A&A 378, L33-L36 (2001)
DOI: 10.1051/0004-6361:20011248

Irregular amplitude variations and another abrupt period change in the ${\delta}$ Scuti star V1162 Ori

T. Arentoft^1, C. Sterken^1, - M. R. Knudsen¹ - G. Handler² - P. Niarchos³ - K. Gazeas³ - V. Manimanis³ - M. B. Moalusi^2,4 - F. F. Vuthela^2,4 - P. Van Cauteren⁵

1 - University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium
2 - South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
3 - Department of Astrophysics, Astronomy and Mechanics, University of Athens, 157 84 Zografos, Athens, Greece
4 - Department of Physics, University of the North-West, Private Bag X2046, Mmabatho 2735, South Africa
5 - Beersel Hills Observatory, Belgium

Received 29 August 2001 / Accepted 6 September 2001

Abstract
We report that the intermediate amplitude ${\delta}$ Scuti star V1162 Ori has changed its main pulsational period in the course of the year 2000. This new period change falls in a sequence of period changes observed during the last 5 years. While the average amplitude value of all our new data, 63 mmag, fits a cyclic amplitude variation suggested by Arentoft et al. (2001), splitting the data up in smaller subsets discloses significant deviations from regularity, with stretches of constant amplitude during short intervals of time. The new data show that the amplitude of one of the secondary frequencies, f₂, has in 3 years dropped from more than 3 mmag to now about 1 mmag, and that the previously obtained f₅probably is a 1 d^-1 alias of the real frequency. We present the newly acquired times of minimum and maximum light as support for subsequent observing campaigns.

Key words: stars: variables: ${\delta}$ Scuti - stars: individual: V1162 Orionis - techniques: photometric - methods: data analysis

  
1 Introduction

In a recent paper, Arentoft et al. (2001) discussed period and amplitude changes in V1162 Ori, a highly interesting intermediate amplitude ${\delta}$ Scuti star. These authors showed by using data collected from 1998 to 2000, that the semi-amplitude of the dominant frequency, f₁, varied between 55 and 75 mmag in an apparently cyclic manner, on a time scale of about 280 d. However, deviations from cyclicity were seen, and the possible cyclic behaviour did not explain amplitude values quoted in the literature (Lampens 1985, 92 mmag; Poretti et al. 1990, 98 mmag; Hintz et al. 1998, 72 and 50 mmag). The variation in period appeared quasi-cyclic, in the sense that the O-C diagram of the times of maximum and minimum light prewhitened with a constant period revealed period changes alternating between period increases and decreases, on top of a slow, secular period change. A further result was the detection of 5 previously unknown low-amplitude frequencies ( f₂-f₆) as well as 3f₁, all having amplitudes of 1-3 mmag. We refer to Arentoft et al. (2001) for descriptions of the background, philosophy and methods of data collection and analysis applied in the present paper. We analyse new data obtained in 2000-2001, and discuss the diagrams presented by Arentoft et al. (2001) in the light of the newly acquired data.

   
2 The data

The data were collected from October 2000 to March 2001, using 6 telescopes at 4 different sites, as outlined in Table 1. In total, we have collected 158 new light extrema during 182 hours of time-series observations. The final data set used for Fourier analysis and investigation of amplitude variability consists of 5911 new datapoints, and the total data set, including the data discussed in Arentoft et al. (2001), constitutes 13463 individual datapoints and 607 light extrema, covering 583 hours of time-series photometry.

 

 

Table 1: List of sites supplying the new data, obtained from October 2000 to March 2001. Telescope diameters are given in meters.

Observatory	Location	Observer	#extrema	Telescope	Detector	#hours
SAAO	S. Africa	M. Knudsen, T. Arentoft, G. Handler	73	1.00	CCD	84
SAAO	S. Africa	G. Handler	11	0.75	CCD	11
SAAO	S. Africa	M. Moalusi, F. Vuthela	14	0.50	PMT	19
ESO	Chile	M. Knudsen, C. Sterken	17	1.54	CCD	16
Athens University	Greece	P. Niarchos, K. Gazeas, V. Manimanis	38	0.40	CCD	47
Beersel Hills	Belgium	P. Van Cauteren	5	0.40	CCD	5
Total			158			182

