Over the period 2012-2016 we have obtained accurate V and R band
photometry and 3400-8000 Ang spectroscopy which show the poorly studied
variable GH Lib to be a multi-periodic Mira and not an Algol-type eclipsing
binary.  The main pulsation periods are 157 and 1180 days, with amplitudes
of 3.5 and 1.0 mag, respectively.  The spectral type change from M2III at
maximum to M7III at minimum.  The intensity of Halpha and Hbeta
emission lines in GH Lib is much larger than in normal Miras, suggesting
that the region of Balmer line formation is located at a larger radius (more
external atmospheric layers) as if mixed with and not deeply below the
region where absorption by TiO molecules occurs

Graziani, Mauro

Munari, Ulisse

Righetti, Gian Luigi

Siviero, Alessandro

Information Bulletin on Variable Stars

Alessandro Siviero

Ulisse Munari

Gian Luigi Righetti

Mauro Graziani

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GH Lib: A multi-periodic Mira, not an eclipsing binary

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COMMISSIONS G1 AND G4 OF THE IAUINFORMATION BULLETIN ON VARIABLE STARSVolume 62 Number 6183 DOI: 10.22444/IBVS.6183Konkoly ObservatoryBudapest13 September 2016HU ISSN 0374 – 0676GH Lib: A MULTI-PERIODIC MIRA, NOT AN ECLIPSING BINARYSIVIERO, ALESSANDRO1; MUNARI, ULISSE2; RIGHETTI, GIAN LUIGI3; GRAZIANI, MAURO31 Dipartimento di Fisica ed Astronomia, Universitá di Padova, 36012 Asiago (VI), Italy2 INAF Osservatorio Astronomico di Padova, Sede di Asiago, I-36032 Asiago (VI), Italy3 ANS Collaboration, c/o Astronomical Observatory, 36012 Asiago (VI), ItalyGH Lib (≡BV 1623, α = 14h36m07.s82, δ = −24◦21′50.′′9 J2000) was discovered onsky patrol plates as a variable star and classified as an eclipsing binary of the Algoltype by Carter (1975), who remarked that too few minima were observed to derive anorbital period. Based on Carter’s report, Kukarkin et al. (1977) included GH Lib inthe General Catalog of Variable stars and Malkov et al. (2006) in their compilation ofeclipsing binaries, where no information appear in addition to what already given in thediscovery paper.Intrigued by the lack of suitable follow-up observations, we have obtained photometricand spectroscopic observations of GH Lib from 2012 to 2016. They show GH Lib to bean oxygen-rich Mira variable, of the multi-period type, and not an eclipsing variable. Inspite the conclusion by Kharchenko (1985) that nearly all Miras with a peak magnitudebrighter than 14 have already been discovered, here we have a brand new Mira variablepeaking at V=11 mag.V and RC optical photometry of GH Lib has been obtained with two ANS Collaboration0.30-m f/10 Meade LX200 telescopes located in Granarolo (Bologna, Italy) and Alfonsine(Ravenna, Italy). Aperture photometry was carried out at both telescopes against thesame local photometric sequence extracted from the APASS survey (Henden et al. 2012).The photometry we collected on GH Lib during 50 independent nights, is given in Table 1(provided as online data to the paper). Our V -band data are plotted in Figure 1 togetherwith archive V -band data extracted from the ASAS database (Pojmanski 1997).The light curve of GH Lib in Figure 1 is that of a Mira pulsating with more than oneperiodicity. A Fourier analysis using the code by Deeming (1975), shows the main periodto be 157 days, with the following ephemeris providing times of maxima in the V band:JDVmax,primary = 2454299 + 157 × E (1)where E is the progressive cycle number. The corresponding phased light curve is givenin Figure 2. The several branches which are clearly visible in Figure 2, the ∼1 magspread similarly present at all phases, and the apparently regular up-and-downs in thepeak brightness along the 157 day cycles in Figure 1, all points to the presence of a secondperiodicity. We have searched the residuals from the mean light curve in Figure 2 (thered continuous curve) and again performed a Fourier analysis that returned 1180 days as2 IBVS 6183– by large margin – the strongest peak. The following ephemeris provides the maxima forthis secondary periodicity:JDVmax,secondary = 2454225 + 1180 × E (2)The sinusoidal combination of these two pulsation periods is over-plotted to the data asthe continuous line in Figure 1, with an amplitude of 3.5 mag for the 157 day and 1.0mag for the 1180 day cycle. The total amplitude of variation of GH Lib is 4.5 mag in V .Secondary periods have been observed in several pulsating M giants, either of the Mira(Percy and Bagby 1999) or semi-regular types (Kiss et al. 1999). In the majority of cases,however, the cycle-to-cycle differences seems due to low-frequency stochastic variability(Eddington and Plakidis 1929, Percy and Colivas 1999, Templeton and Karovska 2009)and not to secondary periods.Figure 1. Long-term V -band light curve of GH Lib obtained by combining our observations(JD>2456400, in red) with ASAS archive data (JD<2455200, in blue). The bi-periodic curve is thesinusoid combination of ephemerides 1 and 2. The four crosses mark the time of the spectra in Figure 3.Four low resolution spectra of GH Lib have been obtained at different epochs with the1.22m telescope + B&C spectrograph operated in Asiago by the Department of Physicsand