2 Ground-based Cousins VI photometry

The advantages of the broadband Cousins VRI photometric system such as high internal precision and maintaining this precision over the whole range of spectral types are discussed by Bessell (1979). This system emerged with the advent of Ga-As photocathode photomultipliers in the early 1970s. There are two issues which should be considered in the broadband photometry of red stars. First, the majority of cool red stars are variable and no standard stars are available redder than $V-I\approx3$. Second, the presence of numerous molecular bands in the spectra of red stars requires stable and easily reproducible bandpasses in order to avoid possible nonlinear transformations from the instrumental to the standard system. In other words, to exclude the transformation uncertainties, such stars must be observed in the natural Cousins VRI system, i.e. using the same filters and detector. Examination of the published sources of Cousins VRI photometry indicates that many extremely red Hipparcos stars actually lack this photometry. Therefore, we have obtained new sets of UBVRI photometry of the Southern carbon stars and BVI photometry for the reddest M and C spectral-type stars.

2.1 Carbon star photometry at SAAO

The observations of 85 carbon stars including a few hydrogen-deficient (Hd) stars were made in 1984 and 1987 using the single channel Modular Photometer on the 0.5 m reflector at the Sutherland station of the SAAO. The photometer uses a Hamamatsu R943-02 GaAs photomultiplier and a filter set which reproduces the Johnson UBV and Cousins $(RI)_{\rm C}$ photometric systems, with a need for only very small linear and non-linear terms in transformations onto the standard system. The observations were made with frequent reference to the E-region standard stars of Menzies et al. (1989). The results of $UBV(RI)_{\rm C}$ photometry are provided in Table 1. The CGCS numbers are those in Stephenson (1989). The last column indicates the total number of observations, usually obtained over 2-3 nights. The standard error of individual observations is about 0.01 mag. It was necessary, however, to extrapolate the color system as some of the stars here are redder than any standard star in one color or another and in any case these are carbon stars (or helium stars in the case of Hd stars) whose colors differ systematically from the oxygen-rich M spectral type standard stars. In addition, most of our programme stars are variable to some degree. All cases with apparent variability or uncertain photometry are marked by (v) or (:), accordingly. Since in the UBVRI photoelectric photometry the aperture size varied from $20\hbox{$^{\prime\prime}$ }$ to  $40\hbox{$^{\prime\prime}$ }$, a nearby optical component, marked in Table 1, may affect the accuracy of our photometry.

Figure 1: Differences between the V-magnitude (top panel) and V-I color index (bottom panel) from Table 1 and that of Walker (1979). A relatively large spread visible in the top panel is mainly due to thevariability.

The generally good agreement (Fig. 1) with the data of Walker (1979), whose observations were made with separate blue and red sensitive photomultipliers and a different filter set, give added confidence to the results.

 

 

Table 1: SAAO photometry of selected carbon stars.

