Two-site CCD observations and spectroscopy of HD 207331: a new δ Scuti variable in Cygnus
L. Fox Machado, W.J. Schuster, C. Zurita, J.S. Silva, R. Michel
aa r X i v : . [ a s t r o - ph . S R ] J u l Two-site CCD observations and spectroscopy of HD 207331: a new δ Scuti variable inCygnus
L. Fox Machado a, ∗ , W.J. Schuster a , C. Zurita b , J. S. Silva a , R. Michel a a Observatorio Astron´omico Nacional, Instituto de Astronom´ıa – Universidad Nacional Aut´onoma de M´exico, Ap. P. 877, Ensenada, BC 22860, Mexico b Instituto de Astrof´ısica de Canarias C / V´ıa L´actea s / n, 38205 La Laguna, Tenerife, Spain Abstract
The results of an observational campaign on the new δ Scuti pulsator HD 207331 are reported. The star was observed photo-metrically from August 26 to September 2, 2009 from the Observatorio San Pedro M´artir (0.84-m telescope, Mexico) and theObservatorio del Teide (0.80-m telescope, Spain). An overall run of 53.8 h of useful data was collected from the two sites. Fouroscillation frequencies for HD 207331 have been found above a 99% confidence level. These results confirm the multiperiodicpulsation nature of the star suggested in previous observations with sparse data. Spectroscopic observations carried out in 2009allowed us to derive its spectral type and luminosity class as well as to estimate its rotation rate. A simple comparison with modelsis performed.
Keywords: stars: δ Sct – techniques: photometric, spectroscopic – stars:oscillations – stars: individual: HD 207331, BD +
42 4208,7 Aql, 8 Aql.
PACS:
1. Introduction
The δ Scuti-type pulsators are stars with masses between 1.5and 2.5 M ⊙ located at the intersection of the classical Cepheidinstability strip with the main sequence. They have spectraltypes A and F, a period range between 0.5 h and 6 h, and gener-ally pulsate with a large number of radial and nonradial modesexcited by the κ -mechanism associated with the second Heliumionization zone. These oscillation modes penetrate to di ff erentdepths inside the star. Thus, δ Scuti stars provide a good op-portunity to probe the internal structure of intermediate massstars.Since most of the δ Scuti stars are short period variables withtypical photometric amplitude of 20 mmag, their oscillationscan be easily detected from the ground. In fact, several δ Scutistars have been discovered accidentally when taken as refer-ence stars for observations of well known δ Scuti stars [e.g.Fox Machado et al. (2002, 2007); Li et al. (2002)]. Howeverto resolve their rich oscillation spectrum from the ground, highquality long time series are required which can only be obtainedby means of observations from di ff erent sites distributed in lon-gitude around the Earth (e.g. Li et al. 2004; Costa et al. 2007).Therefore, to characterize the pulsation spectrum of a new δ Scuti star additional observational e ff orts are needed.In this paper we present the results of photometric and spec-troscopic follow-up observations of HD 207331, a new δ Scutistar recently discovered in Cygnus, aimed at characterizing itspulsation behavior more accurately. ∗ Corresponding author. Tel.: +
52 6461744580; fax +
52 6461744607
Email address: [email protected] (L. Fox Machado)
2. The object HD 207331
The star HD 207331 ( = SAO 51294, BD +
42 4207,HIP 107557) was discovered to be a new δ Scuti star bySchuster et al. (2008) when observing a sample of A-type starson the night of September 27, 2007, in order to test the six-channel uvby - β spectrophotometer attached to the H. L. John-son 1.5-m telescope of the San Pedro M´artir Observatory, BajaCalifornia, Mexico. The variability of the star was clear, despitethe fact that it was observed less than 2 h. Six hours of CCDphotometric observations on the night of September 30, 2007,with the 0.84-m telescope of the same observatory confirmedits variability (Schuster et al., 2008).Di ff erential and standard Str¨omgren ( uvby ) photometry ofHD 207331 was obtained by Fox Machado et al. (2008) usingthe same equipment as the former observations. In particu-lar, the star was monitored for about 11 h between November11 and November 19, 2007. As a result, Fox Machado et al.(2008) found evidence of at least two close frequencies whichmight explain the resulting beating behavior of the light curve.The derived standard photometry is the following: ( V , b – y , m , c ) = (8.329, 0.125, 0.150, 1.018). Fox Machado et al. (2008)also estimated a stellar reddening from the reddening maps ofSchlegel et al. (1998) via the web-page calculator of the NED(NASA / IPAC Extragalactic Database). The derived intrinsiccolours of HD 207331 are the followings ( V , ( b – y ) , m , c ) = (7.980, 0.044, 0.174, 1.002), and β = . M V = .
