Previous Works

Recognition of young stellar objects as members of stellar clusters has been a topic of several researches in the past. Photometric methods have been used widely for the reliable detection of YSOs in the clusters. Circumstellar disks surrounding pre-main–sequence (PMS) stars of solar and intermediatemass have been known and studied, firstin a roundabout way, through theirvague emission in the near and far infrared and at millimeter wavelengths, and in recent times by direct imaging in the optical, near infrared and millimeter. Such studies have thrown much light to the mystery about star formation process. Current chapter reviews some literatures which are concerned with the study of pre main sequence stars in stellar clusters.

Study about the cluster M17 by Hanson et al. (1997) resulted in the discovery of a population

of YSOs of very high mass, in the range 5-20M ʘ. They have used optical and NIR spectral classification for the determination of masses. They claim the presence of circumstellar disks around these massive YSOs by observing the NIR excess along with other techniques like optical veiling. Considering the disk life time of other clusters, M17 was categorised as avery young cluster, with an age less than 1 Myr.

NGC 2024 was studied by Haisch et al. (2001). Mid-infrared data were combined with JHKL photometry and SED of all detected sources was constructed. The main aim of the study was to investigate the nature of YSOs in the cluster and to study about circumstellar disk. Study detected one class I, 28 Class II and five Class III sources. Study proved that JHKL color- color diagrams are very much efficient in detecting stars with circumstellar disks. Circumstellar disk fraction for the cluster was found to be ~85%+/-15%. This proved that earlier studies which declared that the cluster is formed with disks and these disks still exist in the present time.

Star formation region of the cluster NGC 7129 was studied by Muzerolle et al. (2004). Multiband Imaging Photometer for Spitzer (MIPS) and Infrared Array Camera (IRAC) data has been used. Color-color diagrams and SED of some 60 stars are taken. A significant population of PMS has been found. Cluster was found to contain stars belonging to Class 0, I,

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II and III evolutionary stages. Three class II candidates where showing evidences for inner disk clearing.

Open cluster NGC 225 was studied by Subramaniam et al. (2006a) using optical UBV data and Two Micron All Sky Survey (2MASS) JHK data. Study was done to detect the presence of PMS in the open cluster region. About 15 stars which showed near infrared (NIR) excess were classified as PMS. The above also revealed that many of the upper main sequence stars (MS) has not actually reached main sequence as they showed near infrared (NIR) excess. The age of the cluster was found to be less than 10 Myr and it is declared as a very young cluster.

A combined X-ray-optical-infrared study of the rich young cluster NGC 6530 was done by Damiani et al. (2006). 333 stars in the cluster were found to have optical-IR excess. The cluster has been previously studied using CHANDRA -ACIS I observation. The current study was able to find out 196 stars with optical-IR excess which the previous study failed to detect. SED of some of these objects showed enormous excess which indicated the presence of circumstellar disks with very small inner holes and high accretion rate. Study concluded that in the northern region of the cluster star formation has been taking place for a long period of time, than that of the cluster centre. In the central region, most massive stars were found.

Near and mid IR photometry using Spitzer Space Telescope of around 300 members of the cluster IC 348 was done by Lada et al.(2006). Ground based optical and NIR photometry was also used for the study. The space and ground data were then used together for the construction of optical-IR SEDs of the cluster members. The total frequency of the disk bearing stars in the cluster was determined as 50% ± 6% by observing the IR excess between 3.6 and 8.0 µm. Frequency of stars having optically thick disks and optically thin disks were found out separately. Large fraction of stars was classified as classical T-Tauri stars as they posses strong, optically thick disks and only very few stars were founf to be diskless.

Hernandez et al. (2007) studied σ Orionis cluster. Using optical and infrared color magnitude diagrams 336 stars were identified as the members of the cluster. Classification of stars as non-excess stars, stars with optically thick disk, class I stars and stars with evolved disk was done based on the slope of SED. 7 transition disk candidates were also identified. Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 14

Disk around stars in the cluster NGC 2362 was done by Dahm et al. (2007). SEDs were constructed for the candidate members using optical, NIR and Spitzer measured fluxes. 47 stars were observed to posses IR emission in excess as compared to pure stellar photospheres. The study set an upper limit of ≈7 % ± 2 % for stars with primordial disk. As no star with

mass greater than 1.2 Mʘexhibited significant IR excess emission, the study proposed that the presence of circumstellar disks is strongly depending on mass. The disk bearing stars were

having mass in the range 0.2 to 1.2Mʘwith majority having mass between 0.8 to 0.3 Mʘ.

Balog et al. (2007) did Spitzer/IRAC survey of NGC 2244. Study used wavelengths 24 µm, 8 µm and 36 µms and detected large amount of cool dust visible only in 24 µm. Using mid IR color color diagram they identified 337 Class II and 25 Class I objects out of 1084 objects. When 24 µm data was included 213 Class II and 20 Class I sources were identified.

Hernandez et al. (2008) made a study of γ Velorum cluster. Spitzer space telescope observations were used for the study of 579 candidate members of the cluster. Optical and 2MASS photometry were employed. IR excess were observed for 29 stars. Five A-type stars and five solar type stars were reported to have debris disks. One solar type stars was found to have a very large IR excess which indicates the presence of a very massive circumstellar disks. There were 17 low mass stars with disks, and these disks were showing a range of properties. YSOs in the cluster were classified into three different classes by using IR excess at 5-8 µm, and the SED slopes. Nine Class II stars, seven stars with evolved disk, and one transitional disk object were identified. As the disk frequency observed in these cluster were much less as compared to other young clusters, study proposed that there may be occurring very rapid disk dissipation or the age of the cluster may slightly more than the proposed age, i.e. 5 Myr.