   
3 Analysis and results

3.1 The O-C diagram

The O-C diagram of times of maximum and minimum light is shown in Fig. 1. The computed values were obtained using a constant pulsation period of 0.07868910 d (Arentoft et al. 2001) and the cycle count scheme of Hintz et al. (1998). This figure is the updated version of Fig. 14 in Arentoft et al. (2001). Data from before 1998 are from Hintz et al. (1998), from 1998 to mid-2000 from Arentoft & Sterken (2000) and Arentoft et al. (2001), and the later data are from this study. The upper panel shows the long-term evolution of the period. The overall parabolic shape, and thus the presence of a slow, secular period change as found by Arentoft et al. (2001) is still valid when including the new data. Using the larger data base now available, the period change rate of f₁ is refined to $(1/P)({\rm d}P/{\rm d}t)=-1.6\times10^{-5}\pm4.3\times10^{-7}$ y^-1.

The constant change in period has been subtracted in the middle and lower panels, where the middle panel displays the O-C values of all available times of extreme light, and the lower panel the same data combined in bins of 155 cycles (about 12 days). The bin size was chosen to ensure that all bins included a reasonable number of data points. The new data show that another period change has taken place somewhere between May and October 2000. Several period changes have occured from 1996 to 2001, but although the period changes appear to alternate around a mean value on a time scale of about 280 d (3560 cycles), the deviations from a simple sinusoidal shape discussed in Arentoft et al. (2001) are very clearly confirmed by the new data. A model of a secular period change combined with a simple sinusoidal variation definitely does not fit all the variability seen in the data.

In Fig. 2 we show the O-C values (corrected for the secular period change) phased with the best-fit period value (277 d) of the sinewave superimposed in Fig. 1, middle and lower panels. The upper panel of Fig. 2 shows again all available data while the lower panel plots the binned values. Although the upper panel indicates that a cyclic variation may be present in the O-C values, the binned data disclose significant deviations from regularity. Figure 2 does not show the kind of regularity one would expect from a kinematic cause of the observed changes, like e.g. a light-time effect in a simple, non-interacting binary system.

	Figure 1: New O-C diagram (in days) for V1162 Ori. The superimposed parabola in the upper panel corresponds to a linear period change rate of $(1/P)({\rm d}P/{\rm d}t) \sim$ $-1.6\times 10^{-5}$ y-1. The sinewave with an amplitude of 0.003 d superimposed in the middle and lower panels has a period of 277 d, in agreement with the best-fit value of the binned data.
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Figure 2: The O-C diagram (in days) phased with the period of 277 d superimposed in Fig. 1, middle and lower panels. ($\diamond$) are data from Hintz et al. (1998), ( $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...x{\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi$) data from ESO 1998, (+) data from ESO 1999, ($\circ$) data from the 1999-2000 multisite campaign, and ($\times$) are the new data. Error bars are not shown in the lower panel but are very small as can be assessed from the lower panel of Fig. 1.

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3.2 Amplitude variations of f₁

The amplitude variations of f₁ and 2f₁ are shown in Fig. 3, upper and lower panel, respectively. Data prior to HJD2451800 were discussed by Arentoft et al. (2001, along with the superimposed sinewave), the later data are from this study. To obtain the new amplitudes plotted in Fig. 3, the data were prewhitened for the low-amplitude frequencies ( f₂-f₆) and subdivided into 6 subsets, as described by Arentoft et al. (2001). Although the average amplitude value of the new data, about 63 mmag, agrees well with the superimposed sinewave, the values from the individual subsets do not follow the predicted relation. In fact, it appears that the amplitude of f₁ remained constant for at least two months, before starting to vary again. Actually, this could also be the case near the previous maximum around HJD2451500-2451600, and near the first minimum shortly after HJD2450800. It is thus possible that there are epochs with little or no amplitude variability of f₁, and other epochs with very large changes.

