Differential photometry of delta Scorpii during 2011 periastron
aa r X i v : . [ a s t r o - ph . S R ] D ec IL NUOVO CIMENTO
Vol. ?, N. ? ? Differential photometry of delta Scorpii during 2011 periastron
C. Sigismondi ( )( )( )( )( ) ( ) Sapienza University of Rome, P.le Aldo Moro 5 00185, Roma (Italy) ( ) ICRA, International Center for Relativistic Astrophysics, P.le Aldo Moro 5 00185, Roma(Italy) ( ) Universit´e de Nice-Sophia Antipolis (France) ( ) Istituto Ricerche Solari di Locarno (Switzerland) ( ) GPA, Observatorio Nacional, Rio de Janeiro (Brasil)
Summary. — Hundred observations of Delta Scorpii over 200 days, from April 2to October 16, 2011, have been made for AAVSO visually and digitally from Rio deJaneiro, Rome and Paris. The three most luminous pixels either of the target starand the two reference stars are used to evaluate the magnitude through differentialphotometry. The main sources of errors are outlined. The system of Delta Scorpii,a spectroscopic double star, has experienced a close periastron in July 2011 withinthe outer atmospheres of the two giant components. The whole luminosity of DeltaScorpii system increased from about Mv=1.8 to 1.65 peaking around 5 to 15 July2011, but there are significant rapid fluctuations of 0.2 - 0.3 magnitudes occurring in20 days that seem to be real, rather than a consequence of systematic errors due tothe changes of reference stars and observing conditions. This method is promisingfor being applied to other bright variable stars like Betelgeuse and Antares. AfterAugust the magnitude remained constant at Mv=1.8 until the last observation onOctober 16 made in twilight from Rome.PACS – Digital imaging astronomy.PACS – Binary stars.PACS – Variable stars.PACS – Accreting binary systems explosive burning.
1. – Introduction
Delta Scorpii is a double giant Be star in the forefront of the Scorpio, well visible tothe naked eye, being normally of magnitude 2.3. In the year 2000 its luminosity rose upsuddenly to the magnitude 1.6, changing the usual aspect of the constellation of Scorpio.This phenomenon has been associated to the close periastron of the companion, orbitingon an elongate ellipse with a period of about 11 years. The new periastron, on basisof high precision astrometry, occurred in the first decade of July 2011, with the secondstar of the system approaching the atmosphere of the primary, whose circumstellar disk c (cid:13) Societ`a Italiana di Fisica C. SIGISMONDI has a H-alpha diameter of 5 milliarcsec, comparable with the periastron distance [1].The results of a photometric campaign visual and digital, with observations made in Riode Janeiro and in Rome are presented. The method of data analysis of the differentialphotometry, based on Poisson statistics, it is suitable to be used also with other brightvariable stars like Betelgeuse by a large public and with commercial cameras. This is inthe spirit of fostering the ”citizen astronomy” of AAVSO.
2. – Why Delta Scorpii?
When in Rio de Janeiro, visiting the Observatorio Nacional, I realized that in myhome at 142 rua Marquˆes de Abrantes, the window aimed exactly the azimut of thecelestial objects rising to the zenith: Delta Scorpii shined there during one clear eveningof March 2011 and I identified it from this remarkable property. Only later with theapparition of Antares I recognized the usual geometry of Scorpio, upside down becauseof the Southern hemisphere.The forthcoming periastron of delta Sco expected in July 2011 [1, 2, 3] led me tomonitor this star, and the AAVSO to select it as the star of that month [4].Delta Scorpii is an easy target to be found with naked eyes, it is visible even in urbancontexts with heavy luminous pollution, and therefore it opens the field of naked eyevariable stars which is far from being saturated by the citizen astronomers.
3. – Differential photometry for visual observations and airmass corrections
The choice of beta (
M v = 2 .
50, from Simbad website( )) and pi ( M v = 2 .
89) Scorpiias reference stars for the images taken with my CMOS is due to their color index B − V ∼
0, similar to the one of delta Scorpii. When pi Scorpii was too low and not visible becauseof the hazes near the horizon I used alfa Scorpii (
M v = 1 .
09, but also slightly variable)as reference.When, for visual observations, I changed the reference star to eta Ophiuchi (
M v =2 . M v = 2 .