Caramazza et al.(2008) studied whether star formation process depends on the environment, by observing the PMS stars in the cluster NGC 1893, a young cluster in the outer part of the galaxy. According to theoretical studies, environment for the star formation process in the outer galaxy is less contributory to the process as compared to the inner part. In this study also IR observations obtained using Spitzer Space Telescope is used to look for the presence of stars with IR excess. By analyzing color-color diagrams, 242 stars were identified to have disks and there were 7 ClassProperty 0/I protostars. of Christ Masses University. of these stars were also determined by Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 15

employing this IR data. They have set an upper and lower limit for the masses of these PMS

stars as 28-46M ʘ and 1.4-3M ʘrespectively. 3% of the candidate members were Class 0/I protostars. So from their studies they have concluded that intense star formation process actively takes place in the cluster NGC 1893 even in spite of the so called unfavourable conditions exixting in the outer galaxy.

Sung et al. (2009) did the mid-infrared photometric study of the young open cluster NGC 2264 using images obtained from SPITZER and MIPS. Classification of the young stellar objects using color-color diagram was done and the result was compared with classification done on the basis of the slope of SED. Most of the disked stars were found to be class II objects. Subclustering of class I stars were detected. Study also revealed the presence of stars with transition disks.

Investigation of the properties of YSOs of open cluster NGC 6823 was done by Riaz et al. (2011). Around 80 percent of the stars in the cluster were showing Class III classification and only 20 belonged to Class I/Class II group. The PMS mainly includes of young disc sources with ages between ˜1 and 5 Myr, and at lower masses of approximately 0.1-0.4 M . Optical ʘ VRI and near infrared photometric data has been used for the study

Membership and fundamental parameters of the young open cluster NGC 1893 was investigated by Prisinzano et al. (2011). A multi wavelength approach was adopted for the study which extended from X-ray to NIR. 1061 members were found to have circumstellar disks and were classified as Class II YSOs. 415 candidate members were stars without disk. Many parameters of the cluster like age, distance etc, were determined. Study concluded that NGC 1893 includes a rich population of young stars with general properties similar to those found in young clusters.

Majaess et al. (2012) studied a cluster of young stellar objects (J2000 02:54:31.4 +69:20:32.5). 2MASS and Wide |Field Infrared Survey Explorer (WISE) photometry was used. 19 class I and 21 class II stars were found to lie at a distance < 3 arcmin from cluster center. 11 class I and 31 class II objects were detected at a distance > 3 arcmin.

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Chameleon I star forming region was surveyed by using HST by Robberto et al. (2012). 31 T- tauri stars were detected. Optical, IR and sub-mm data were compiled from literatures to build the SED for 19 objects. Using online SED fitting tool, many stellar and disk parameters

were determined. Most of the sources were found to have mass between 1.7-0.12 Mʘ. Age ranges between 0.5-5Myr. Mass accretion rate was found to decrease with stellar age and increases with stellar mass. Mass accretion rate was found to follow the same scaling relation 2 as proportional to M ʘfound in other PMS clusters. Some outliers were also found which found to have dissipated thei circumstellar disk in a very shorter time scale.

2.2 NGC 7419

Figure 2.1 NGC 7419; an open cluster in the constellation of (Picture Courtesy: NASA)

A star catalog is a kind of categorisation of stellar objects. The classification may be based on some common properties that they share like their origin, way of detection etc. Different kinds of catalogs are currently used in astronomy. General Catalog of Nebulae and cluster of stars was prepared by astronomer Johan Ludvig Emil Dryer. It was first published in 1888 and was later modified in 1895 and 1908. Here classification is done on the basis of right ascention values of the stellar objects. New General Catalog (NGC) is a revised version of General Catalog. It includes about 7840 objects. These NGC numbers are now widely used in astronomy for identification of stellar objects.

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NGC 7419 is an open cluster that is present in the constellation of Cepheus. It has a right ascension and declination of 22:54.3(hours:minutes) and +60:50(degrees:minutes) respectively and with a dimension of about 3.4’ x 3.4’.

There are many conflicting reports about the distance to NGC 7419. Based on studies done using RI photometry, Blanco et al. (1955) established that the cluster should be at a distance of 6 kpc. But distance established by Vande Hulst et al. (1954) was only half as that of this value; only 3.3 kpc. Beauchamp et al.(1994) claims a distance of 2 kpc and Subramaniam et al.(2006b) establishes a distance of 2.9 kpc. Regarding the age also, there are many discripencies. 7419 was established as a cluster of 40 Myr by Bhatt et al.(1993) where as Beauchamp et al. (1994) claimed a much younger age of 14 Myr. Subramaniam et al. (2006b) estimated its age to be in a range of 20-25 Myr. A similar value was established by Joshi et al. (2008) as 22.5 +- 3 Myr.