CGCS	HIP	GCVS	V	B-V	U-B	R-I	V-I	n	CGCS	HIP	GCVS	V	B-V	U-B	R-I	V-I	n
177		AM Scl	12.33	2.16	3.42:	1.02	1.85	3	3810			10.39	1.41	0.90	0.83	1.63	5
196	5809		10.02	1.33	1.34	0.59	1.09	5	3813			11.02v	2.39	2.61	1.30	2.45	4
258			10.19	1.31	1.16	0.61	1.12	6	3842	85750		9.37	1.88	2.02	0.88	1.66	5
327	10472	V Ari	8.71	2.19	2.45	1.16	2.15	6	3855a			11.20	1.30	0.91	0.64	1.19	2
378	12028		8.16	1.24	0.77	0.60	1.09	4	3864a		V450 Sco	10.30v	2.38	3.56:	1.50	2.82	4
576	17933		8.30	1.65	1.72	0.75	1.41	6	3938	88584	W CrA	9.95	1.89	1.83	0.99	1.81	4
639	19269		10.66	1.23	0.68	0.72	1.43	4	3939		V1783 Sgr	10.53	1.54	1.01	0.94	1.74	3
725	21051		8.91	1.14	1.12	0.55	1.04	6	3957	88887		9.80	1.52	1.18	0.96	1.87	4
1380	31725		9.37	1.37	1.28	0.58	1.07	3	3958			10.45	1.34	0.95	0.66	1.23	6
1460	33042	KY CMa	10.75	2.73	4.00:	1.33	2.42	4	3966a			11.06v	1.95	1.76	1.05	1.95	4
1489	33550	RV Mon	6.88	2.65	7.16:			3	3992	89783	FO Ser	8.42	1.85	1.85	1.15	2.21	3
1507	33794	V614 Mon	7.32v	1.76	2.14:	1.13		4	4021	90694		9.90	1.39	0.92	0.81	1.60	3
1659	35549	MY CMa	10.63	2.44	3.08:	1.36	2.55	3	4042			11.13	2.08	2.01	1.05	1.90	3
1790			9.58	1.85	2.15	1.11	2.07	3	4070			9.33	1.29	1.00	0.58	1.06	3
1871a			10.16	1.23	0.82	0.56	1.02	3	4086	91929	RV Sct	10.02	2.35	2.52	1.54	2.92	3
1968	38787	V406 Pup	7.62v	3.20:	4.60:	1.40		4	4094	92115		9.49	0.83	0.47	0.42	0.67	3
2153	40805	V433 Pup	9.54v	1.67	1.66	1.07	2.05	3	4145	93181	V4152 Sgr	9.33	1.16	0.84	0.57	0.94	1
2331	43093	UZ Pyx	7.32	2.01	2.99	1.09		4	4168			9.95	1.29	1.22	0.59	1.10	4
2449	45295	GM Cnc	8.65	1.57	1.50	1.00	1.93	3	4179a	94049		10.29	1.26	0.83	0.60	1.14	3
2759	50994		9.53	1.30	1.07	0.59	1.09	4	4194	94294	V1445 Aql	11.31	2.08	2.35:	1.31	2.51	3
2787			9.48	1.29	0.96	0.60	1.11	5	4196			10.82	1.42	1.24	0.68	1.31	3
2829	52271		7.08	1.33	1.16	0.59	1.11	4	4229	94940	V1942 Sgr	7.06	2.56	4.45:			1
2852	52656	TZ Car	8.71v	2.10	2.60	1.30	2.50	4	4247	95289		6.96	1.07	0.58	0.57	0.97	1
2925	53810		8.33	1.16	1.08	0.55	1.05	4	4498			11.14	1.32	1.19	0.61	1.12	3
2975	54806		10.16	1.44	1.14	0.85	1.64	4	4524a	98117		9.18	1.21	0.61	0.54	1.01	3
2986		DI Car	10.5 v	1.4 v	1.30v	0.64v	1.2v	6	4567	98223		9.35	2.03	2.06	0.92	1.73	4
3001	55448	V905 Cen	10.51v	1.80	1.87	1.15	2.20	4	4595	98542	V1468 Aql	10.36	2.04	2.55	1.16	2.13	3
3066	56551		8.76	1.06	0.51	0.51	0.92	4	4598	98538	V1469 Aql	8.37	2.08	2.52	0.96	1.77	3
3141	58513	DD Cru	8.87	2.20	2.94:	1.04	2.03	4	4614	98958		8.05	1.07	0.97	0.51	0.98	3
3199		TV Cen	8.02v	2.74	2.89	1.42	2.57	5	4873	101277	BI Cap	9.67v	1.42	1.09	0.95	1.85	3
3227	60534	S Cen	7.66v	1.89	2.70:	1.11	2.10	5	4972	102726		10.30	1.29	0.92	0.63	1.14	3
3286	62401	RU Vir	9.97v	4.63	5.10:	1.99	3.42	4	4978	102706		8.16v	1.28	0.94	0.58	1.14	3
3335	63955		8.50	1.17	1.03	0.54	1.01	4	5147	104522		9.82	1.56	1.49	0.97	1.86	6
3405	66070	V971 Cen	8.50	1.87	2.12	1.02	1.94	5	5227	105212		9.67	1.26	0.87	0.57	1.06	5
3492	70339	RS Lup	9.62v	2.69	4.70:	1.35	2.46	5	5408	107349	BU Ind	10.15v	1.45	1.28	0.95	1.85	4
3545			10.95	1.40	0.80	0.77	1.44	5	5420	107490	RR Ind	9.34v	2.84v	5.29v	1.31v	2.36v	7
3558			10.42	1.51	1.26	1.01	1.98	5	5561	108953	HP Peg	8.89	1.45	1.13	0.61	1.15	2
3606	75694	HM Lib	7.48v	1.20	0.86	0.61	1.07	4	5627			10.71	1.72	1.63	0.82	1.50	3
3657			9.84	1.59	1.32	0.69	1.28	5	5761	113150		10.82	1.17	0.55	0.59	1.11	6
3672	79484		10.36	1.69	1.46	0.77	1.42	5	5823	114509		9.26	1.22	0.81	0.60	1.11	5
3707	81254	LV TrA	8.30	0.95	0.67	0.45	0.72	5	5937	117467		8.48	1.37	1.30	0.62	1.15	5
3756	83387	T Ara	9.03v	2.78	4.90:	1.40	2.55	5	5980	168		9.55	1.12	0.48	0.51	0.96	4
3765			9.11	1.39	1.26	0.65	1.25	4

