2 Observations

 

 

Table 1: Spectroscopic observations of AS 78 and MWC 657

Object	JD	Obs.	Exp.t	Range	$\Delta \lambda$
	2400000+		min	Å	Å
AS 78	49358.32	SAO	32	3530-5530	4
AS 78	49358.34	SAO	32	4930-6950	4
AS 78	49374.20	SAO	24	3895-4920	2
AS 78	49374.22	SAO	24	5705-6780	2
AS 78	51388.49	SAO	80	4540-7761	0.6
AS 78	51419.03	Lick	$^{\rm a}$	$8000-13\, 500$	14
AS 78	51419.03	Lick	$^{\rm a}$	$14\, 000-24\, 820$	36
MWC 657	49592.58	OHP	60	3900-6815	0.2
MWC 657	49941.59	OHP	60	3900-6815	0.2
MWC 657	51066.72	DSO	48	3800-5600	3.6
MWC 657	51186.16	SAO	60	4735-6800	0.6
MWC 657	51542.50	DSO	60	3800-5600	3.6

The observatory name is listed in Col. 3, the spectral resolution in Col. 6.
• $^{\rm a}$ See text for the explanations.

The log of spectroscopic observations is presented in Table 1. The low-resolution observations of AS 78 were obtained on 1994 January 5 and 21 at the 6-m telescope of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences with a photoelectric TV-scanner (Balega et al. 1979) mounted at the Nasmyth focus. The high-resolution spectrum of AS 78 was obtained on 1999 July 28 at the same telescope with the échelle-spectrometer PFES (Panchuk et al. 1998) and a 1140$\times$1170 pixel CCD detector.

The infrared observations of AS 78 were acquired on 1999 August 28 with the 3-m Shane reflector of Lick Observatory and the Aerospace near-infrared imaging spectrograph. The spectrograph, which is described in more detail by Rudy et al. (1999), uses two channels to provide wavelength coverage from 0.8 to 2.5 $\mu$m. The blue (0.8-1.4 $\mu$m) and red (1.4-2.5 $\mu$m) spectra, which are acquired simultaneously, are each combinations of 6 individual spectral segments taken at different wavelength positions. Of the 6 positions, there are 3 that are well separated in wavelength; for each of these, 2 distinct positions are used that are separated by 2.5 pixels. The latter provides both oversampling and protection from dropouts due to bad pixels. Each of the 6 individual spectral segments are in turn formed from 4 individual spectra, each of 5 seconds duration. These spectra are taken at two separate locations along the slit ("plus beam'' and "minus beam'') in the following sequence: plus, minus, minus, plus. The spacing along the slit was 20'' and the slit was oriented North-South.

Figure 2: Brightness and colour variations of MWC 657 folded onto the 86 day period. The 1996 data are marked by filled circles, while those of 1998 are open circles

A 2 $\hbox{$^{\prime\prime}$ }$ slit was employed for the observations of AS 78 and its calibrator star HR 1276, an F5 dwarf. The instrumental response and most of the effects of atmospheric absorption were removed by dividing the spectrum of AS 78 by that of the calibrator. To remove the intrinsic spectrum of the calibrator from this ratio, a model from Kurucz (1994) appropriate to an F5 V star was used. The absolute flux level was set from the K-magnitude (5.17) of HR 1276. The K-magnitude was derived from the V-magnitude tabulated in the Bright Star Catalog (Hoffleit & Jaschek 1982) and the nominal V-K colour for an F5 dwarf given by Koornneef (1983). Using this absolute calibration, we estimated the star's brightness in the JHK bands. The results are listed in Table 2 and are in good agreement with the CST data for the J and H bands. The continuum flux at 0.9 $\mu$m (1.110^-16 Wcm^-1$\mu$m^-1) agrees well with the results of our I-band photometry ( $\overline{I}$ = 9.6 mag that corresponds to 1.210^-16 Wcm^-1$\mu$m^-1 according to Strajzhys 1977).

