Studying Variable Stars using Small Telescopes Sudhanshu Barway SAAO, Cape Town What are Variable Star? Stars showing variations in their brightness as a function of time. Brightness variation: 0.001– 20 mag. (in V band λeff=5500Å & ∆λ=850 Å) Period: seconds to years Over 30,000 variable stars are known today, and roughly equal number are suspected to be variables! Types of Variable Star 1. Intrinsic variables – Stars which vary their light output, hence their brightness, by some change within the star itself. Provide a wealth of information about the internal structure of stars, models of stellar evolution and distance determination. Further classified as: Pulsating Variables – Cepheids, RR Lyrae, RV Tauri, δ-Scuti Eruptive Variables – Cataclysmic Stars, Supernovae, Novae Types of Variable Star 2. Extrinsic variables – Light output varies either due to processes external to the star itself or due to the rotation of the star. Further classified as: Eclipsing Binaries – Algol, W Uma, β-Lyrae Rotating Variables – RS CVn, BY Dra, Light curve of Variable Stars Light curve – brightness variation of star over time Light curve of Pulsating Variables - Cepheids Variation in brightness is due to the periodic expansion and contraction of the surface layers of the stars i.e. the star actually increases and decreases in size periodically. Light curve of Eclipsing Binaries - Algol A binary system of stars in which the orbital plane lies edge-on to us so that the component stars periodically eclipse one another. Light curve of Eclipsing Binaries – more examples Light curve of Rotating variables – RS CVn binaries Brightness variation is due to the spots (similar to sun- spots) on the surface of stars. As a star with spots rotates, its brightness changes. Why should one study Variable Stars? Systematic observations over a longer period helps: To determine the short-term and long-term behavior of the stars. To construct theoretical models. Research on variables stars is important for deriving stellar properties, e.g., mass, radius, luminosity, temperature, internal and external structure, chemical composition and their evolution. Studying Variable stars using Small Telescopes What is a small telescope? Telescopes with aperture ≤1 m; typically ~ 6 – 16 inch 16-inch Meade LX 200 Telescope A typical Modern Small Telescope Studying Variable stars using Small Telescopes State-of-the art Small Telescopes 8-inch Celestron CGE 800 Telescope 14-inch Celestron CGE 1400 Telescope Studying Variable stars using Small Telescopes Detectors for Small Telescopes A typical solid state photometer (Model SSP 3A) SSP 3A is a basic level instrument for the measurement of stellar magnitude. Detector: P-N photodiode Effective area: 1mm For details: http://www.optecinc.com/ Studying Variable stars using Small Telescopes Detectors for Small Telescopes A small format CCD camera from SBIG (Model - ST7 XME imaging camera) CCD - Kodak KAF-0402ME +TI TC-237 Pixel Array - 765 x 510 pixels CCD Size - 6.9 x 4.6 mm Pixel Size - 9 x 9 microns square Exposure - 0.12 to 3600 seconds, Computer Interface - USB 1.1 Computer Compatibility - Windows 98/2000/Me/XP and Mac OS For details: http://www.sbig.com/ Studying Variable stars using Small Telescopes Advantages of having Small Telescopes – 1. Convenient access to a telescope. 2. For sufficiently bright stars, small telescopes achieve same photometric accuracy as that of large telescopes. 3. With advanced increasing sophistications in optics and electronics it is possible for smaller telescopes to do what larger telescopes could do in the past. e.g.. APT (Automated Photometric Telescope) and Robotic telescopes. Studying Variable stars using Small Telescopes Advantages of having Small Telescopes – 4. Photometry of variable stars using small telescopes is very convenient and easy. Long term projects are specially good for small telescopes. 5. The geographical location of an observing place can make it critically important, independent of telescope aperture. Studying Variable stars using Small Telescopes What can we do with small telescopes? (a) Teaching : observational projects for graduate students Excellent choice for conveying excitement of observational Astronomy to the young minds! (b) Research: suitable projects may be undertaken for M Phil or Ph D students. Publications in reputed astronomical journals! Studying Variable stars using Small Telescopes Observational Projects for graduate students- 1. Measurement of extinction coefficient. 2. Calibration of telescope-detector system. 