Variable Stars Advanced Lab

Transcription

1 Variable Stars Advanced Lab Introduction In this lab, you will be observing several variable stars to determine the period and classification of the star. A variable star is considered any star which goes through periodic or semi-periodic variations in luminosity. Several examples of the types of variable stars are listed in Table 1: Type Prototype MV Spectral Class Pulsation Period Range Characteristic Period Classical δ Cephei -0.5 to -6 F6 to K2 1d to 50d 5d to 10d Cepheids Population II W. Virginis 0 to -3 F2 to G6 2d to 45d 12d to 28d Cepheids RR Lyrae Stars RR Lyra 0.5 to 1 A2 to F6 1.5h to 24h 0.5d Long Period σ Ceti 1 to -2 M1 to M6 130d to 500d 270d Variables (Mira) RV Tauri Stars RV Tauri -3 G K 20d to 150d 75d â Canis Majoris β Canis Majoris -3 B1, B2 4h to 65h 5h Stars Semiregular σ Herculis -1 to -3 K, M, L, T 100d to 200d 100d Red Variables Dwarf Cepheids δ Scuti 4 to 2 A to F 1h to 3h 2h ZZ Ceti Stars ZZ Ceti 30s to 1500s Table 1: Some Classes of Pulsating Stars (Zeilik, Gregory & Smith, 1992) In this lab you will make the measurements of several variable stars, over several nights. The main instrument you will be using for this experiment will be the Braeside Observatory. The Braeside telescope is a 16 inch telescope located in Flagstaff, AZ. A CCD, or Charged Coupled Device, situated at the end of the telescope converts the light which enters the telescope into the digital image. The CCD camera used on the Braeside telescope is called a SITe CCD. Each pixel making up the CCD chip is a light sensitive device which measures the amount of light falling on it. The plate scale for the camera is arcsecond/pixel which gives a field of view of 6 arcmin square. The CCD camera has several filters. Each filter selects out a region of light known as a passband. The filters are used to measure the color of astronomical objects. If an object emits more light in the blue than in green (known as visual light) then the object will appear brighter in the B filter than in the V filter. The filter wheel has 8 slots open for filters which are described in Table 2. Filter # Filter Name Wavelength Description (nm) 1 Open (no filter) 2 U 365±66 Near Ultraviolet Light 3 B 445±94 Blue Light 4 V 551±88 Visual Light 5 R 658±138 Red Light 6 I 806±149 Near Infrared 7 Pinhole 8 Neutral Density Table 2: Filters of the CCD camera and descriptions

2 Making Measurements To make measurements you will need to acquire each night: Calibration Files Standard star images Images of the Variable Star Calibration Files Calibration files will be necessary each night data is acquired. The images are used to remove obscuring dust on the optics, light sensitivity variations and additional random noise from the CCD camera. Improperly calibrating the images can cause results with larger error. Explained below are three kinds of calibration images which will be used. The bias frame is an exposure of zero exposure time. The bias frame will measure the amount of noise in each image. The electronics of the CCD camera will cause each pixel to register some amount of charge. The charge can then be misinterpreted as signal if not accounted for with the bias frame. The dark frame is an image of the same exposure time as the object, but receives no light. At a given exposure time a CCD chip will accumulate a certain amount of signal that is not related to an image. That is, if you repeat the same exposure under the same circumstances (temperature & time) the signal can be repeated. This means that if we take a dark frame and then take an image at the same temperature and for the same time as the dark frame the unwanted signal that is accumulated and recorded during the exposure on the dark frame will be on the image as well. So, if we subtract the dark frame from the image we can remove all the unwanted signal. Flat fields are images of evenly illuminated surfaces such as the twilight sky. Ideally, we would like to use flat field images taken that evening, however in case of poor weather conditions at sunset, you might use flat field images from a previous session. The images are useful in measuring the amount of dust or dirt in the optical system. A speck of dust on the mirror will be out of focus and appear on the image as faint a doughnut shaped region. Flat field images also take into account vignetting and other light varying problems. To remove these imperfections we divide the final astronomical image by the normalized flat field. Standard Stars Standard stars are stars which have well known magnitudes in several colors. Standard star measurements will allow you to calibrate the observations of the variable star to the magnitude system. Use a standard star which is not too far from the variable star (<5 ), at about the same airmass, and probably brighter than the variable star. And make sure the star is up! Alternate observations of the standard star with the variable star, making sure you have at least three sets of standard star measurements at a variety of airmasses and several images per filter in each set. Obtain measurements of the standard star in each filter used for the variable star. Standard stars are listed in section H of the The Astronomical Almanac, provided in the control room. Use standard stars that are calibrated for the Johnson UBVRI filter system. Variable Stars You will observe several variable stars in the V filter. Listed in Table 3 are possible target stars, their positions in J2000 coordinates, mean apparent visual magnitude (V), availability at 11:00 p.m. MST, and reference to a paper which studies the star. The papers referenced and many others may be found by looking at the ADS website at You may pick another star, but you will need to do a literature search for background information.

