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Herbig Ae/Be Star Accretion Rates and the Hertzsprung Russell Diagram

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Herbig Ae/Be Star Accretion Rates and the Hertzsprung Russell Diagram Department of Physics and Astronomy BSc Project PHYS3150 Herbig Ae/Be Star Accretion Rates and the Hertzsprung Russell Diagram Author: Alison McSloy Supervisor: Student number: Professor Rene Oudmaijer 200549097 Submission date: 11th February 2013 Word count: 3850 1 Abstract The accretion rates of a sample of 90 Herbig Ae/Be stars were calculated, using photometry data from the X shooter project at the European Southern Observatory's Very Large Telescope. Accretion rate estimates for Herbig stars have been studied far less than those for low mass stars, demonstrating the importance of conducting further research in this area. The strength of the observed hydrogen α lines for each star were used to calculate the accretion rate values. The data for the Herbig stars was plotted on a Hertzsprung- −5 −11 −1 Russell diagram. The accretion rates calculated varied from 10 - 10 M yr . The median mass accretion rate was found to be 5 x 10−8 M yr−1. A positive correlation was found between stars' accretion rates and their bolometric luminosities, as well as between stars' accretion rates and their temperatures. 2 Introduction day. There have been very few direct observations of Herbig star magnetic fields to date [4], meaning that the origin of these magnetic fields still remains The formation of stars from the interstellar medium a mystery [5]. This further supports the view that is a physical process that is still not fully understood more research must be carried out in order to im- by astronomers today. There are many parameters prove astronomers' understanding of this intermedi- that affect the rate of star formation that must be ate mass star formation. This project investigates considered, such as angular momentum, metallicity, the accretion rates of Herbig Ae/Be stars, which are stellar and circumstellar masses, the environment in an example of these intermediate mass stars. which the star formation occurs, and time [1]. In recent years, low mass star formation, namely the Herbig Ae/Be stars are intermediate mass stars (2- formation of T Tauri stars, has been well studied; 10M ) that are still in the pre-Main Sequence con- as a result the physical processes involved in creat- traction phase [6]. Herbig stars are defined as stars ing these stars are now fairly well understood. High with spectral type A or B (with emission lines), that mass star formation is slightly less well understood, exhibit an infrared excess, and are within luminos- but the real gap in knowledge arises at the interface ity classes III-V. This definition will unavoidably in- between the two. The physical processes present in clude some evolved massive stars, so it is imperative formation of intermediate mass stars, i.e. Herbig that astronomers must take care to differentiate be- stars, remains largely unknown. It is this lack of tween these and Herbig stars [8]. current knowledge that emphasises the importance It is worth taking a moment to define accretion of continuing research into this topic, in order to al- rates themselves. Stars form within giant molecu- low astronomers to construct theories that could ex- lar clouds (hereafter referred to as GMCs), which plain how these intermediate mass stars are formed. have masses in the range of 103M -7M [9]. The Low mass star formation (typically <2M ) is ac- average density of these GMCs is 102-103 particles cepted to occur by a process called magnetospheric per cm3 [10]. A large proportion of the matter found accretion [2]. In contrast, high mass star forma- between these forming stars is gas, where hydrogen tion is not understood quite as well, similar accre- exists in atomic form [11]. When a protostar of mass tion rates would result in high mass stars leaving M and radius R accretes some mass δm during the the Zero Age Main Sequence before becoming fully formation process, its gravitational potential energy formed. As this is not observed, another form of decreases. This accretion of mass occurs over a time accretion must occur in order for high mass stars to δt, which allows the average rate of gravitational en- form [3]. The point where the magnetospheric accre- ergy release to be calculated. This average value is tion process stops and another accretion process be- proportional to the average mass accretion rate by gins is currently unknown, but it is clear that study- the following equation: ing these intermediate mass (2-10M ) stars will al- GMM_ low astronomers to further their understanding in Egrav = (1) this area. The point where magnetospheric accre- R tion stops and another accretion mechanism begins In order to remain in thermal equilibrium, the rate is currently