10) wasa better reference [4], even if farther than eta Oph.With the star near the horizon there are great differential effects even for small angulardistances like between beta and delta Scorpii.Near the horizon the airmass estimate with Garstangs [5] model does not fully explainthe loss of luminosity of the star with respect to its references: when for visual obser-vations I used alpha Ophiuchui, to apply the airmass correction I preferred the classicalcosecant law, even with its infinity at zero altitude: ∆ M = 0 . / [ sin ( h ) − sin ( h )] with h the height of delta Sco and h of alpha Oph. A layer of humid air near the groundafter sunset in summer time, for observations made in the last few degrees above thehorizon can act as 60 airmasses of clear atmosphere [6] while for Garstang there are only5 airmasses which determine a loss of ∆ M = 0 .
65 magnitudes.Moreover I have selected reference pairs of stars in order to compare visually withthem the difference in magnitude between delta, alpha and beta Scorpii. These pairs arealpha and beta Centauri (∆
M v = 0 .
70) for the Southern hemisphere and alpha Lyrae ( ) http://simbad.u-strasbg.fr/simbad/ IFFERENTIAL PHOTOMETRY OF DELTA SCORPII DURING 2011 PERIASTRON Fig. 1. – Plot of hundred observations of Delta Scorpii: visual (blue diamonds) and digital (redsquares). The star reaches its maximum luminosity Mv ∼ .
65 around July 5-15, 2011. The restof the time the luminosity remains around the magnitude Mv ∼ .
8, with some fluctuations,possibly physical, of ∆ M = 0 . − . and alpha Aquilae (∆ M v = 0 .
74) for the Northern one. Delta was for most of the timeexactly intermediate (i.e. of
M v = 1 .
8) between alpha and beta so that the differencebetween alpha and delta was ∆
M v = 0 . M v = 0 . ◦ above the horizon.Delta Scorpii undergoes variations of luminosity with a 60 days period [4], this couldexplain the changes in luminosity of ∆ M = 0 . − . M v = 2 . M v = 2 .
27 on June 14.05, 2.17 on July 11.9 and 2.18 on August 7.8 all with the Moonat 1 ◦ − ◦ , even if occulted) which seem to be spurious.
4. – Conclusions
The digital method requires always calibrations and the stars have not to saturatethe detectors. Moreover the detectors have to work in their linear regime.
C. SIGISMONDI
Fig. 2. – The scheme of the techniques adopted for the observations and for the analysis of them.
The visual method shows that the errors are reduced by using always the same refer-ence stars.Beyond some real physical phenomenon appearing, the intrinsic scatter of the databasesof such stars shows the complexity of the task to reduce the visual and digital errorbarbelow 0.1 magnitudes.Alpha Orionis and Alpha Scorpii itself, are very interesting targets of similar ob-servational campaigns, as well as other bright stars. Being the brightest stars in theirsurroundings the calibration of the detectors is crucial for the success of these studies.
REFERENCES[1]
Meilland A., et al. , Astron. Astrophys. , (2011) A80.[2] Sigismondi C. , http://arxiv.org/abs/1107.1107 , (2011) .[3] Ames A.,Tycner C., and
Zavala R. , Proc. International Astronomical Union Symposium , (2010) 278.Fig. 3. – The stars with their brightest 3 pixels, used for the estimation of the magnitude. Hereis the image of July 12, 2011, with Delta very close to the full Moon. The intensity of each staris proportional to the sum Σ of the three brightest pixels in the G channel. The relative errorof such hypothesis of proportionality to the sum Σ, due to Poissons statistics, is 1 / √ Σ. Thischannel is the closest to Johnsons V band [7]. The luminosity of delta Scorpii with respect toits reference stars β and π is Mvβ, π − . δ/ Σ β, π ). The value published in the AAVSOwebsite is the average ( Mvβ + Mvπ ) / Mvβ and
Mvπ . IFFERENTIAL PHOTOMETRY OF DELTA SCORPII DURING 2011 PERIASTRON [4] Otero S. , , (2011) .[5] Garstang R. H. , Proc. Astron. Soc. Pacific , (1989) 306.[6] Sigismondi C. , http://arxiv.org/abs/1106.2514 , (2011) .[7] Lanciano O. and
Fiocco G. , Applied Optics ,46