Using data from the Joan Oró telescope (located in Montsec, Catalonia) and the reference catalogue from Max Planck Institute, we will observe the evolution of different novae in the M81 galaxy.
This article explains the data analysis that has been performed and comments the obtained results2020/202

Jumilla Lorenz, Jordi

Real Fraxedas, Fèlix

Sala Cladellas, Glòria

English

UPCommons. Portal del coneixement obert de la UPC

M81 extragalactic nova explosionsFèlix Real Fraxedas and Jordi Jumilla LorenzProjectes d’Enginyeria F́ısica 2 - Engineering Physics - Universitat Politècnica de CatalunyaTutor: Gloria Sala Cladellas(Dated: June 4, 2021)Using data from the Joan Oró telescope (located in Montsec, Catalonia) and the reference cata-logue from Max Planck Institute, we will observe the evolution of different novae in the M81 galaxy.This article explains the data analysis that has been performed and comments the obtained results.I. INTRODUCTIONIn a binary system formed by a white dwarf and a starthat has left the principal sequence, a matter transfer isproduced from this last star due to its transformationinto a red giant. An outburst is produced, caused byexplosive hydrogen burning on the accreted envelope ofthe white dwarf in a cataclysmic variable [1]. The out-ermost layers of the red giant are expanded and ejected,and are gravitationally captured by the white dwarf (thisphenomenon is called accretion). The accumulated ma-terial, composed mostly of hydrogen and helium, is com-pacted at the surface of the white dwarf due to the intensegravitational field. The temperature of the material in-creases progressively as more and more material buildsup at the surface of the star until it eventually gets to acritical temperature and the nuclear fusion starts. Thisexplosion transforms large quantities of hydrogen and he-lium into heavier elements. It causes the brightness ofthe star to increase to maximum luminosities up to 105Lsun. Nonetheless, a fraction of the hot envelope can re-main in steady hydrogen burning on the surface of thewhite dwarf [2] [3], powering a supersoft X-ray source(SSS) that can be observed directly once the ejected en-velope becomes optically thin to X-rays. The duration ofthe SSS phase is related to the white dwarf mass and thechemical abundances of the envelope, whereas the timeneeded by the envelope to become transparent and letthe SSS be observable is related to the ejected mass.Our position within the Milky Way introduces largebiases in Galactic population studies. Therefore investi-gating novae in other galaxies not only gives a better rep-resentation of the overall nova population of those galax-ies but also allows us to study how the properties of anova population vary with the Hubble type of a galaxy.For example, by studying novae in the Local Group(LMC, M31 and M33), it has been shown that novae inlater-type galaxies appear to be faster fading and morelikely to belong to the He/N spectroscopic class [4].The Galaxy M81, with a nova rate of 33± 13 per yearand at the distance of only 3.6 Mpc, is the obvious can-didate for the first detailed multi-wavelength study ofnovae beyond the Local Group. Its comparatively earlyHubble-type (SAab) also allows us to study novae in adifferent environment than can be found in the LocalGroup.Its orientation is almost face-on, which makes it afavourable target to distinguish between bulge and disknovae. To date, 240 nova candidates have been discov-ered in M81, the first one being observed on plates takenin 1950 [5].FIG. 1. M81 image obtained with TJO, with a total of 3200seconds of exposure, in R filter, Red circles indicate the loca-tions of all novae discovered in M81 since from 2000 to 2019.We will treat the M81 galaxy, also known as Bode’sgalaxy. It is a spiral galaxy relatively close to Earth (12milion light years), very attractive for the astronomers forits large size and activity. It is located in the constellationof Ursa Major, and its coordinates are right ascension of09h 55m 33.2 s and declination +69º 3’ 55”.The observational data has been obtained from JoanOró Telescope (TJO), of the Observatori Astronòmic delMontsec (OAdM). It is the largest telescope in Catalonia(0.8 m). The TJO is equiped with the LAIA instrument,which is the optical imager of TJO. The field of view atTJO without vignetting is 30 arcmin. The CCD camerahas a pixel size of 15 x 15 µm (0.4 x 0.4 arcsec). Theobservations have been done using the red (R) filter ofthe Johnson-Morgan standardized photometric system.Novae emit large amount of light in the Hα range of thespectrum, which the R filter covers. This filter is used todetect novae since standard stars do not emit with such2intensities in the Hα line.After this introduction, now we can start explainingwhat we have done in this project to clear, treat andanalyze the given data.II. DATAAs we have commented, the data we will treat comefrom observations of the galaxy M81. We have used thecatalogue of Max Planck für extraterrestrische Physikas a reference, which contains the registry of M81 de-tected novae since 1950 until nowadays: https://www.mpe.mpg.de/~m31novae/opt/m81/. It is a catalogue ofnovae with their correspondent position and discoverydate.This catalogue was the first step in our project: firstly,we had to understand the magnitudes and units used init. In astronomy, there are coordinates similar to latitudeand longitude, but they are called declination and rightascension. Right ascension (α) is the angular distance ofa particular point measured eastward along the celestialequator and it is usually measured in degrees, minutesand seconds. The declination (δ) is measured north orsouth of the celestial equator along the hour circle pass-ing through the point to measure. It is measured between+90º i -90º. Another