3 Name RA Dec V Availability SW And Jul-Dec HU Tau Sep-Mar DY Ori Oct-Mar W Gem Nov-Mar ζ Gem Nov-Mar RR Gem Nov-Mar W UMa Feb-Jun GW Vir Feb-Jul W Vir Feb-Jul RS Boo Mar-Jul AH Her Apr-Sep BL Her Apr-Sep EP Lyr Apr-Oct RU Peg Jul-Dec δ Cep May-Nov RV Lac Jun-Oct Table 3: Table of target stars Data Acquisition To control Braeside from ASU, you will use the program XBOBS (X-based Braeside OBserving System). The program was specifically written to operate the telescope, dome and camera. You can view the telescope via the internet at To observe from ASU and control the observatory, do the following Log into the Braeside computer located in PSH-563-A2. Login name: observer, password: braeside Open an xterm by typing xterm & at a Konsole prompt. At the observer prompt, type in idl. At the idl prompt, type in xbobs In the region marked Telescope Control go to the Dome/Tel section, hit the pulldown button marked Select and hit the Auto Open selection. This will initialize the telescope and set it up for observing. You may watch and listen to the telescope as it sets up by turning the room lights and audio on in the region marked Dome Control. When the telescope is ready for you to use, the top row of red and green lights should be all green, and any warning boxes should have disappeared. Remember to turn the room lights off before taking data. In the region marked Target Acquisition and Pointing turn the Pointing Updates on. The boxes below the switch will fill in with numbers indicating the location of the telescope and where it is pointed. In the Dome Control panel, link the dome by selecting the Link option near the Dome buttons. If this is not done then the dome will not track with the telescope. Find the location of a bright star with in one hour east or west of the meridian at the time of observing. There is a catalog of bright stars built into the program. In the same section of the widget, find the region marked Catalogs and press the button marked Local. In a few seconds a window will appear. Scroll down to find the name of the star near the meridian. Highlight the star. Hit select. The information should appear the in boxes for the Destination. Then hit exit. Move the telescope to the destination by pressing the button marked Go There. You will be able to hear the telescope move into place. Once at the location, enter the observers names in the Observer box and press the Set button all located in the Observing Interface section. Then take a short exposure (~0.5s) of the region. Enter the exposure time in the box marked Exptime, and take the exposure by pressing the button marked Expose. The image will appear in the window. You want the star as close to the center of the frame as possible. If the star is not in the frame, take a longer exposure (~15s). If the star is bright enough (>4th magnitude) then spikes of star light should enter the frame. Move the camera in small increments by going to the Manual Positioning and pressing the N, S, E, or W buttons, and adjusting the increment in the center. You can move the telescope between arcseconds.