unknown; this topic is still debated to- of gravitational energy released must equal the ac- I cretion luminosity; if this energy is not efficiently nosity is illustrated in figure 1, for a protostar of radiated away, the protostar's temperature would in- M∗=2.5M and R∗=2.6R : crease. If this assumption is followed, an accretion luminosity can be defined [12]: GMMacc Lacc = (2) R∗ In the case of Herbig stars, material accretes onto the star via an accretion disc, which in some cases can be directly observed. For example, the dust compo- nent of the disc can be observed via the scattering of optical and near-infrared light [11]. The geometry of Herbig star accretion discs is still under investi- gation; however, it is widely accepted that as dust grains coagulate, they settle towards the midplane of the disc, making them easier to observe [13]. How- Figure 1: Example of total flux due to accretion and ever, this only applies to Herbig stars close enough to a protostar [15] the observer; the majority of the time, these accre- In Figure 1, the black line represents the observed tion discs cannot be directly observed. As accretion spectra of a star. The grey line represents the contri- discs are often not directly observable, other meth- bution to the total flux from the star itself, and the ods must be used in order to provide proof their dashed line illustrates the contribution to the total existence. An example of such a method is the mea- flux from the accretion luminosity. surement of disc velocity profiles, which imply the presence of a rotating disc around a star. These Some of this blackbody radiation (due to the accret- profiles reveal velocity gradients, which indicate the ing material) acts to ionise the hydrogen atoms in presence of material orbiting a protostar in a disc- the surrounding disc. When the hydrogen then re- like configuration [14]. combines, emission is produced, accounting for the Emission lines can be used to calculate mass accre- emission lines present in the observed spectra. The tion rates for stars. From the above explanation of strength of the hydrogen α emission line has been accretion rates, it is not immediately obvious how shown to increase with the accretion luminosity of a one can calculate an accretion rate given an emis- star [15]. For this reason, the hydrogen α line acts sion spectrum. In order to understand how measur- as an excellent tracer of accretion rates. ing emission lines can result in a calculated accretion Now that the reason for using the hydrogen α emis- rate, one must consider the physical processes that sion line as a tracer of accretion rates has been deter- occur when a protostar accretes mass from its sur- mined, the process of calculating an accretion rate rounding disc. As described by equation 2, when can be explained. The first step in this process is matter accretes onto a protostar, the resulting loss to measure the equivalent width of an emission line, in gravitational potential energy is released as lumi- for example the hydrogen α emission line at 6562.6A.˚ nosity. When material accretes onto a protostar, all This equivalent width is a way of measuring the rel- of this material is heated to approximately the same ative strength of the emission line. The flux of the temperature, resulting in the observation of black- hydrogen α line can then be calculated by multi- body radiation (due to the accreting material). The plying the equivalent width by the flux density of protostar itself also produces a certain luminosity, so the R band. From this, the line luminosity can be it follows that the observed spectra is a combination calculated from the following equation: of the two different luminosities. When the spec- tra are observed, these luminosities are measured as L = 4πd2f (3) fluxes; the relation between these two parameters is shown in equation 3. An example of the difference where f is the line flux, d is the distance to the star, between a star's luminosity and the accretion lumi- and L is the line luminosity. The line luminosity II can then be converted into an accretion luminosity, at all observable, so it could be the case that all which in turn leads to the calculation of an accre- 90 stars analysed were affected by this This higher tion rate. Parameters such as distance d, mass M equivalent width value would then result in a higher and radius R are usually found in photometry data, value of the accretion rate. or can be sourced from other papers. Once the equivalent width has been found, the flux density must also be calculated in order to achieve 3 Data and Flux Measurements a value for the line flux. The photometry data con- sisted of two tables provided by the supervisor. They contained data acquired from various sources, which The photometry data and spectra of the 90 Her- was then compiled into the photometry data used.
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