essential magnitude is the mag-nitude of observation (similar to the intensity). It canprovide information of what process is the the nova fol-lowing. This magnitude goes in the inverse of usual: themost intense observations have a magnitude of approxi-mately 18 and the most weak ones a magnitude over 22.Finally, an important magnitude is what is called modi-fied Julian date (MJD). The Julian date is the continuouscount of days since the beginning of the Julian period,and it is often used in astronomy and software to bettertreat the dates. In our work, we will use modified Juliandate, that subtracts 2400000.5 to the Julian date to workwith smaller numbers.In order to treat all the data, we decided to usePython programming language, as it is very completeand has a lot of libraries that ease the work withfiles, its treatment and the creation of plots. To bet-ter treat this data, we created a first program, calledmax planck DB to csv.py, where we transferred themagnitudes we wanted from the previous database to.csv format, easier to treat later on. Also, in this scriptwe converted the right ascension and declination to de-grees, to facilitate the comparison with the observations.Moreover, we were given two data folders, one with files.reg and the other one with files .log, with photometricdata from 22 March 2016 to 10 December 2018 takenfrom the Montsec telescope. The former ones are fileswith a format ready to represent the observations in aplot or image. In this way, we had information, separatedby days with the coordinates of every source that hadbeen detected and its corresponding magnitude. For eachday of observation we also had a file with the observationsfiltered to select only the ones that could correspond tonovae. On the other hand, the .log files (also one foreach day of observation) give a summary of the performedobservations, uncertainties, date and time of observation,filter,...In order to better compare the observation files withthe reference table, we followed a similar procedure tothe previous one: we wrote an script that converted the.reg files to .csv files, creating the folder REG modified.III. NOVAE DETECTION ALGORITHMIn order to identify novae among the stars detectedin our Montsec data, we compared the coordinates (dec-lination and right ascension) of the sources detected inthe observations (.reg files) with the ones in the MaxPlanck catalogue. Our approach is to compare the dis-tance between the observed and the catalogued positionswith a certain threshold. If their distance is smaller thanthis threshold value, we have a coincidence, otherwise wediscard the observation.Our first approach was to compare both declinationand right ascension differences to a constant tolerance,that is:|αdb − αobs| < tol and |δdb − δobs| < tol (1)We took tol = 10−3 deg and it worked quite good. Itwas able to filter out most of the non-novae stars. How-ever it had some obvious drawbacks and could be easilyimproved. First of all, we were considering a square ofside 2·tol around the database coordinated of the star,but it makes more sense to compare the actual distance.Moreover, each observation has its own error in α and δand we are not considering it. Our final algorithm consid-ers an ellipse with semi-axis depending on the particularerror for α and δ of that day.(αdb − αobs)2(8σα)2+(δdb − δobs)2(8σδ)2< 1 (2)Since the fraction of the sky that the M81 galaxy oc-cupies is small, we approximate its coordinates (α and δ,which are in the end spherical coordinates) as cartesiancoordinates.With this algorithm, we still get some anomalous re-sults, but they are few compared to the correct ones.Now what we had was a folder with one file for eachday of observation containing all the candidates to novaobserved that day.IV. FINAL LIGHT-CURVE SYNTHESISAs we have mentioned, until this step we work with theobservations separated by date. The goal of the project3is to follow the evolution of different novae in the M81galaxy and, to do so, it is much better to classify theobservations by nova, not by date. This was the followingstep of the project.We created a script, called results.py which, search-ing by the name of the nova, travels through the folderof candidates and creates a file for each nova containingall the observations of it (including the date of discoveryfrom the Max Plank catalogue too).After that, the only step left is to plot the results. Todo it, we created a script, called plots.py. To plot theresults, we took the files generated by results.py andgenerate a .png file for each nova.As it can be seen in figure 4, we have plotted the ob-servations made by the Montsec telescope, the discoveryobservation, taken from Max Planck catalogue and foreach day in the range of the observations where the novahas not been detected, an arrow in the minimum mag-nitude observed that day. This provides and upper limitfor the magnitude of the nova if it has not been detectedthat particular day. It is possible that that specific daythe sky was not very clear and thus very faint magni-tudes could not be observed. Also in the same figure,fore each observation, an estimation of the error is plot-ted. With that, we obtain the range of possible values ofthe magnitude.In the annexed material, a scheme of all the steps andscripts and their organization is shown.V. RESULTSNow we can comment the results of our project. Wehave collected 157 observations of possible novae. How-ever, as we have commented, some of them have to be dis-carded. Some of the stars were discovered around 1950,according to the initial database and, as the observationsof Montsec telescope we work with start in 2016, the co-incidences in coordinates with that novae are accidents.With that discarded observations, we end up with 29observations of 14 different novae. In