058 mag, and d =
242 pc.The Hipparcos catalogue (Perryman, et al., 1997), on theother hand, provides a parallax of 3.31 ± ±
81 pc can be estimated. This corre-
Preprint submitted to New Astronomy November 1, 2018 igure 1: CCD imagen of the HD 207331 FOV at the 0.84m telescope. 1 standsfor HD 207331, 2 for the comparison star, and 3 for the check star. Someproperties of the stars are listed in Table 1. North is up and East is left. sponds to a distance modulus of 7.40 ± M v = . ± σ error withdistance reported by Fox Machado et al. (2008). The Hippar-cos catalogue also lists for HD 207331 a magnitude of H p of8.3970 ± ± V T of 8.335 ± ff
3. Observations and data reduction
The two observatories involved in the observational cam-paign are the following:-Observatorio del Teide (OT, Spain): the data were collectedusing the IAC 80 telescope (0.80-m). The images wereacquired with CAMELOT, a CCD detector of 2048 × × B filter.Figure 1 shows a typical image of the CCD’s field of view(6 . . ′ × . ′ ) at the 84-cm telescope of the San Pedro M´artirobservatory. The target star is labeled with number 1; com-parison and check stars with 2 and 3, respectively. As canbe seen this is an uncrowded star field. Although the CCD’s Figure 3: Normalized spectrum of HD 207331. For sake of clarity only theregion between 3800 Å − FOV at the IAC-80 telesope of the Teide observatory is larger( ∼ ′ × ′ ), no other suitable comparison stars could be ob-served. Table 2 shows the main observational parameters cor-responding to the target and comparison stars as taken from theSIMBAD database operated by the CDS (Centre de Donn´eesastronomique de Strasbourg).Sky flats, dark and bias exposures were taken every night atboth sites. All data were calibrated and reduced using standardIRAF routines. Aperture photometry was implemented to ex-tract the instrumental magnitudes of the stars. The di ff erentialmagnitudes were normalized by subtracting the mean of di ff er-ential magnitudes for each night. In Figure 2 the entire lightcurves, HD 207331 - Comparison, are presented. As can beseen from the fourth panel (from top to bottom) no overlappingof the observations was obtained for the one night of observingat OT, the 28th of August 2009. Spectroscopic observations were conducted at the 2.12-mtelescope of the SPM observatory during September 27, 2009(UT). We used the Boller & Chivens spectrograph installed inthe Cassegrain focus of the telescope. The 600 lines / mm grat-ting was used to cover a wavelength range from 3900 to 6000Å. A dispersion of 2.05 Å per pixels with a resolution of 5.0Å was employed. The SITe3 1024 × µ m pixel size was attached to the spectrograph. The spec-tra were reduced in the standard way using the IRAF package.Fig. 3 shows a close-up of the normalized spectrum at region3800 Å − v sin i ) can beestimated from the empirical calibration v sin i vs FWHM givenby Bush & Hintz (2008) who determined the v sin i of 118 δ Scuti variables. To do so, we have measured the FWHM for the2 igure 2: Di ff erential light curves of HD 207331. The name of the observatory involved is indicated in each panel. able 1: Log of observations. Observing time is expressed in hours. Day UT Date 2009 Start Time End Time OT SPM(HJD 2455000 + ) (HJD 2455000 + )1 Aug 26 69.93 70.02 - 2.1852 Aug 27 70.77 71.00 - 5.3352 Aug 28 71.67 72.70 7.086 7.7314 Aug 29 72.72 73.01 - 7.0765 Aug 30 73.65 73.99 - 8.2186 Aug 31 74.69 74.88 - 4.5367 Sep 01 75.66 75.89 - 4.5928 Sep 02 76.68 76.93 - 7.085Begin End Total Time OT SPMAug 26 Sep 02 53.844 7.086 46.758 Table 2: Position, magnitude, and spectral type of target, comparison, and check stars observed in the CCD frame.
Star ID RA Dec V SpTyp(2000.0) (2000.0) (mag)Target HD 207331 21 47 02 +
43 19 19 8.3 A +
42 4208 21 47 12 +
43 19 51 9.4 A +
43 18 58 10.9 -4501 Å Ti II and 4508 Å Fe II lines. Then, we have applied therelationship (Bush & Hintz, 2008): v sin i = − . FWHM + . FWHM − . v sin i = ±
10 km / s was derived for HD 207331.In order to test the validity of the calibration for our tele-scope system we also have derived the v sin i values for δ Scutivariables 7 Aql and 8 Aql whose spectra were recorded byFox Machado et al. (2010) with the same telescope, equipmentand configuration as ours. These stars have accurately deter-mined v sin i in the literature. We have found very good agree-ment between the derived projected rotational velocities fromequation (1) and those listed in the literature for 7 Aql and8 Aql.
4. Period analysis
The period analysis has been performed by means of stan-dard Fourier analysis and least-squares fitting. In particular, theamplitude spectra of the di ff erential time series were obtainedby means of Period04 package (Lenz & Breger, 2005), whichutilizes Fourier as well as multiple least-squares algorithms.This computer package allows us to fit all the frequencies si-multaneously in the magnitude domain.The spectral window in amplitude of the observations isshown in the first plot of Fig. 4. The amplitude spectrum of thedi ff erential light curve, HD 207331 - Comparison is depicted inthe next plot. The subsequent plots in the figure, from left toright, illustrate the prewhitening process of the frequency peaksin each amplitude spectrum.The frequencies have been extracted by means of a standardprewhitening method. In order to decide which of the detected Table 3: Frequency peaks detected in the light curve: HD 207331 - Comparison.S / N is the signal-to-noise ratio in amplitude after the prewhitening process.
