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SPECTRAL CHANGES in at MICROSCOPII ATTRIBUTED to FLARING Adam Clarke

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SPECTRAL CHANGES in at MICROSCOPII ATTRIBUTED to FLARING Adam Clarke SPECTRAL CHANGES IN AT MICROSCOPII ATTRIBUTED TO FLARING Adam Clarke 2011 Abstract Spectroscopic analysis of AT Microscopii has been conducted during a period of flaring with estimated magnitude increase of three. The spectrum is shown to undergo considerable changes including doppler broadening, area increase and some potential pattern in peak value which require further analysis to confirm. Delayed changes in the full width at half maximum compared with the continuum is linked to flare activity triggered by single magnetic loop formation. A decrease in peak wave- length of approximately six angstroms has been detected and cannot be determined to be either physical or erroneous in nature without further analysis and observation. −1 The FWHM is converted to a velocity change of ∆VH gamma = (92:100±5:700)kms −1 and ∆VH delta = (122:000±24:000)kms from quiescent level to the maximum value during the flare outburst. 1 Contents 1 Introduction 3 1.1 What is a dMe Type Star? . 3 1.2 Flares . 3 1.2.1 Rise Time . 3 1.2.2 Decay Time . 3 1.2.3 Decay Time Constant . 4 1.3 AT Mic . 4 1.4 Data Reduction . 5 1.4.1 Debiasing . 5 1.4.2 Flat Fielding . 5 1.4.3 Other Reduction Techniques . 5 2 Methodology 5 2.1 IRAF . 5 2.1.1 Image Reduction . 5 2.1.2 Spectra Extraction using IRAF . 6 2.1.3 Wavelength Calibration Using IRAF . 7 2.2 IDL . 10 2.2.1 Reading in the Calibrated Files . 10 2.2.2 Curve Fitting . 11 2.2.3 Area . 11 2.3 Time . 12 2.4 Wrapper . 12 3 Results 12 4 Analysis 15 5 Discussion 19 6 Conclusion 20 7 Acknowledgements 21 A IDL Functions and Procedures 23 A.1 Averaging FITS . 23 A.2 Line Fitting . 24 A.3 Area . 25 A.4 Time . 26 A.5 Wrapper . 27 A.6 Multiple Plots . 28 2 1 Introduction In this paper observations of dMe type star AT Microscopii (AT Mic) are considered during periods of flaring, in particular with a flare of estimated magnitude increase of three. Comparison with quiescent period spectroscopy to the period of flaring, was conducted to determine any changes attributed. During the period of flaring Doppler Shifting or broadening is expected which would indicate matter being accreted either toward or away from the line of observation, or in the case of broadening could indicate atmospheric turbulence. The change in magnitude and the ratios between quiescent and outburst levels are also considered to determine trends between the two. 1.1 What is a dMe Type Star? Stars are classified dependant on their spectral type and the Harvard system, founded by Cannon and Pickering [1912], which lists them in order of atmospheric temperature. An \M" type star on its main sequence, like AT Mic, typically has a temperature of ≤ 3,700K and is red in visual colour. The preceding \d" marks it as a dwarf star and the succeeding \e" specifies that the star has emission lines. 1.2 Flares Stellar activity is based upon the rotation of stars. Rapidly rotating stars seem to offer more activity than those with longer rotational periods due to a magnetic dynamo type effect. One type of activity associated with rotation is flaring. Flares occur when magnetic energy in the stellar atmosphere is suddenly released, relating to a sudden increase in brightness of the star. Flares have many features associated with them; 1.2.1 Rise Time The rise time of a flare is defined as the time taken for the intensity of the star to be enhanced from the quiescent level to its maximum intensity. Because of the natural variation in some stars a flare is defined as the scenario where at least two consecutive observation points lie 3σ above the average quiescent intensity, where σ is the standard deviation of the quiescent stars level, taken during the same observations. 1.2.2 Decay Time The decay time is the time taken between the peak of the flare period intensity and the end of the flare, located when the intensity returns to the quiescent intensity. Due to the nature of flares, the decay time is much longer than the Rise Time which leads 3 to difficulty in estimation, since stars observed can often have set before the star has returned to quiescent levels. 1.2.3 Decay Time Constant Hence it is much preferable to ascertain the decay time constant, which is defined as the time taken for the flare to decrease in intensity by a factor of (1=e) This is fitting since the flare decay often fits an exponential decay. Whilst the scope of this experiment is not to determine the values of this constant, its exponential nature will help confirm that the observations are of a flare event. 