figure 2 we can seea distribution of this stars by number of observations. Aswe can see, most of them contain only one observation(apart from the discovery date) and three of them havethree or more observations.Another interesting parameter to observe is the max-imum magnitude observed for each star. In figure 3 wesee that most of the novae are observed with a maxi-mum magnitude over 20 (remind that bigger magnitudeimplies weaker observation) and this weak observationsusually coincide with the novae with less observations.In the following table we summarize the results pernovae. We have only included the novae from the MaxPlanck database with which we have found real coinci-dences. The first column is the nova name, the secondcolumn indicates the maximum observed magnitude andthe third column counts the number of matches we havefound for each nova in our data.FIG. 2. Novae classified by number of observations of each.FIG. 3. Histogram of the number of novae classified by max-imum magnitude of observation.Novae observationsNova name Max. obs. mag. # of obs.2018-04b 19,8 12018-03b 20,5 22018-03a 20,4 12018-02a 18,5 72018-01a 20,2 22017-04b 20,2 12017-04a 20,5 22017-02a 19,4 42017-01a 19,4 12016-12b 20,8 12016-11d 20,8 32016-11a 20,6 12016-10a 20,0 22016-06a 19,5 1TABLE I. Novae coincidences summary.In the next two figures (2 and 3) we show the resultsobtained for the novae M81 2017-02a and M81 2018-02a,respectively and then we will comment them.4FIG. 4. Observations for two different novae: 2017-02a(above) and 2018-02a (below).In the previous section, we commented the informationthat appears in the plots, so we can proceed on comment-ing the results for each nova.In the case of the M81 2017-02a nova we have an obser-vation around 2 days after the discovery day. We detectit passed 4, 7 and 8 days. We see that in the obser-vation between the 6th and the 7th day we do not de-tect the nova despite the minimum magnitude that daywas around 20.7 whereas the observed magnitude for thisnova was around 19.5-20 the previous and following daysand so, it should had been detected. This can be due toan imprecision of the algorithm or in the data that day,but we cannot know it. Except for that, the evolution ofthe magnitude of the nova seems correct. We start withlow magnitude, increase a little and then descent again(note that the magnitude of the discovery date cannotbe directly compared with the observed, as it does notcome from the same telescope).As for the second figure, from M81 2018-02a, after thediscovery day we have 6 observations. They follow a sim-ilar evolution of magnitude as in the case of the M812017-02a, with the 6 observation in the 7 days after thediscovery. Around 21 days before the discovery day, wehave an observation match with the same coordinates asM81 2018-02a and a very low magnitude of around 21.8.The days after in which we had observations, the min-imum magnitude observed those days was greater that21.8. Therefore, we suggest that this nova could havebeen discovered before its official discovery day, althoughit was with a very low magnitude.A parameter that is usually used for studying the evo-lution of the novae is the time that it takes for the mag-nitude to decay 2 units from its maximum value. In ourcase, as we have not got many observations, it does notmake much sense to calculate it, but we can give an es-timation of the time that it takes for the magnitude todecay 1 unit. For example, in the first case, the novaM81N 2017-02a has its maximum magnitude in the sec-ond observation, in around 19.4 and the last observationin around 20.4. So, it takes approximately 4 days to de-crease 1 point of the magnitude. In the other nova, themaximum value is obtained in the 4th observation afterthe discovery, at around 18.5. The last observation hasa magnitude of approximately 19.2 and between themthere are two days. Making a simple linear estimation, itwould take almost 3 days from the maximum magnitudeto decay 1 unit.VI. CONCLUSIONSThe goal of the project was to observe the evolution ofnovae with the photometric data from Montsec telescopeand comparing it with the catalogue of Max Planck Insti-tute. We have not obtained a big quantity of novae withgood observations. However, in the ones that we have(or we have found) good observations, we have clearlyseen the evolution of the magnitude of observation of thenovae.The project has brought us the opportunity to get toknow better what are novae, what is the work that isperformed in order to study them and also to refreshsome of our programming skills.VII. BIBLIOGRAPHY[1] Jordi José; Margarita Hernanz. Nucleosynthesis in classi-cal novae: Co versus one white dwarfs. https://arxiv.org/abs/astro-ph/9709153, February 1998.[2] Sumner Starrfield. Cno abundances and hydrodynamicmodels of the nova outburst. ii-1.00 solar mass models withenhanced carbon and oxygen. http://adsabs.harvard.edu/pdf/1974ApJS...28..247S, 1974.[3] Glòria Sala; Margarita Hernanz. Models for thesoft x-ray emission of post-outburst classical no-vae. https://www.aanda.org/articles/aa/pdf/2005/33/aa2622-04.pdf, 12 April 2005.[4] A. W. Shafter et. al. A spectroscopic and photo-metric survey of novae in m31. https://iopscience.5iop.org/article/10.1088/0004-637X/734/1/12/pdf, 19May 2019.[5] Michael M. Shara et. al. The early palomar program(1950–1955) for the discovery of classical novae in m81: Analysis of the spatial distribution, magnitude distribu-tion, and distance suggestion. https://iopscience.iop.org/article/10.1086/316449/pdf, 27 May 1999.

M81 Extragalactic nova explosions

https://upcommons.upc.edu/bitstream/2117/393806/1/Final_project_PEF2_v3.pdf

M81 Extragalactic nova explosions

Abstract

Similar works

Full text

Available Versions

UPCommons. Portal del coneixement obert de la UPC