3.3 New results on f₂-f₆

	Figure 3: Amplitude variations of f1. The superimposed sinewave has a period of 282 d, as discussed by Arentoft et al. (2001).
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The residual amplitude spectrum after subtracting f₁ and harmonics (2f₁, 3f₁) from all available data is shown in Fig. 4, upper panel. In the lower panel f₂ (12.9412 d^-1) and f₃ (19.1701 d^-1) have also been subtracted. f₂ was found to have an amplitude of 3.2 mmag by Arentoft et al. (2001), but the amplitude in Fig. 4 is only about 2 mmag. This difference is explained by Fig. 5 where the evolution in amplitude of f₂ and f₃is shown in the upper and lower panels, respectively. Whereas the amplitude of f₃ has remained constant over a time span of more than 3 years, this is not the case for f₂, whose amplitude is decreasing to the extent that it is barely detectable in the new data.

f₄ has also an amplitude lower than the 2.4 mmag quoted in Arentoft et al. (2001), due to an amplitude of less than 1 mmag in the new data. It furthermore appears that the previously found frequency value of f₅ (15.9901 d^-1) most likely is an 1 d^-1 alias, and the real frequency appears to be 16.9901 d^-1. The period ratio with f₁ is then 0.748 instead of 0.795, and thus still not in agreement with the expected value of 0.77-0.78 for the ratio of the fundamental to first overtone oscillation (e.g. Petersen & Christensen-Dalsgaard 1996). The amplitude in the new data is 1.5 mmag, as compared to 2.1 mmag in Arentoft et al. (2001). f₆, which had been detected with very low amplitude (1.1 mmag), is not confirmed after inclusion of the new data. The reality of these effects was tested by analysing subsets of data combined in several different ways.

	Figure 4: Residual amplitude spectrum after subtracting f1 and harmonics (upper panel), and f1(and harmonics), f2 and f3 (lower panel), from the total dataset described in Sect. 2.
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	Figure 5: The decreasing amplitude of f2.
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4 Conclusions

 

 

Table 2: New times of maximum and minimum light (HJD-2450000). The cycle count scheme is based on 1998.

$T_{\rm max}$	E	$T_{\rm max}$	E	$T_{\rm max}$	E	$T_{\rm min}$	E	$T_{\rm min}$	E	$T_{\rm min}$	E
1823.5645	59892	1900.3650	60868	1925.3878	61186	1823.6086	59892	1908.3562	60969	1955.3337	61566
1859.7619	60352	1901.3900	60881	1946.3183	61452	1859.7252	60351	1909.3774	60982	1956.3543	61579
1860.6250	60363	1902.4105	60894	1949.2322	61489	1860.6711	60363	1912.3686	61020	1959.2690	61616
1861.6490	60376	1905.3213	60931	1949.3096	61490	1879.5537	60603	1912.4452	61021	1960.2902	61629
1861.7279	60377	1906.3456	60944	1950.2544	61502	1880.4995	60615	1913.3891	61033	1964.3006	61680
1862.7504	60390	1908.3905	60970	1950.3316	61503	1884.4317	60665	1914.3335	61045	1973.2742	61794
1863.8532	60404	1911.3794	61008	1950.3338	61503	1884.5111	60666	1914.4138	61046	1975.2426	61819
1864.7969	60416	1912.3246	61020	1954.3477	61554	1884.5913	60667	1915.3572	61058	1978.5463	61861
1879.5892	60604	1912.4037	61021	1955.2904	61566	1885.3776	60677	1915.4380	61059	1982.5604	61912
1880.4568	60615	1913.3476	61033	1956.3140	61579	1885.4575	60678	1916.3825	61071	1985.2354	61946
1880.5338	60616	1913.4264	61034	1959.3029	61617	1885.5364	60679	1916.4581	61072	1989.2483	61997
1884.3916	60665	1914.3713	61046	1960.3292	61630	1887.3437	60702	1917.3255	61083	1990.2718	62010
1884.4692	60666	1914.4499	61047	1965.2853	61693	1888.4438	60716	1917.4031	61084
1885.3346	60677	1915.3163	61058	1965.3633	61694	1888.5264	60717	1918.3486	61096
1885.4128	60678	1915.3929	61059	1973.2296	61794	1889.5455	60730	1918.4272	61097
1885.4929	60679	1916.4176	61072	1973.3116	61795	1890.3341	60740	1919.2911	61108
1885.5727	60680	1917.3608	61084	1975.2760	61820	1890.4148	60741	1919.3710	61109
1887.3796	60703	1917.4407	61085	1975.5933	61824	1890.4899	60742	1921.3381	61134
1888.4806	60717	1918.3042	61096	1977.5606	61849	1890.5713	60743	1921.4138	61135
1888.5627	60718	1918.3860	61097	1978.5018	61861	1894.5049	60793	1921.4150	61135
1889.5032	60730	1919.3284	61109	1978.5830	61862	1896.3943	60817	1923.3039	61159
1889.5818	60731	1919.4070	61110	1980.5494	61887	1896.4744	60818	1924.3272	61172
1890.3708	60741	1921.2946	61134	1982.5182	61912	1898.4421	60843	1925.3522	61185
1890.4500	60742	1921.3742	61135	1985.2727	61947	1898.5167	60844	1946.2832	61451
1890.5284	60743	1921.3744	61135	1989.2863	61998	1903.3961	60906	1946.3625	61452
1894.4618	60793	1922.3184	61147	1990.2272	62010	1904.3430	60918	1949.1978	61488
1896.3485	60817	1922.3988	61148			1904.4209	60919	1949.2728	61489
1896.5096	60819	1924.2855	61172			1905.3657	60931	1950.2185	61501
1898.3971	60843	1924.3637	61173			1906.3883	60944	1950.2964	61502
1898.4761	60844	1925.3073	61185			1907.3304	60956	1954.3102	61553