^a

Close companion: 1871 ( $9\hbox{$^{\prime\prime}$ }$ separation, bright); 3855 ( $15\hbox{$^{\prime\prime}$ }$); 3864 ( $11\hbox{$^{\prime\prime}$ }$); 3966 ( $15\hbox{$^{\prime\prime}$ }$, bright), 4179 ( $14\hbox{$^{\prime\prime}$ }$), 4524 ( $13\hbox{$^{\prime\prime}$ }$ & $18\hbox{$^{\prime\prime}$ }$).

2.2 Photometry of red stars at Siding Spring Observatory

In March-April 2002 additional $BVI_{\rm C}$ photometry for 47 very red Hipparcos carbon and M stars was secured at the Siding Spring Observatory, Australia. The data were obtained using the 24 inch reflector and a single channel photometer. A cooled unit containing a Hamamatsu GaAs photomultiplier tube and a set of filters allow us to match closely the Cousins photometric system, in the same way as was done at SAAO. Each night a set of the E-region standards (Menzies et al. 1989) was measured to obtain the atmospheric extinction coefficients and the transformation coefficients to the standard system. Mean transformation coefficients for this run were as follows: $\xi_{V}=0.005$, $\xi_{B-V}=1.010$, and $\xi_{V-I_{\rm C}}=1.015$ (see Berdnikov & Turner 2001, Eq. (2)). Hence the instrumental system is very close to the standard BVI_C system, which greatly alleviates the problem of color-related extrapolation in the reductions of very red programme stars. Every 60-90 min two standard stars (red and blue) were used to define instantaneous zeropoints in the transformation relations. Some very bright programme stars were observed with the addition of an Oriel 50550 neutral density filter. The $BVI_{\rm C}$ photometry is presented in Table 2.

 

 

Table 2: BVI photometry at Siding Spring.

HIP	GCVS	JD -2 450 000	V	B-V	V-I
23203	R Lep	2353.915	11.63	4.60	3.75
23636	T Lep	2376.883	12.18	1.75	5.92
24055	U Dor	2376.977	8.61	1.62	4.17
		2378.889	8.62	1.60	4.17
25004	V1368 Ori	2376.871	10.07	3.53	3.48
25673	S Ori	2376.874	8.71	1.65	4.66
28041	U Ori	2376.870	10.22	1.87	5.40
		2378.880	9.94	2.00	5.45
29896	GK Ori	2353.919	9.96	4.22	3.52
34413	W CMa	2361.980	6.74	2.69	2.43
35793	VY CMa	2353.922	8.19	2.28	3.28
39967	AS Pup	2376.928	9.01	1.50	4.61
		2378.919	9.01	1.48	4.60
40534	R Cnc	2376.925	11.22	2.26	5.77
		2378.931	11.31	2.30	5.81
41061	AC Pup	2376.908	8.99	3.23	2.78
		2378.933	9.04	3.31	2.80
43905	T Cnc	2353.926	8.23	4.31	3.29
48036	R Leo	2353.929	7.28	1.71	5.02
53085	V Hya	2354.036	7.34	4.66	3.61
53809	R Crt	2354.038	8.43	2.01	4.81
57607	V919 Cen	2354.039	6.93	1.59	4.15
63642	RT Vir	2354.175	8.25	1.81	4.67
64569	SW Vir	2354.178	7.09	1.72	4.53
67419	W Hya	2354.179	8.42	2.44	5.64
69754	R Cen	2354.178	7.48	1.94	4.22
70969	Y Cen	2354.181	8.12	1.60	4.50
75393	RS Lib	2354.182	10.79	1.96	5.41
80365	RT Nor	2354.183	10.08	1.01	0.94
80488	U Her	2379.168	8.68	1.60	4.84
80550	V Oph	2357.168	9.21	4.13	3.19
82392	V TrA	2364.266	8.16	2.23	2.24
84876	V1079 Sco	2354.185	9.40	3.31	3.34
85617	TW Oph	2357.168	7.86	4.24	3.32
85750		2357.174	9.36	1.93	1.65
86873	SZ Sgr	2357.170	8.78	2.36	2.73
87063	SX Sco	2357.172	7.65	2.86	2.65
88341	V4378 Sgr	2379.172	10.37	2.97	3.24
88838	VX Sgr	2379.174	9.20	2.82	4.34
89739	RS Tel	2357.254	10.01	0.85	0.77
90694		2357.252	9.93	1.37	1.61
93605	SU Sgr	2357.258	8.33	1.73	4.39
93666	V Aql	2357.260	6.78	3.98	3.07
98031	S Pav	2379.201	7.82	1.64	4.63
99082	V1943 Sgr	2379.195	7.67	1.77	4.58
99512	X Pav	2357.256	8.97	1.91	4.92
100935	T Mic	2357.265	7.68	1.78	4.76