3. Photometry* of well known variable stars of various types: . Delta-Cepheid variables - δ-Cepheid, ζ- Gem . β- Cephei type variable - ν Eri . δ- Scuti variable - V474 Mon . Eclipsing Binary - Algol-type: β- Per, λ- Tau . Eclipsing Binary - β-Lyrae type: β-Lyrae . Eclipsing binary - W UMa type: 44 Bootes . RS CVn binaries - V711 Tau, UX Ari * Differential photometry technique is normally use for photometry of variable stars. Observational Projects for Graduate Students Light curve of Cepheid variables - • Periods : 1-70 days • <Te> - 4000-5000 K • <L> - 300 – 40,000 LΘ • Amplitude – 0.1 - 2 mag • Massive stars of spectral type F or G-K Object: δ Cephei • Giant star • Period : 5.4 days • ∆V = 0.89 • Steep rise but slow decline Observational Projects for graduate students- Observational Projects for Graduate Students Variable Star Comparison Check Star Star Name δCep εCep ζ-Cep RA (J2000.0) 22h 29m 24.21s 22h 15m 15.23s 22h 11m 04.43s Dec (J2000.0) +58° 26' 35.6" +57° 4' 15.1" +58°13' 42.2" Sp. Type F5 Iab F0 IV K1.5 Ib B-V +0.757 +0.261 +1.590 V 4.13 4.18 3.34 Amplitude V 1.0 Period 5.37 days Epoch 2451621.151 Variability δCep Type Observational Projects for Graduate Students Cepheid variables exhibit a relationship between their period and their intrinsic luminosity known as “Period-Luminosity relation”. Period-Luminosity relationship can be used to determine the distance to a Cepheid. This relationship also used to determine the extragalactic distance scale by observing Cepheids in other galaxies. Studying Variable stars using Small Telescopes Research projects in observational astronomy – 1. Variable stars/suspected variable stars Photometry of rotating variables – RS CVn binaries, Chromospherically Active Stars or spotted stars. 2. Nova, Supernova search and follow up observations 3. Meteor and Asteroid observation 4. Lunar and Planetary Occultation 5. Comet observation Research projects in observational astronomy Photometry of rotating variables – • Rotating variables are normally late type stars with enhanced solar like magnetic activities such as dark spots, highly energetic flares, facular networks, chromospheric plages, emission from transition region and corona. • Starspots can blanket up to 50% of a hemisphere of the active star, whereas sunspots cover only about 0.2% of the solar hemisphere during the sunspot maximum. Research projects in observational astronomy Photometry of rotating variables – • Presence of Strong Ca II H & K and Hα emission lines. The rotating variables are also known as Chromospherically Active Stars include single and binary stars. Different groups of chromospherically active stars are - RS CVn binaries, BY Dra, W Uma, T Tauri, FK Com These stars needs continuous monitoring - Short term variation: Light curves vary over couple of rotation periods Long term variation: Light curve varies over long periods like sun spot cycle Research projects in observational astronomy Photometry of rotating variables – • Short-term and long-term activity cycles in Chromospherically Active Stars. • To investigate the relationship among various activity indicator with dynamo parameters. • Comparative study of various starspot models and explore the role of various least square optimization technique in starspot models. • Search for new Chromospherically Active Stars. Research projects in observational astronomy Photometry of rotating variables – Short-term variation Chromospherically Active Stars display appreciable changes in amplitude, shape, phase of minimum light as well as mean light level within a short period of time. Spot modeling analysis of photometric data reveals the variation in spot parameter within very short period of time. Light curve of UX Ari Research projects in observational astronomy Photometry of rotating variables – Long-term variations In addition to the short-term variation due to rotational modulation of stellar brightness in rotating variables, it is believed that there should also exist a systematic long-term variations. These long-term variations or Stellar Activity Cycles are very similar to the solar 11-year sunspot cycle or 22-year magnetic cycle. The study of stellar activity is one of important areas where knowledge of existence of activity cycle can provide a better understanding of the vital force responsible for a variety of atmospheric magnetic activities occurring in the Solar as well as other stellar systems. Research projects in observational astronomy Photometry of rotating variables – Photometry of suspected variables- Padmakar et al. identified new RS CVn type rotating variable star HD 61396 using 16” telescope and SSP- 3A photometer later verified by Barway et al. 2004. Barway et al. identified 4 new rotating variables using IUCAA’s 16” Meade telescope & SSP- 3A photometer. Light curve of suspected variable star HD 61396 Research projects in observational astronomy Research publications using small telescopes • Barway, S., Pandey, S.K., Parihar, P.S. and Chaware, L., Long-term spot activity