Study of NGC 7419 by Subramaniam et al. (2006b) was based on optical and near-IR photometry. 83 stars (42%) were found to have an NIR excess. They were identified as intermediate mass PMS stars. Age of these stars was found to be between 0.3 and 3 Myr whereas cluster was estimated to have an age of 25 Myr. These two results are contradicting. One possible explanation is that cluster may have gone through two different episodes of star formation, one at 25 Myr and other at 3 Myr. 83 stars involved in these study may be belonging to the 3 Myr star formation period. Circumstellar disc was detected around 25 stars through the analysis of slit spectra. Study also raised the possibility of finding very high mass PMS stars which can be as young as 0.3 Myr. Multiwavelength study of NGC 7419 conducted by Joshi et al. (2008) involves studies done in NIR region using 2MASS data. They also support the existence of very young stellar population, of age less than 2 Myr, and a second episode of star formation in the cluster region.

Thus existing literatures establishes that NGC 7419 is a reasonably populated young open

cluster. Cluster includes high mass (>10Mʘ), intermediate mass (2-10M ʘ) and low mass stars

(< 2Mʘ) (Joshi et al. 2008). Excess emission in the near IR support the fact there exist a very young population of Herbig Ae/Be stars (Subramaniam et al. 2006b). Studies have not been done regarding the circumstellar disks that can be possibly present around these young stars in the cluster. Literatures citedProperty above clearly of Christ explain University. how combined optical-IR photometry Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 18

can be useful in the study about disks present around pre-main sequence stars. By incorporating mid IR data, a deeper insight can be obtained about those YSOs. Emissions due to the presence of circumstellar disks can be more clearly identified using WISE data which provide fluxes measured in the mid-IR region and the current study aims at incorporating the mid-IR data for getting a detailed analysis of pre main sequence stars and circumstellar disks in the cluster NGC 7419

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Chapter 3 Data and Methodology

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3.1 Infrared Data

Those regions in space which are deeply embedded in gas and dust will be hidden from optical telescopes. Infrared wavelength has to be used for the study of such regions as they can propagate through those dusty regions of space without much scattering, their wavelength being much greater than that of optical radiations. As protostars are obscured within dusty clouds, they can be detected using infrared wavelengths only and such studies will also help to acquire much information about circumstellar disks.

3.1.1 Two Micron All Sky Survey (2MASS)

Two Micron All Sky Survey (2MASS) was an investigation of all sky at three infrared wavelengths 1.25 µm, 1.65 µm and 2.17 µm which are respectively named as J, H and K bands. Study was done for a period of four years; from 1997 to 2001. Survey used two infrared telescopes; one located atMt.Hopkins, Arizona for the survey of Northern Hemisphere sky and the other one at Cerro Tololo observatory in Chile for Southern Hemisphere sky. 2MASS was a combined project of University of Massachusetts at Amherst and NASA. Data was released in 2003.A huge amount of raw data has been processed and many astronomically useful images and a list of infrared brightness and positions were obtained. It contained information about 500 million sources. Canis Major Dwarf Galaxy was discovered in the 2MASS data. Many brown dwarfs, whose existence was previously unknown, were also discovered.

3.1.2 Wide Field Infrared Survey Explorer (WISE)

Wide Field Infrared Survey Explorer (WISE) is NASA’s infrared wavelength space telescope. It was launched in December 2009. Mission placed an infrared telescope of diameter 40 cm in earth’s orbit which surveyed the sky at four infrared wavelengths; 3.4, 4.6, 12 and 22 µm that are accordingly named as W1, W2, W3 and W4. It was 500 times more sensitive than NASA’s previous infrared mission Infrared Astronomical Satellite (IRAS). WISE extended 2MASS survey to thermal infrared. It acquired information about various processes like evolution of protoplanetary debris disc and also about star formation in Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 21

galaxies. Many previously unseen asteroids and comets were discovered. First Earth’s Trojan asteroid was also discovered from WISE data.

3.2 Optical Data

UBV CCD photometric study of cluster NGC 7419 was done by Beauchamp et al. (1994). The aim of the study was to confirm the presence of five red giant stars in the cluster. U, B, V magnitudes were measured and U magnitude is available only for the brightest stars in the cluster due to instrumental inefficiency. So U magnitude is not considered and only fluxes in two optical band 0.44 µm (B band) and 0.55 µm (V band) is used in the present study.

3.3 Methodology

3.3.1 Cross matching of Optical and Infrared data

B and V magnitudes for NGC 7419 are obtained from Beaucham et al. (1994). List includes 518 stars. Infrared data for the cluster in the JHK and WISE band for a target dimension of 10 arc min includes 344 stars along with RA and DEC coordinates. 66 stars were lacking data in the JHK band and those stars are excluded from studies. Optically identified stars are then cross matched with IR catalogs to identify the counterparts. Matches between catalogs were found by comparing their RA and DEC coordinates. Cross matching is done with an upper limit of 2.5 arc seconds. Final cross matched list involves 40 stars with flux values in 9 bands (B, V, W1, W2, W3, W4, J, H, K). Stars are provided with numbers for easy identification. Table 3.1 gives the crossmatched list of stars with their magnitudes in 9 bands.