2.3 VRI photometry of red variables with APT

Since 1996 the University of Vienna has been obtaining UBV(RI)_Cphotometry in Arizona using two 0.75 m automatic photoelectric telescopes (APT) located on the grounds of Fairborn Observatory. The photometer of the APT dubbed Amadeus (Strassmeier et al. 1997), has an EMI-9828 S-20/B multi-alkali cathode photomultiplier, which is sensitive up to $\sim$900 nm. This photomultiplier in combination with filters close to those suggested by Bessell (1976) reproduces a $V(RI)_{\rm C}$ system close to the one used by Walker (1979). In 1997 a monitoring programme of nearly 60 late spectral type semiregular and irregular variables was initiated. Typical light curves resulting from this programme can be found in Lebzelter (1999) and Kerschbaum et al. (2001). A complete sample of light curves will be published elsewhere (Lebzelter et al., in preparation). In Table 3 we present median V, $V-I_{\rm C}$, and an intercept a₀ and slope a₁ from the fit V-I vs. V for 45 selected Hipparcos variables used in the following calibration (Sect. 3). The total number of observations n is indicated in the last column.

 

 

Table 3: APT photometry of selected red variables.

HIP	GCVS	V	$V-I_{\rm C}$	a0	a1	n
4008	VY Cas	9.49	4.14	0.66	0.366	217
5914	Z Psc	6.85	2.54	-0.18	0.396	49
6191	AA Cas	8.24	3.47	0.00	0.422	206
10472	V Ari	8.52	2.07	-1.15	0.379	30
17821	BR Eri	7.15	3.16	-0.15	0.465	270
21046	RV Cam	8.16	3.81	0.38	0.420	326
22667	o1 Ori	4.84	2.50	-0.10	0.536	83
32083	VW Gem	8.32	2.41	-0.86	0.391	36
33369	BG Mon	9.66	2.46	-1.40	0.400	35
41061	AC Pup	9.05	2.83	-1.42	0.474	360
41201	FK Hya	7.29	3.48	0.22	0.446	388
43063	EY Hya	9.60	4.49	1.01	0.366	85
44601	TT UMa	9.02	3.68	-0.17	0.427	425
44862	CW Cnc	8.70	4.03	0.90	0.360	67
56976	AK Leo	8.54	2.87	-1.37	0.497	68
57504	AZ UMa	8.50	3.97	0.57	0.400	440
59108	RW Vir	7.33	3.63	0.66	0.405	377
61022	BK Vir	7.81	4.24	2.13	0.268	98
61839	Y UMa	8.39	4.40	1.92	0.295	411
66562	V UMi	7.91	2.92	-0.95	0.488	78
69449	EV Vir	6.91	2.62	-1.05	0.533	223
70236	CI Boo	6.48	2.93	-0.63	0.549	182
70401	RX Boo	7.43	4.33	2.97	0.184	105
71644	RV Boo	8.24	4.06	1.30	0.333	190
73213	FY Lib	7.24	3.65	0.30	0.460	225
74982	FZ Lib	7.10	3.04	-1.00	0.570	367
78574	X Her	6.28	3.92	1.60	0.371	346
80259	RY CrB	9.63	4.02	0.24	0.393	255
80704	g Her	4.86	3.47	1.23	0.461	291
81188	TX Dra	7.26	2.96	-0.58	0.488	153
81747	AX Sco	8.73	4.00	-0.60	0.527	120
82249	AH Dra	7.54	3.52	0.00	0.465	301
84027	CX Her	9.86	4.04	1.85	0.225	33
84329	UW Her	7.97	3.42	-0.29	0.464	298
84346	V438 Oph	9.12	4.26	2.41	0.199	164
93989	V398 Lyr	7.39	3.30	-0.32	0.490	265
95173	T Sge	9.29	4.66	2.45	0.236	276
96919	V1351 Cyg	6.56	3.06	0.00	0.466	226
102440	U Del	6.77	3.61	0.88	0.402	299
103933	DY Vul	7.09	3.58	0.55	0.425	207
107516	EP Aqr	6.63	4.01	2.16	0.279	183
109070	SV Peg	8.67	4.47	0.18	0.490	69
110099	UW Peg	8.89	3.39	-0.82	0.473	207
112155	BD Peg	8.66	3.82	0.56	0.376	159
113173	GO Peg	7.37	2.66	-0.76	0.464	168