Freq. Freq. Period A ϕ / (2 π ) S / N (c / d) ( µ Hz ) hours (mmag) f f f f
5. Discussion
From the di ff erential light curves it can be noted that the beat-ing behavior seen in the short time series of Fox Machado et al.(2008) is present. This is a common characteristic of the lightcurves of δ Scuti stars. The amplitude spectrum shows a con-centration of high signal-to-noise peaks between 18–28 cycleday − . In particular, the highest amplitude peak is located at22.49 cd − (260.19 µ Hz), and the next significant frequency islocated at 20.09 cd − (232.47 µ Hz ).Fox Machado et al. (2008) detected a significant frequencypeak at ∼ − (244.1 µ Hz) with an amplitude of 6 mmagin Str¨omgren y band. As can be seen in Table 3, this is well4 igure 4: Spectral window (first plot). Pre-whitening process in HD 207331(from second plot). In each plot, from left to right, the highest amplitude peakis selected and removed from the time series, and a new spectrum is obtained. within the period of the modes detected in this campaign, tak-ing into account that the complex window function of those ob-servations does not allow a precise comparison. Nonetheless,this peak is most likely a side-lobe of the second frequency ( f )detected in our campaign. No equally spaced close frequencypairs were found in the frequency pattern of HD 207331. Figure 5 shows the observed position of HD 207331 (aster-isk) in the HR diagram and its associated uncertainty (crossupon the asterisk). Since the Hipparcos distance for HD 207331has a large relative error, we have considered in Fig. 5 theSt¨romgren absolute magnitude derived by Fox Machado et al.(2008), namely M V = . M V and0.02 mag for ( B − V ) have been adopted.Two main sequence evolutionary tracks that approximatelymatch the observed position of HD 207331 are shown bydashed and dot-dashed lines. These evolutionary sequenceswere computed as explained in Fox Machado et al. (2006) byusing the CESAM evolution code (Morel, 1997) with inputphysics appropriate to δ Scuti stars and a chemical initial com-position of Z = .
02 and Y = .
28. Also shown are the observedinstability strip boundaries from Rodr´ıguez et al. (1994). Ac-cording to the models depicted in Fig. 5, for a solar metallic-ity, HD 207331 would have a mass between 2.20 M ⊙ and 2.25 M ⊙ . The age of HD 207331 corresponding to these evolution-ary models is ∼
527 Myr and ∼
513 Myr, respectively.
Figure 5: Position of HD 207331 in the HR diagram. Evolutive sequences ofnon-rotating models without overshooting are shown by dashed ( M / M ⊙ = . M / M ⊙ = . We now use these models to derive possible radial and non-radial modes in order to obtain some insights into the pulsa-tion behaviour of HD 207331. The adiabatic eigenfrequen-cies were computed using the code FILOU (Su´arez & Goupil,2008). The following identifications of the modes are possible,but not unique: f – ( l = n = f – ( l = n = f –( l = n = f – ( l = n = ff ects in spite the fact that HD 207331is most likely a high rotating star. In fact, the rotation e ff ects areimportant not only in the position of the star in a HR diagram(P´erez Hern´andez, et al., 1999) but also in the computation ofoscillation frequencies (Su´arez et al., 2009).Nevertheless, these simple computations indicates thatHD 207331 may pulsate in radial and non-radial modes typi-cal among δ Scuti stars. Since the star is located near blue edgeof instability strip, it could be pulsating in higher overtones thanthe fundamental.
6. Conclusions
A summary of the follow-up observations which led to thecharacterization of the new δ Scuti star HD 207331 has beenpresented. The star was observed from two observatories dis-tributed in longitude around the Earth. An overall run of 53.8h of useful data was collected from the two sites on nine ob-serving nights. A period analysis reveals that HD 207331 is amultiperiodic pulsating star with at least four oscillation modes.A simple comparison with theoretical modes has been per-formed. The star shows complicated pulsations as do most ofthe δ Scuti stars. HD 207331 seems to pulsate with low-order- p modes typical among δ Scuti stars.The analysis of a few low resolution spectra points to it beinga fast rotating δ Scuti star of spectral type A0V.5o date our observations represent the most extensive workon HD 207331.
Acknowledgements
This work has received financial support from the UNAM viaPAPIIT grant IN114309. WJS acknowledges financial supportfrom CONACyT by way of grant 49434-F. Based on observa-tions collected at the 0.84 m telescope at the Observatorio As-tron´omico Nacional at San Pedro M´artir, Baja California, Mex-ico, and at the IAC-80 telescope operated by the Instituto deAstrof´ısica de Canarias in the Spanish Observatorio del Teide.Special thanks are given to the technical sta ff and night assis-tants of the San Pedro M´artir and Teide Observatories. Thisresearch has made use of the SIMBAD database operated at theCDS, Strasbourg (France). References