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Table 3.1 Cross matched list. First column give the star number. Magnitude in two optical band (B, V), three near-IR bands (J, H, K) and four mid- IR bands are listed. Star B V J H K W1 W2 W3 W4 No 77 20.74 18.73 14.40 13.65 13.36 13.12 12.93 10.93 8.17 81 18.85 17.05 13.16 12.62 12.35 11.75 11.31 10.18 7.36 82 19.02 14.81 6.98 5.31 4.41 3.68 4.70 2.95 2.65 87 18.51 16.47 12.14 11.49 11.17 10.90 10.69 9.54 7.68 92 18.35 16.50 12.50 11.96 11.66 11.29 10.71 9.61 6.84 136 20.04 18.07 14.00 13.38 13.12 12.30 12.45 9.29 6.05 137 16.18 14.93 13.19 12.77 12.65 12.11 12.31 9.41 7.24 140 16.14 14.22 10.52 9.98 9.67 9.13 9.01 8.13 6.27 142 18.75 16.90 13.06 12.54 12.27 11.40 11.25 9.12 6.50 143 18.92 16.99 12.96 12.41 12.11 11.56 11.50 9.33 7.18 145 19.18 17.31 13.09 12.37 11.89 11.67 11.28 9.12 7.49 147 21.13 19.07 14.87 14.15 13.77 13.40 13.38 10.05 7.30 148 18.55 16.59 12.44 11.75 11.24 10.53 10.20 8.36 6.47 152 17.98 16.30 12.82 12.34 12.11 11.54 11.39 8.69 6.16 154 15.64 13.94 10.45 9.96 9.72 9.56 9.42 8.96 7.40 156 18.76 17.02 13.45 12.78 12.57 12.18 12.00 8.91 6.22 158 21.65 19.50 15.23 14.62 14.36 13.83 13.52 9.76 6.75 161 20.99 18.99 14.52 13.84 13.59 13.07 12.78 9.06 6.46 165 17.81 16.14 12.70 12.18 11.95 11.72 11.54 8.94 7.03 168 16.18 14.34 10.41 9.81 9.36 8.78 8.35 7.72 4.93 182 17.26 15.25 10.98 10.29 9.77 9.67 9.25 7.83 5.94 181 16.01 14.27 11.03 10.53 10.16 8.81 8.72 10.84 6.63 182 17.26 15.25 10.98 10.29 9.77 9.67 9.25 7.83 5.94 185 16.44 14.65 11.10 10.57 10.28 9.23 9.04 8.86 6.35 186 21.41 19.45 13.69 13.13 12.58 11.83 11.82 8.88 6.78 189 15.55 13.75 9.94 9.32 8.84 9.46 9.34 7.77 7.01 190 19.01 16.29 11.05 10.05 9.68 8.87 8.83 7.34 6.30 191 16.84 15.10 11.43 10.85 10.35 9.64 9.15 9.39 5.62 192 15.38 13.73 10.52 10.09 9.86 9.31 9.06 8.97 6.36 195 17.77 13.87 6.06 4.51 3.94 5.36 3.36 2.87 2.27 196 16.78 14.76 10.69 10.11 9.79 9.38 9.44 9.80 7.44 197 15.82 14.06 10.46 9.94 9.64 8.54 7.93 8.05 5.43 199 11.89 10.49 8.38 7.79 7.67 7.50 7.52 7.24 5.43 201 21.02 18.98 14.92 14.29 14.10 13.23 12.71 9.95 7.45 202 16.34 12.86 5.64 4.59 4.00 3.46 4.32 2.90 2.19 203 20.09 18.00 13.74 13.12 12.68 12.13 11.69 9.22 6.31 205 16.05 14.18 10.36 9.70 9.21 8.75 8.30 7.82 5.40 206 16.42 14.57 10.99 10.49 10.27 9.57 9.35 8.62 5.93 289 17.31 15.06 10.75 9.98 9.46 9.11 8.70 7.89 6.24 292 17.84 16.02 12.62 12.05 11.77 11.19 10.85 8.41 5.91 294 18.55 16.57 12.74 12.15 11.83 11.75 11.57 9.11 6.33

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3.3.2 Color-Color Diagram

Color-color diagram (CCD) is a tool for comparing magnitude of a star at different wavelength. Color of a star is the difference in magnitudes measured in two different wavelengths. Color defined by two wavelengths will be plotted on x-axis and color defined by another two wavelength will be plotted on y-axis. Color of a star is actually the ratio of fluxes. So when color is calculated for the same star or cluster at two different wavelengths, the distance to the star/cluster cancel out. So as colors are independent of distance, CCDs are also independent of distance.

In WISE CCDs, difference in magnitudes measured in different WISE bands will be plotted on the axes. Those stars that have circumstellar disk around them will be having more emission in the longer wavelength region as they reradiate by absorbing radiations from the stellar core, even though at much lower temperatures. Due to this lower temperature, emission will be in infrared wavelength. Such stars will be having very high value of WISE colors on both axes and they occupy top right region of WISE CCDs. As time passes, the circumstellar material will get dissipated. As a result, infrared emission also gradually decreases which in turn results in lower value of WISE colors. A star which has completely dissipated its circumstellar disk (prominent emission in shorter wavelength) thus occupies lower left region in WISE CCD.

For stars with transition disk, excess will be shown in W4 band only indicating absence of material in the inner region and the presence of an optically thick disk in the outer region. In the WISE 1, 2, 4 CCDs such stars will be showing excess in W1 – W4 color only and hence will be seen in the lower right region

3.3.3 Spectral Energy Distribution (SED)

SED of an object is a graph which shows how much energy a body emits at different wavelength; i.e. how the energy emitted by the body is distributed across electromagnetic spectrum. SEDs are found to be really informative tool for studies regarding YSOs. By looking onto to the structure of SEDs we can determine, through what evolutionary stage the YSO is going through. The spectrum of a star is usually approximated to black body Property of Christ University. spectrum. If a dusty disk is surrounding the core, the radiations emitted by the central core of Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 24

the star will be absorbed by this disk and will be re-emitted in infrared wavelength. So for a star with circumstellar disk, SED shows an excess in the IR region.