2.4 Published sources of VI photometry

Only two large surveys of relatively bright red stars are available in the $VI_{\rm C}$system - a survey of the Southern carbon stars (Walker 1979) and the recent photometry of nearly 550 Hipparcos M stars (Koen et al. 2002). Additional literature on the $VI_{\rm C}$ photometry of Hipparcos red stars is not rich, therefore we included some other sources containing Johnson $VI_{\rm J}$photometry. We used normal color indices for M0 to M8 spectral type stars (Celis 1986, Table 4) to obtain the following relationship between the Johnson $V-I_{\rm J}$ and Cousins $V-I_{\rm C}$:

$$V-I_{\rm C}=-0.359+0.894(V-I)_{\rm J}-0.0087(V-I)_{\rm J}^{2},$$ (1)

defined for the giants of M spectral type. This is valid for zirconium (S-type) stars, and probably usable for carbon stars as well, throughout the $V-I_{\rm J}$ range from 1.9 to 8.7 mag. Note that this relationship yields a bluer color index, by $\sim$0.1, than a similar relationship from Hipparcos Catalogue (ESA 1997, vol. 1). A list of all sources used in this paper to calibrate V-I photometry is given in Table 4. It contains the reference, the number of stars n, spectral type, photometric system, and remarks. This list is not complete since we deliberately left out a few sources for the further independent comparisons.

 

 

Table 4: Selected sources of VI photometry.

Source	n	Type	System	Remarks
Bagnulo et al. (1998)	1	C	Cousins
Barnes (1973)	11	M	Johnson	narrow-band I
Celis (1982)	24	M	Kron(?)	$\sim$Johnson I
Celis (1986)	20	M	Cousins
Eggen (1972)	30	C	Eggen	$\sim$Cousins I
de Laverny et al. (1997)	2	C	Cousins
Kizla (1982)	36	C, M	Johnson
Koen et al. (2002)	80	M	Cousins	only V<8.4
Lee (1970)	43	M	Johnson
Mendoza & Johnson (1965)	33	C	Johnson
Olson & Richer (1975)	11	C	Johnson
Percy et al. (2001)	16	C, M	Johnson
Walker (1979)	119	C	Cousins
Table 1	61	C	Cousins	this study
Table 2	42	C, M	Cousins	this study
Table 3	45	C, M	Cousins	this study