1.3 AT Mic AT Mic is a visual binary system both of which are of the type dMe. Table 1 specifies its significant properties necessary for observation. This particular star was observed by a team consisting of myself and four colleagues, using the 1.9m Grubb-Parsons telescope, situated at the South African Astronomical Observatory as part of a UCLan funded student experience. Time-sampling intervals of 1000 seconds were taken, until flaring was detected using a secondary 0.5m telescope conducting real time photometry. Upon detection of flaring the time-sampling period was reduced to a period of 500 seconds for approximately 2.5 hours, which should cover the flare and the stars return to quiescent level. AT Mic was not being observed at the onset of flare, instead once a flare was detected using the smaller photometric telescope the larger telescope had to be manoeuvred to the flaring target. This meant that the spectroscopic data will not show the two consecutive points at 3σ, however since the photometric data does show this, we can say with certainty that a flare is being observed. Table 1: Observational Data for AT Mic Name AT Mic RA 20 41 51.1586 Dec -32 26 06.830 Visual Magnitude 10.25 Spec Type. M4Ve 4 1.4 Data Reduction The spectroscopic data from the telescope needed to be reduced to remove additive effects such as background noise and differences in pixel sensitivity. For the SAAO data gathered this comprised of debiasing and flat-fielding. 1.4.1 Debiasing The bias level is an offset added to the signal readout of the CCD which makes sure the input value to the Analogue-to-Digital converter is always positive [Kilkenny and Worters, 2010]. It is an intrinsic noise which must be subtracted from all output CCD images. This is done by taking bias-frames, 0 second exposure readouts, averaging these and subtracting it from the objective CCD images. 1.4.2 Flat Fielding CCD response varies across the instrument, resulting in individual pixels have differing sensitivities. To overcome this, flat fields are taken during observations of photomet- rically flat source. In the case of the data gathered at SAAO, the 1.9m telescope was positioned targeting a white section of the dome, which was illuminated uniformly. Once several flat field images are found, an average is taken, and then all objective CCD frames are divided through resulting in a constant level of sensitivity across the images. 1.4.3 Other Reduction Techniques Dependant on the instrument used, there is sometimes a requirement for Dark frames to be taken. However due to the nature of the SAAO CCD used and its cooling levels, this is not a necessity for the data gathered. The SITe CCD used had a dark count of a few counts per pixel per hour and so this can be ignored.There was also a slight issue with the spectrograph becoming slanted during observations, for some unknown reason, which needed to be extracted with care to avoid loss of data. 2 Methodology 2.1 IRAF 2.1.1 Image Reduction As described earlier, the first part of the project involved taking raw telescope data and correcting the CCD frames for additive effects. Many modern computing programs are 5 available across differing platforms, one of the more widely available being the Image Reduction and Analysis Facility (IRAF) created by the National Optical Astronomy Observatories (NOAO). This is free-ware and is renowned for its cross-platform compat- ibility. IRAF is no longer supported by an official funded team, and as such technical support is given by volunteers. Since the program was not available on the university network, it had to be installed on a personal laptop. Problems with the downloaded binaries and the installation script, coupled with volunteer based support created un- foreseen delays in the beginning of the data analysis, although the problems that arose gave good experience of the issues astronomers can have to deal with, when installing older programs that have passed out of funded service and maintenance. IRAF is set up to work for certain CCD types \out of the box" however parameters had to be refined to work with the header information set by the SAAO telescopes. This mainly comprised of editing translation tables so that IRAF could understand the SAAO specific header information. This was needed for specific packages, for example allowing the program to determine the difference between flat, image and bias frames. Over the period of the observing week bias frames were taken which needed to be com- bined to form a master bias. This was done using the zerocombine procedure within the noao, imred, ccdred package. Similarly the flat fields were combined to form a master flat, using the flatcombine procedure within the same package.
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