For studies regarding YSOs, the main concern will be about the IR region in SEDs. YSOs usually include a central core surrounded by an envelope of dust and circumstellar disk. As all these regions will be having different temperatures, the amount of infrared radition emitted by them also varies.

Figure 3.1 SED in the presence and absence of circumstellar disk

Class 0 sources are in the early main accretion phase and are deeply embedded. They are at low temperatures (< 70 degree K) and emits in microwave ranges. For Class I sources, late accretion phase will have started. Their temperature will be in the range 70-650 K and they start emission in the infrared. Class II stars are PMS stars with protoplanetary disk around them. They have temperature of 650-2880 K with NIR and MIR excess. Class III will have almost completely expelled their disk. Their temperature will be greater than 2880 K and will be having only very slight NIR excess.

Thus spectral energy distribution is an ideal tool for understanding different stages of evolution of protostars and also for checking for the presence of circumstellar disks. SED plots are fitted using the online fitting tool (http://caravan.astro.wisc.edu/protostars/) developed by Robitaille et al. (2007). It is also useful in determining physical parameters of the sources like age, mass, temperatureProperty ofetc. Christ SEDs ofUniversity. the stars are compared with the models Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 25

3 7 available for the range of mass 0.1 to 50 Mʘand range of age 10 to 10 Myrs. Radius and temperature of central source are obtained from evolutionary tracks. Once the age of central source is determined, disk and envelope parameters are arbitrarily sampled from the ranges that are functions of the age of the central source. For each fitted SED, 10000 possible fit models are available. Each model is provided with χ2value. Model with minimum χ2value is chosen as the best fit model. Best fit model parameters are used as the stellar parameters. In order to find the error in the estimated parameters, those models which satisfy the condition 2 2 χ min - χ2best< 3, where χ minis the statistical goodness of fit parameter per data points, are chosen. Standard deviation of the parameters of those models from the parameters of the best fit model gives the errors in the estimated parameters.

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Chapter 4 Results and Discussion

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4.1 Classification based on WISE Color-Color Diagrams

Koening et al. (2012) proposed a method of classifying YSOs based on their location on the color-color diagram. Stars can be classified as Class I, Class II, Class III or as stars with transition disk . Method was developed on the basis of position of objects that are already classified.

Figure 4.1 shows ([W2] – [W3]) Vs ([W1]-[W2]) color-color diagram for NGC 7419. Figure shows the distribution of Class I, Class II and Class III/disk less objects (Source that appear just below the Class II area are the Class III source). Class I stars are protostars with infalling disk. As they are having thick material around it, emissions in the longer wavelength will be stronger and they occupies top right region of the diagram which indicates an excess in both W2 and W3. Class II stars are more evolved than that of Class I. The excess that they show in W3 and W2 will be less than that of Class I. It is clear from the figure that we do not have any Class I sources. Stars appear to be crowded in the Class II and Class III regions. Stars with numbers 82, 181 and 202 are removed from the diagram as they are found to be out of range.

Figure 4.2 shows ([W2]-[W4]) Vs ([W1]-[W2]) color-color diagram. It shows the distribution of Class I and transition disk objects. Transition disks are stars with disk that has an inner hole. Due to the presence of the hole, excess is shown only for one color, i.e W2 -W4. This indicates greater flux in W4 waveband. Thus they occupy lower right region of the diagram. A good number of transition disk objects are found to be present in the cluster.

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Figure 4.1 WISE band 1, 2 and 3 color-color diagram showing the distribution of Class I and Class II objects.

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Figure 4.2 WISE band 1, 2 and 4 color-color diagram showing the distribution of Class I and Transition Disk object

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4.2 Classification based on Spectral Energy Distribution (SED)

Stars can be classified on the basis of the shape of their SEDs. Class I stars will be having a rising SED from 3.3 to 22 µm. Class II stars shows excess beyond 10 µm. Class III stars are diskless objects and their SED will be similar to that of a black body. Transition disk stars shows an excess beyond 20 µm which is an indication of the presence of large gap between the central core of the star and the disk. SED of 38 stars have been taken. 18 stars were showing Class II behaviour and 8 stars were showing transition disk nature. Four stars were showing a very slight excess in W3 and a large excess in W4. Due to this slight excess in W3 they cannot be precisely called Class II or Transition disk objects. Such stars are categorised as Class II/Transtion disk.

Figure 4.3, 4.4 and 4.5 shows SEDs of Class II, transition disk and Class II/Transition disk objects. The solid black line in the figure represents the best fitting model and the grey lines 2 shows those models which obeys the criteria χ min - χ2best< 3. Interstellar extinction Av is assumed to have a magnitude in the range 2-10 and distance range of 2.2 to 2.4 Kpc is taken. SEDs of stars having numbers 154, 189, 190, 195, 196 and 199 are avoided as their best fit model was having χ2value above 30.

Thus stars are classified based on their position in the WISE color-color diagram as well as depending on the shape of their SEDs. Classification done on the basis of SEDs is taken as the final classification.