The low-resolution optical spectra of MWC 657 were obtained on 1998 September 10 and 1999 December 30 at the 0.8-m telescope of the Dark Sky Observatory (DSO) of Appalachian State University. We used the Gray/Miller spectrograph and a 600 lines mm^-1 grating in the first order, with a Tektronics 1024 $\times$ 1024 thinned, back-illuminated CCD. These spectra have a spectral range of 3800-5600 Å, and a resolving power R of about 1300 (3.6 Å/2 pixels). Two high-resolution spectra of MWC 657 were taken on 1994 August 28 and 1995 August 12 with a fiber-fed spectrograph ÉLODIE at the 1.93-m telescope of the Observatoire de Haute-Provence (south of France), in the framework of an extended search for spectroscopic binaries among young early-type emission-line stars (Corporon & Lagrange 1999). Another high-resolution optical spectrum of MWC 657 was obtained on 1999 January 7 at the SAO 6-m telescope with PFES.

The simultaneous UBVRIHK observations in the Johnson photometric system were obtained between January 1993 and March 1999 with two 1-m telescopes at the Assy and Tien-Shan Observatories of the Fesenkov Astrophysical Institute of the National Academy of Sciences in Kazakhstan, equipped with a two-channel photometer-polarimeter (Bergner et al. 1988). The 26 $\hbox{$^{\prime\prime}$ }$ diaphragm and a 85 Hz chopper throw of 84 $\hbox{$^{\prime\prime}$ }$ in the north-south direction were used in all the runs. The results of the optical photometry are presented in Table 2 for AS 78 and in Table 6 for MWC 657. Typical errors of these observations do not exceed 0.03 mag (0.05 mag in the U-band for MWC 657), and also include those of the transformation from the instrumental to the standard photometric system. The data with larger errors are marked with a colon. Only four K-band and one H-band observations were obtained for AS 78, as its brightness turned out to be very close to the limiting magnitude of the equipment. These results are shown in Table 3 and discussed in Sect. 3.1.1. The results of our first UBVRIK photometric observations of MWC 657 obtained in 1996 September-December were presented by Miroshnichenko et al. (1997).

On 1999 September 17 we obtained JHK observations of both stars at the 1.55-m Carlos Sánchez Telescope (CST), operated by the Instituto de Astrofísica de Canarias at the Spanish Observatorio del Teide (Tenerife, Spain). We used a CVF infrared spectrophotometer equipped with an InSb photovoltaic detector, operating at the temperature of liquid nitrogen, with a photometric aperture of 15 $\hbox{$^{\prime\prime}$ }$ and a chopper throw of 30 $\hbox{$^{\prime\prime}$ }$ in the RA direction to subtract the contribution from the background sky. The Teide photometric system is described in Arribas and Martínez-Roger (1987), as well as its relations with other standard photometric systems. Since this system is very close to the Johnson system used at Tien-Shan, no additional corrections have been applied to the CST data. For the flux calibration we used several standard stars taken from the list given by Koornneef (1983), observed at different air masses to determine the atmospheric extinction in each filter during the night of the observation. Each star was observed 4 times in each band. The averaged data for AS 78 are included in Table 3, while those for MWC 657 are presented in Sect. 3.1.2.

On 2000 February 25 and 26, we obtained IR observations of AS 78 at the 3-m NASA IRTF, equipped with a single-element gallium-doped germanium bolometer, at Mauna Kea (Hawaii, U.S.A.). The photometric aperture of 10 $\hbox{$^{\prime\prime}$ }$ and an 11 Hz chopper throw of 15 $\hbox{$^{\prime\prime}$ }$ in the N-S direction was used. The measurements in the broad-band KLM filters were taken on the first night, while both the broad-band KLMN and narrow-band filters centered at 8.7, 9.8, 11.6, and 12.5 $\mu$m were used on the second night. The accuracy of the measurements for AS 78 was close to 1 per cent in the broad-band filters, and about 4 per cent in the narrow-band ones. The magnitudes for HR 1457 and HR 1641 from the IRTF standard stars list were used to calibrate the KLM-data, while the results of Hammersley et al. (1998) obtained at IRTF for HR 2077 were used for the N-band and narrow-band data. The mean resulting accuracy of the object's magnitudes after calibration is 2 per cent for the broad-band and 5 per cent for the narrow-band data. Our IRTF data are listed in Table 4.