Newly acquired data on the ${\delta}$ Scuti star V1162 Ori reveal that the period of the main oscillation has yet again changed, in between May and October 2000. Although the period changes seem to alternate between period increases and decreases, they are definitely not regular. The time scale of the period variations of about 280 d, as discussed by Arentoft et al. (2001), is supported by the new data. The presence of a secular period change is confirmed, and the rate of change is refined to $(1/P)({\rm d}P/{\rm d}t) \sim-1.6\times10^{-5}\, \pm \,4.3\times10^{-7}$ y^-1. Although lower than the value quoted in Arentoft et al. (2001), it is still much higher than what is expected from evolutionary changes (Breger & Pamyatnykh 1998).

The evolution in amplitude of f₁ over the 2000-2001 observing season shows that also the amplitude changes are irregular, which is in agreement with the fact that amplitude values found in earlier studies are not explained by the cyclic variation reported by Arentoft et al. (2001), as mentioned in the introduction. A new possible feature resulting from our data is the presence of short intervals of constant amplitude in between the large amplitude variations. Of the low-amplitude frequencies only f₃ has remained constant in amplitude, while f₂, f₄ and f₅ all have lower amplitude as compared to Arentoft et al. (2001). f₆ is not confirmed by the new data.

Spectroscopic observations covering at least one full observing season are needed to search for possible radial velocity variation in the 280 d cycle. Equally important, continued photometric monitoring is crucial for determining the evolution in period and amplitude. We give in Table 2 our new times of maximum and minimum light, in order to allow observers in the coming season to assess the evolution in period.

Acknowledgements

T.A., C.S. and M.R.K. acknowledge financial support from the Belgian Fund for Scientific Research (FWO). This project was supported by the Flemish Ministry for Foreign Policy, European Affairs, Science and Technology, under contract BIL 98/11/52, and the National Research Foundation of South Africa. P.V.C. is grateful to the Royal Observatory of Belgium for putting at his disposal equipment acquired through a project of the Belgian FWO. This research was supported by the Danish Natural Science Research Council through the centre for Ground-Based Observational Astronomy. The observations by M.B.M. and F.F.V. were made possible by a Mobility Grant of the South African National Research Foundation (NRF).

References

 
• Arentoft, T., & Sterken, C. 2000, A&A, 354, 589 In the text NASA ADS  
• Arentoft, T., Sterken, C., Handler, G., et al. 2001, A&A, 374, 1056 In the text NASA ADS  
• Breger, M., & Pamyatnykh, A. A. 1998, A&A, 332, 958 In the text NASA ADS  
• Hintz, E. G., Joner, M. D., & Kim, C. 1998, PASP, 110, 689 In the text NASA ADS  
• Lampens, P. 1985, IBVS, 2794 In the text  
• Petersen, J. O., & Christensen-Dalsgaard, J. 1996, A&A, 312, 463 NASA ADS  
• Poretti, E., Antonello, E., & Le Borgne, J. F. 1990, A&A, 228, 350 In the text NASA ADS