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Figure 4.4 SEDs of transition disk objects

Figure 4.5 SEDs of sources classified as Class II/Transition Disks

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4.3 Parameter Estimation using SED

Parameters of the sources and error in the estimated parameters are determined using the online fitting tool using method described before. Assumptions used in the model would have affected the estimated parameters. Estimated parameters for Class II, transition disk objects and Class II/Transition disks are listed in table 4.1 to 4.6. Disk parameters like disk mass, disk accretion rate are also determined.

Out of the 31 sources, 21 sources found to have age less than 1 Myr. 8 candidate members have age between 1 to 5 Myr. There are two outliers (star no: 161, 158) which are as old as 9 Myr. 74 % of the candidate members can be categorised as intermediate mass PMS stars as

their masses are in the range 2-10 Mʘ. 26 % are low mass stars (M < 2 Mʘ). Massive stars (>

8Mʘ) in the PMS stage are not observed. The 2-5M ʘstars can be Herbig Ae/Be stars. Thus stars included in the current study shows very young age, and the estimated value of masses also seconds the fact that candidate members are in their pre-main sequence stage of evolution. A detailed discussion about the estimated disk parameters is given in the following sections.

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Table 4.1 Estimated parameters for Class II source. Column description given below

Star No A E_A MCS E_MCS IRAU E_IRAU TC E_TC

81 0.796 0.052 2.84 0.09 0.237 2.742 4883.0 100

92 0.621 0.029 3.52 0.14 0.322 0.009 4975.0 41

142 0.677 0.032 2.49 0.05 0.219 0.016 4770.0 74

143 0.554 0.037 2.93 0.06 1.907 0.308 4818.0 73

145 0.322 0.038 1.52 0.07 1.452 0.069 4425.0 29

147 3.560 0.260 1.95 0.05 7.278 2.378 5020.0 256

148 0.972 0.036 3.57 0.04 0.403 0.024 5352.0 46

156 0.458 0.057 1.27 0.09 4.421 0.764 4380.0 40

158 8.745 0.890 1.61 0.11 6.321 9.562 5528.0 619

161 9.085 0.591 3.14 0.11 30.810 2.217 12260 538

182 0.617 0.314 4.40 0.08 0.673 0.019 5780.0 334

185 0.617 0.062 4.40 0.08 0.673 0.087 5780.0 326

197 0.386 0.000 4.71 0.00 0.743 0.000 5185.0 0

201 4.747 0.367 1.82 0.12 1.675 1.182 5087.0 714

203 0.453 0.179 1.13 0.12 0.148 0.104 4314.0 396

205 0.453 0.179 1.13 0.12 0.148 0.104 4314.0 396

289 0.617 0.000 4.40 0.00 0.673 0.000 5780.0 0

292 0.624 0.036 3.25 0.06 1.863 0.695 4914.0 113

Column Description

1. Star No 2. A – Age of central source (Myr) 3. E_A – Error in A 4. MCS – Mass of central source (MCS) 5. E_MCS – Error in MCS 6. IRAU – Inner radius of star (AU) 7. E_IRAU – Error in IRAU 8. TC – Temperature of central source (K) 9. E_TC - Error inProperty TC of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 35

Table 4.2 Estimated parameters for Class II source. Column description given below

Star No DM E_DM DAR E_DAR DIST E_DIST Av E_Av F1 N

81 -5.13 -3.98 -10.50 -9.70 2.29 1.00 2.21 0.11 2.1044 91

92 -4.35 -3.73 -10.19 -9.91 2.34 1.01 2.74 0.06 2.4900 57

142 -3.92 -2.88 -9.46 -8.46 2.24 1.00 2.66 0.05 1.8433 218

143 -5.17 -3.02 -11.91 -9.37 2.40 1.00 2.71 0.05 1.1433 225

145 -4.22 -3.01 -10.38 -9.34 2.19 1.00 2.22 0.05 0.8833 294

147 -2.41 -3.05 -8.70 -8.52 2.29 1.00 3.66 0.13 0.7578 130

148 -4.33 -2.62 -9.11 -8.20 2.29 1.00 3.80 0.05 0.4578 207

156 -1.49 -3.12 -6.60 -7.69 2.19 1.00 2.18 0.04 1.1033 133

158 -4.37 -2.42 -9.00 -7.38 2.34 1.00 4.32 0.29 3.0778 27

161 -6.03 -3.90 -11.75 -9.18 2.19 1.01 6.70 0.22 1.1933 75

182 -3.32 -2.40 -9.50 -7.71 2.19 1.01 4.63 0.25 0.2200 89

185 -3.32 -1.84 -9.50 -10.23 2.40 1.05 2.00 0.00 13.8157 2

197 -0.64 -0.79 -7.64 -16.07 2.19 1.00 2.00 0.06 25.6556 2

201 -1.78 -1.83 -7.96 -8.38 2.24 1.01 3.57 0.33 3.4656 33

203 -2.21 -2.81 -8.56 -8.38 2.19 1.01 2.82 0.16 1.3156 94

205 -2.21 -2.82 -8.56 -8.38 2.19 1.01 2.82 0.16 1.3156 94

289 -3.32 -3.33 -9.50 -17.44 2.19 1.00 3.55 0.03 17.9389 5

292 -1.92 -2.77 -7.75 -8.30 2.34 1.00 2.43 0.07 1.0156 131

Column Description 1. Star No 2. DM – Disk mass of star in log of mass in solar mass 3. E_DM – Error in DM 4. DAR – Disk Accretion rate of star in log of mass in solar mass 5. E_DAR – Error in DAR 6. DIST – Log 10 distance 7. E_DIST – Error in DIST 8. Av- Interstellar 9. E_Av – Error in Av 10. F1 – CHI ^ 2 value 11.Property N – No of of models Christ taken University. into account Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 36