Astronomy of the University of Padova. The spectra cover the 3400-8000 Å range ata dispersion of 2.3 Å/pix, and have been treated in a standard fashion in IRAF, includingcorrection for bias, dark and flat fields, subtraction of sky background, and wavelengthcalibration. Flux calibration has been achieved via spectrophotometric standard starsobserved during the same nights as GH Lib itself. The spectra are shown in Figure 3, andtheir corresponding position in the pulsation light curve of GH Lib is marked with crossesin Figure 1. By lucky coincidence, two of the four spectra were obtained very close to themaximum and minimum along the 157-day cycle, and the remaining two at the same 0.20phase. The continuum of the spectra at maximum and minimum are perfectly fitted bythose of M2III and M7III spectral type standards, respectively, and by an M5III at 0.20phase.The primary period of 157 days places GH Lib close to the short term border in theperiod distribution for Miras. Among the 5202 Miras with a pulsation period listed inthe General Catalog of Variable Stars (GCVS) at the time of writing, only 4% of theoxygen-rich type (M spectral type) have a period shorter than GH Lib. An amplitude of4.5 magnitudes and a M2III-M7III spectral range are similar – according to the GCVScompilation – to those exhibited by other Miras of equivalent pulsation periods.IBVS 6183 3Figure 2. ASAS data (crosses) and our V -band observations (dots) of GH Lib folded with the mainpulsation period of 157 days, following ephemeris 1. The continuous curve is a spline fit to the data.Figure 3. Spectra of GH Lib at four distinct epochs. To enhance visibility, an offset in ordinates isapplied as indicated. The phase is relative to the main pulsation period of 157 days, followingephemeris 1.4 IBVS 6183What is not typical is the large intensity of the Hα and Hβ emission lines visible in thespectra of GH Lib in Figure 3. Lower Balmer lines do not generally stand in prominentemission in low resolution spectra of Mira variables, the strongest observed emission beingthat of Hδ (Joy 1926). The reason for such an anomalous ratio in the intensity of Balmeremission lines is due to the overlapping absorption by TiO bands (Scott 1945). Shortwardsof 4300 Å there is essentially no absorption band from TiO, and therefore Hδ (4100 Å),H8 (3889 Å) and H9 (3835 Å) can emerge unabsorbed through the stellar atmosphere(Merrill et al. 1962). On the contrary, at the redder wavelengths of Hα and Hβ theabsorption from many strong and overlapping TiO bands effectively blocks the emergenceof radiation from internal atmospheric layers.In Miras, the ionization of hydrogen responsible for the observed emission lines occursin a shock region in the atmosphere (Joy 1947, Merrill 1955), where infalling materialfrom the previous pulsation cycle collides with that moving outward from the followingcycle (Fox et al. 1984, Gillet et al. 1985). This shock region is usually located deep inthe atmosphere, well below the region from where TiO absorption originates (de Lavernyand Magnan 1995). In GH Lib, the Hδ/Hγ/Hβ/Hα intensity ratio is 1.00/1.34/1.44/1.48,1.00/1.51/1.88/4.84, and 1.00/1.17/1.51/3.25 for the 2013 Feb 8, 2012 May 11, and 2013Mar 22 spectra in Figure 3, respectively. For comparison, this same ratio measuredon some spectra of o Cet (obtained with the same instrumentation as for GH Lib) is1.00/0.40/0.05/0.62 (similar numbers are tabulated by Crowe and Garrison (1988) for alarge sample of southern Miras). We conclude that the region of Balmer line formationin GH Lib seems located at a larger radius (more external atmospheric layers) than intypical Miras, as if mixed with and not deeply below the region where absorption by TiOmolecules occurs, which poses the interesting question of how TiO can survive in the sameplace where hydrogen is collisionally ionized (comparing the 6.8 eV ionization potentialfor TiO with that of hydrogen, 13.6 eV). The relative location within the atmosphere ofGH Lib of the regions where Balmer emission lines and TiO absorption bands form shouldbe investigated via radial velocities from high-resolution spectra.References:Carter, B. S., 1975, IBVS, 994Crowe, R. A., Garrison, R. F., 1988, ApJS, 66, 69 DOIDeeming, T. J., 1975, Ap&SS, 36, 137 DOIde Laverny, P., Magnan, C., 1995, ASPC, 83, 419Eddington, A. S., Plakidis, L. 1929, MNRAS, 90, 65 DOIFox, M. W., et al., 1984, ApJ, 286, 337 DOIGillet, D., et al. 1985, A&A, 150, 89Henden, A., et al., 2012, JAAVSO, 40, 430Joy, A. H. 1926, ApJ, 63, 281 DOIJoy, A. H. 1947, ApJ, 106, 288 DOIKharchenko, N. V., 1985, Sov. Astr. Lett., 11, 25Kiss, L. L., et al., 1999, A&A, 346, 542Kukarkin, B. V., Kholopov, P. N., Kireeva, N. N., 1977, ASSL, 64, 131 DOIMalkov, O. Yu., Oblak, E., Snegireva, E. A., Torra, J., 2006, A&A, 446, 785 DOIMerrill, P. W., 1955, PASP, 67, 199 DOIMerrill, P. W., et al., 1962, ApJ, 136, 21 DOIPercy, J. H., Bagby, D. H., 1999, PASP, 111, 203 DOIPercy, J. H., Colivas, T., 1999, PASP, 111, 94 DOIIBVS 6183 5Pojmanski, G., 1997, AcA, 47, 467Scott, R. M., 1945, ApJ, 101, 71 DOITempleton, M. R., Karovska, M. 2009, ApJ, 691, 1470 DOI

GH Lib: A multi-periodic Mira, not an eclipsing binary

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