2.5 Radial velocities

Although radial velocities have no direct bearing on the photometry, they could be used to identify spectroscopic binaries and hence shed light on possible discrepancies in the photometry caused by duplicity. We selected 19 Hipparcos carbon stars, mostly R type. The radial velocity measurements were made with a Coravel-type spectrometer using the Steward Observatory 1.6 m Kuiper Telescope at Mt. Bigelow, Arizona in February, 2002. Additional measurements were also obtained with the Moletai Observatory 1.65 m telescope in Lithuania and the 1.5 m telescope of the Turkish National Observatory near Antalya. A detailed description of the spectrometer is given in Upgren et al. (2002). On average, the estimated precision of a single measurement is 0.7 km s^-1. A total of 61 measurements of radial velocity are given in Table 5, where columns 1-6 are Hipparcos number, carbon star number from Stephenson (1989), GCVS variable star name (Kholopov et al. 1985-1995), Julian date, heliocentric radial velocity and its estimated standard error, both in km s^-1. More details on the observing and reduction procedure can be found in Upgren et al. (2002). By examining the ratio of external and internal error in accordance with Jasniewicz & Mayor (1988), it is evident that two stars in Table 5, HIP 53522 and 53832, are new SB1 spectroscopic binaries, although the time span is too short for the orbit determination. Both stars are suspected CH-like carbon stars (Hartwick & Cowley 1985), which adds more weight to the paradigm that most CH stars are binaries.

 

 

Table 5: Radial velocities of R and other selected carbon stars.

HIP	CGCS	GCVS	JD-2 450 000	${\it RV}_{\rm hel}$	$\sigma_{\it RV}$	HIP	CGCS	GCVS	JD-2 450 000	${\it RV}_{\rm hel}$	$\sigma_{\it RV}$
26927	1035	$\cdots$	2327.617	42.5	0.6	53832	2919	$\cdots$	2327.946	5.2	0.7
			2332.630	42.2	0.6				2332.843	3.4	0.6
29896	1222	GK Ori	2330.729	54.6	1.5				2363.492	-2.8	0.7
$\cdots$	1226	V1393 Oria	2332.641	34.2	0.6				2368.389	-3.1	0.7
29961	1230	V1394 Ori	2327.658	70.8	0.7				2382.344	-5.5	0.7
31829	1337	NY Gem	2327.732	-123.0	0.8				2386.347	-6.4	0.7
32187	1373	V738 Mon	2327.706	60.3	0.7				2399.392	-8.9	0.7
			2332.650	61.2	0.7				2403.329	-8.4	0.7
33369	1474	BG Mon	2327.752	71.4	0.7				2419.261	-11.5	0.7
			2333.745	71.6	0.7				2423.270	-12.1	0.7
			2350.255	71.4	0.7	58786	3156	$\cdots$	2349.500	-21.3	0.7
34413	1565	W CMa	2330.686	18.9	0.6				2368.400	-21.4	0.7
			2333.737	19.6	0.6				2386.299	-21.2	0.7
35681	1622	RU Cam	2350.266	-24.4	0.6	62944	$\cdots$	$\cdots$	2327.992	8.5	0.6
			2356.253	-26.3	0.7				2332.853	6.5	0.6
			2375.335	-24.9	0.7				2363.504	6.7	0.6
38242	1891	$\cdots$	2327.760	13.7	0.7				2368.416	6.2	0.6
			2332.664	15.7	0.7				2382.382	7.3	0.6
39118	1981	$\cdots$	2327.772	95.5	0.7	63955	3335	$\cdots$	2327.957	-9.2	0.7
			2332.670	96.2	0.7				2332.919	-10.1	0.6
44812	2428	$\cdots$	2327.917	20.2	0.7	69089	$\cdots$	$\cdots$	2330.980	-20.3	0.6
			2350.314	20.1	0.8				2332.906	-20.3	0.7
			2375.300	20.7	0.7				2359.572	-21.4	0.6
50412	2715	$\cdots$	2349.486	-84.8	0.7				2382.449	-20.2	0.6
			2386.284	-84.9	0.7				2399.479	-20.9	0.6
53354	2892	$\cdots$	2330.801	4.7	0.7
			2332.827	5.8	0.8
53522	2900	$\cdots$	2327.938	28.0	0.6
			2369.333	31.5	0.7
			2375.326	33.3	0.7
			2382.336	34.9	0.7
			2386.337	34.3	0.7
			2399.377	36.3	0.7
			2403.322	37.5	0.7
			2419.255	37.5	0.7
			2423.255	38.5	0.7

$\parbox{14cm}{ $^a$\space Not HIP 29899. See Table~\ref{tab:off}.}$