Table 4.3 Estimated parameters for Transition disk source. Column description same as table 3

Star A E_A MCS E_MCS IRAU E_IRAU TC E_TC No

77 1.041 0.074 1.48 0.06 12.180 0.904 4530 37

87 0.834 0.037 3.80 0.02 0.459 0.871 5386 56

137 1.472 0.116 3.93 0.15 97.640 8.350 13030 78

140 0.617 0.000 4.40 0.00 0.673 0.000 5780.0 0

152 1.468 0.081 3.68 0.11 30.310 2.229 9494.0 431

165 2.045 0.175 2.96 0.04 17.430 1.614 5935.0 136

168 0.386 0.025 4.71 0.03 0.743 0.010 5185.0 107

206 0.617 0.078 4.40 0.05 0.673 0.055 5780.0 246

Table 4.4 Estimated parameters for Transition disk source. Column description same as table 4

Star DM E_DM DAR E_DAR DIST E_DIST Av E_Av F1 N No

77 -2.96 -4.01 -9.69 -10.19 2.19 1.01 3.09 0.06 1.1656 96

87 -5.88 -4.66 -13.17 -12.46 2.19 1.00 3.91 0.07 0.9122 20

137 -7.17 -5.66 -12.65 -10.92 2.40 1.00 4.99 0.26 1.3986 64

140 -3.32 -3.56 -9.50 -17.33 2.19 1.00 2.00 0.00 11.4586 3

152 -5.79 -2.90 -11.80 -8.48 2.40 1.01 5.24 0.27 2.0889 109

165 -5.84 -3.11 -11.07 -10.12 2.40 1.01 3.33 0.14 0.4011 49

168 -0.64 -1.29 -7.64 -8.25 2.19 1.00 2.00 0.00 13.7643 9

206 -3.32 -2.14 -9.50 -8.40 2.24 1.01 2.84 0.22 8.2278 9

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Table 4.5 Estimated parameters for Class II/Transition disk source. Column description same as table3

Star A E_A MCS E_MCS IRAU E_IRAU TC E_TC No

136 1.225 0.221 2.73 0.10 28.820 2.017 4982.0 374

185 0.617 0.062 4.40 0.08 0.673 0.087 5780.0 326

191 0.280 0.011 4.25 0.31 0.482 0.025 4824.0 48

192 0.617 0.000 4.40 0.00 0.673 0.000 5780.0 0

294 0.521 0.051 2.91 0.07 18.920 1.221 4802.0 134

Table 4.6 Estimated parameters for Class II/Transition disk source. Column description same as table 4

Star DM E_DM DAR E_DAR DIST E_DIST Av E_Av F1 N

No

136 -2.96 -3.05 -9.78 -8.33 2.40 1.01 3.68 0.13 4.0944 114

185 -3.32 -1.84 -9.50 -10.23 2.40 1.05 2.00 0.00 13.8157 2

191 -1.59 -2.02 -7.84 -8.75 2.40 1.02 2.01 0.08 21.1767 5

192 -3.32 -3.62 -9.50 -17.09 2.19 1.00 2.02 0.00 19.6978 1

294 -4.94 -3.12 -10.27 -9.55 2.34 1.01 2.43 0.10 2.5778 115

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4.4 Parameter Plots

To understand the variations in parameters like disk mass, disk accretion rate etc with age plots are made between estimated values of parameters. Following are the plots.

4.4.1 Age Vs Disk Mass

Following plot indicates how the disk mass of stars varies with age. Only those stars that are classified as Class II and transition disk sources, based on SED, are included in the plots. The disk mass is shown in log scale. Figure shows that stars have a range in disk mass, while all of them are younger than 3 Myr, except two stars. This plot supports the fact that young stars deplete their disk in very short time-scale of 3 Myr. Two stars are found to retain their disk mass even at ages as old as 8-9 Myr.

Figure 4.6 Age Vs Disk Mass of Class II and Transition Disk source. Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 39

4.4.2 Age Vs Disk Accretion Rate

Figure shows a plot between age of Class II and Transition disk sources plotted against accretion rate in log scale. Determined parameters show a large range in the value for disk accretion rate. The stars are younger than 3 Myr, (except two), which suggests that the accretion rate rapidly drops and the stars are found to terminate accretion at 3 Myr. Thus accretion rate as well as the mass of the disk decreases rapidly for these sources.

Figure 4.7 Age Vs Disk Accretion Rate of Class II and Transition Disk sources

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4.4.3 Age Vs Mass of Central Source

6 sources are found to have mass between 1-2 Mʘ, which are younger than 9 Myr. The rest of the sources have masses between 2 – 5 solar mass and are younger than 3 Myr, only one source as old as 9 Myr. The 2-5 solar mass stars are similar to Herbig Ae/Be stars, which are also known to have a pre-main sequence life time of 3 Myr. Hence our result is in tune with the above.

Figure 4.8 Age Vs Mass of Central Source of Class II and Transition Disk sources

4.4.4 Disk Mass Vs Disk Accretion Rate

High disk mass stars are showing high accretion rate and vice versa. We find a linear relationship between the Disk accretion rate (log scale) and disk mass (log scale). This Property of Christ University. Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 41

indicates that a uniform mechanism is operating across the sources in adjusting the disk accretion versus disk mass.