4 Once the star is in the frame, you can center it by pressing the Recenter Image button in the Target Acquisition and Pointing section, then click on the center of the star, and then hit Go There. You will want to reset the coordinates by pressing the Reset Curr to Dest button. If the star looks like a doughnut, then you will need to focus the telescope. Go to Focus in the Observing Interface panel and adjust the focus by short amounts, press Set and take short exposures in between changing the focus. Once the star looks in focus, check the seeing estimate of the field stars by pressing Seeing Estimate in the image display panel and clicking on the center of a star. Typical seeing with Braeside is about arcsec, excellent seeing is 2.5 arcseconds and poor seeing is >~6 arcsec. While you adjust the focus, the seeing will decrease until you are at the best focus possible. Continue to get estimates of the seeing while you are observing. You can record the focus and seeing estimate on the observers log sheet in the comments column (Table 4). Once this is all set, you may go to your target. Enter the coordinates of the target and object name in the RA, Dec, and Object Name boxes in the Destination section each time you go the star, or you may add them to the local catalog. To add a star, open the local catalog, press the Add button, enter the information and then press the Add button in the new window. Move the telescope to the target as described above. Center the telescope on the target, set the filter you will be using and obtain an exposure. Get an image where the counts inside the target are between 50,000-60,000 counts in the raw image. You can check the number of counts by hitting the Track button in the image window panel. The mouse position (x and y) and counts (flux) will be displayed while you are moving the mouse over the image. Click the right mouse button to automatically move the mouse to brightest local pixel. You must click the left mouse button to deactivate tracking before proceeding. If the counts for the target are too low, for example 10,000 in 5 seconds, you can use the fact CCDs respond linearly, that is, if you want 60,000 counts, then extend your exposure time by a factor of 6. As the counts exceed 60,000, the response of the CCD changes from linear to logarithmic. Avoid using data with counts greater than 60,000. Images where the counts reach 65,536 are considered saturated and are usually notable by vertical bleeding of the stars. Finding a good exposure time may take several trials. You want images with as many counts as possible to be able to apply Poisson statistics for error analysis. Once you have the image set up the way you would like it, go to the Camera Control panel and turn on the Auto Increment option. When this is set to Off then the filename displayed in the Filename to be written box inside the Observing Interface panel will never change and you will write over the same image. Once the Auto Increment is set to On, then each exposure will save an image. When you have finished using the telescope for the night, close the telescope by selecting the Auto Close option in the Dome/Tel button in the Telescope Control panel. Use lights and camera to confirm the closure and parking of the telescope. Data Reduction The data should be analyzed using a language which you are comfortable with, such as C, Fortran, or IDL. Each file made at the telescope is written as a.fits files, and so you will need a language capable of reading the file in. The first section of each file is a header containing the observation information (filter, exposure time, RA and Dec, airmass, etc) written in ASCII format. To begin the reduction, subtract the bias image (B) from the data image (I(raw)). Then subtract the product of the dark image (D, which is normalized to one second) and the exposure time (t) of the image. Then divide the normalized flat image (F). I ( ) ( raw) B ( D t) I clean = F Measure the light from the object by summing up the pixels within a circular region around the star. Since the light from a star spreads out like a 2-dimensional gaussian, make sure you have used an area large enough to include fainter pixels. The total number of counts for a star is an instrumental flux which can be converted to a magnitude using the following equation: = 2.5 log m 10 where f is the instrumental flux per second, and m is the instrumental magnitude. ( f )

5 The error in the flux will be assumed to follow Poisson statistics, that is: σ f = f which can be used to obtain the error in the magnitude as: σ m = σ f To convert the measurements from instrumental magnitudes to an actual magnitude and correct for observing at different airmasses, you will need to make a plot of the standard star magnitude vs. airmass. Airmass is the is the secant of the angle between the star and the zenith. This value is stored in the header of each file. There is a linear relationship between magnitude and airmass. Each magnitude reported in literature is given at an airmass of 1, that is if the star was at the zenith. The slope will determine the relationship between magnitude and airmass. The offset between the instrumental magnitude and the accepted magnitude is the y-intercept. Apply the slope and y-intercept to the variable star to obtain magnitudes at 1 airmass. Make a plot of the variable star data of magnitude vs. time. Phase the data to the period to better show the periodicity of the star.

6 Start Time: End Time: Weather Conditions: Image Number Extension Number Filter Object Exposure Time Comments V ζ Gem 30s Focus = -330, Temp=25 F Table 4: Observers log sheet