Figure 4.9 Disk Mass Vs Disk Accretion rate of Class II and Transition Disk sources

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Chapter 5

Summary and Conclusion

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5.1 Summary of work done

A study of the cluster NGC 7419 using optical, 2MASS and WISE data has been done. Color-color diagram for the cluster showed the presence of YSOs. It was confirmed by taking the SED of individual sources. Classification of YSOs into Class I/II/III or transition disk sources was done on the basis of color-color diagrams as well as SEDs. There are some cases were sources identified as transition disk objects on color-color diagrams are classified as transition disk objects based on SED. This indicates that the adopted boundary between class II and transition disk sources is uncertain and introduces some ambiguity in definitively distinguishing between the two types based on their IR colors. However classification done on the basis of SED is taken as the final classification as fluxes measured in 9 bands are used as compared to color-color diagrams which have considered only the mid-IR fluxes. Class I sources are found to be absent in the selected sample. Out of the 40 sources considered, 18 objects were classified as Class II and 8 objects were classified as transition disk sources. 5 sources were included in the category of Class II/Transition Disk objects due to slight excess shown in W3. Remaining 9 sources were excluded from classification due to poor optical-IR cross-correlation and high values χ2 for SED fits.Table 5.1 gives the final classification.

Detection of transition disk objects offers a platform for future studies. According to the theory of accretion, there occurs a slow fading away of the disk due to the photoevaporation of the disk from inside out caused by radiations from the star. But this theory does not match with the observations all the time. Discovery of the transition disk objects in the present study seconds the fact that photoevapouration is not the only process resulting in disk dissipation. Dust grains in the inner region of the disk may stick together by gravitational effects, thus growing in size resulting in voids in the inner region which can be considered as signatures imprinted by a developing planet. A more detailed analysis of the properties of dust grains is required for the confirmation of these facts.

Many stellar and disk parameters are determined using the online SED fitting tool (Robitalle et al. 2006).Out of the 31 stars which are classified, 23 stars are having mass in the range 2-

5Mʘ. This indicates predominance of intermediate mass PMS stars. Remaining 8 stars have

masses less than 2Mʘ, i.e. lowProperty mass stars. of 28Christ stars outUniversity. of 31 have age less than 2 Myr. Thus Use it for fair purpose.Give credit to the author by citing properly, if you are using it. 44

study agrees with the result of Subramanian et al. (2006b) that NGC 7419 is a young open cluster with number of intermediate mass pre main sequence stars. Their study claims that cluster NGC 7419 may have gone through two different episodes of star formation, one at 25 Myr and other at 3 Myr. Stars involved in the current study are mostly having ages less than 3 Myr, so they might be belonging to the second stage of star formation.

In the early phases of PMS stage, process of accretion will be actively taking place from the surrounding circumstellar disk. As the star ages, rate of accretion will be gradually slowed down and finally will be terminated. Observations obtained in the current study agree with the theory. Candidate members are found to dissipate their circumstellar disk within a time period of 3 Myr. Accretion rate is high for stars with age less than 3 Myr. The highest disk mass is for star 156 which of a very young age of 0.4 Myr which also has got the highest accretion rate. A plot made between the disk mass and disk accretion rate confirms these linear relationship. Various mechanisms can be responsible for the dissipation of circumstellar disk at a rapid rate. The mechanism can be higher UV or X-ray flux from the central stellar source, tidal forces caused by the presence of a giant planet or an increase in the photoevapouration rate of the disk.

Current study made the first ever attempt to classify the YSOs of NGC 7419 into different classes using infrared data. Study demonstrated that a combination of optical and infrared data can be used as a very important effective tool for distinguishing pre main sequence stars from other field stars in the cluster. In summary study confirms the presence of young stellar objects in cluster NGC 7419 with circumstellar disk around them; most of them having age less than 3 Myr. Thus NGC 7419 offers a wonderful site for study the processes of stellar evolution and planet formation.

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Table 5.1 Classification of stars based on CCDs and SEDs

Star no Classification based on CCD Classification based on SED/ Final Classification 77 Transition Disk Transition Disk 81 Class II Class II 87 Transition Disk. Transition Disk 92 Class II Class II 136 Transition Disk Class II/Transition Disk 137 Transition Disk Transition Disk 140 Transition Disk Transition Disk 142 Transition Disk Class II 143 Transition Disk Class II 145 Class II Class II 147 Transition Disk Class II 148 Class II Class II 152 Transition Disk Transition Disk 156 Transition Disk Class II 158 Transition Disk Class II 161 Transition Disk Class II 165 Transition Disk Transition Disk 168 Class III Transition Disk 182 Class II Class II 185 Transition Disk II/Transition Disk 186 Transition Disk Class II 191 Class III II/Transition Disk 192 Transition Disk II/ Transition Disk 197 Class III Class II 201 Class II Class II 203 Class II Class II 205 Class III Class II 206 Transition Disk Transition Disk 289 Class III Class II 292 Class II Class II 294 Transition Disk II/Transition Disk

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5.2 Future Scope

Inclusion of far-infrared data will give a better understanding of the circumstellar disk. Spectroscopic observations are planned to determine the membership and PMS nature of these stars. We plan to undertake a spectroscopic investigation of selected stars in the cluster NGC 7419. By analysing the spectra of the stars in the cluster, a double-confirmationof the circumstellar disk can be obtained as there will be emission lines.

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