The results of optical spectroscopic monitoring observations of a young binary system, Z CMa, are presented in this study. Z CMa consists of a Herbig Be star and an FU Orionis object, and it shows irregular light variation in the quiescent phase and exploding brightening in the outburst phase. Medium-resolution spectra were obtained on 21 nights between 2015 and 2019 using the Nayuta telescope in Japan. We also used five high-resolution spectra, obtained between 2008 and 2011, with the Keck Telescope. During the outburst phase, the intensity of the He I absorption line increased with an increase in the luminosity of the system. Because the He I absorption line is a characteristic feature of an early-type star, we considered that the outbursts were caused by the Herbig Be star. The equivalent widths of the [O I] line decreased with an increase in the luminosity of the system. We claim steady mass loss at the rate of
A young stellar object (YSO) gains its weight by accretion of surrounding materials through a circumstellar disk. Steady mass accretion rate of an order of 10−8 Me∙yr−1 has been proposed for a solar-mass YSO [
Z CMa is an YSO that shows irregular light variations. In the past twenty years, Z CMa has repeatedly exhibited 0.5 - 1.5 magnitude changes in the optical wavelengths [
Near-infrared speckle interferometry resolved two components of the binary [
The large scale picture that the Herbig Be star is surrounded by a dust cocoon of variable geometry and the whole system is surrounded by an infalling envelope is widely supported by many observational results (e.g. [
JHKL-band photometry of the individual components was carried out by using the adaptive optics system [
We have carried out a series of optical spectroscopy of Z CMa. We also investigated the relationship between the line intensities obtained from spectroscopy and the luminosity obtained from broad-band photometry.
We conducted spectroscopic observations of Z CMa on 21 nights between October 2015 and November 2019 by using the medium- and low-dispersion long-slit spectrograph (MALLS) mounted on the 2.0 m Nayuta telescope at Nishi-Harima Astronomical Observatory, Japan. Using a grating of 1800 lines/mm and a 0.8” slit, we obtained spectra with a wavelength resolution of ~10,000 between 6280 Å and 6720 Å. The exposure time ranged from 300 s to 1200 s, and 1 - 4 spectra were obtained each night. Flat frames and comparison frames were acquired by using a halogen lamp and an Fe-Ne-Ar lamp in the instrument, respectively. Dark frames were also obtained. Details of the observations are presented in
We also used data obtained by employing the high-resolution echelle spectrometer (HIRES) of the Keck telescope. The data between 2008 and 2011 were obtained from the archive system. The spectral resolution was ~48,000 between 5700 Å and 7150 Å. We obtained object frames, bias frames, flat frames, and Th-Ar lamp comparison frames. Details of the observations are also listed in
The MALLS data were reduced using the IRAF software. The average count of the overscan region was subtracted from the entire region. Ten dark frames were combined with the median, and the combined dark image was subtracted from the object and flat frames. The normalized flat image was used to correct the sensitivity of each CCD element. We identified approximately 50 emission lines of the Fe-Ne-Ar lamp in the comparison image and established a relationship between the column number of the pixel and the wavelengths. The column number of the pixel of the object frame was converted into the wavelength using this relationship. We also corrected the spatial distortion of the object image using the comparison image. We then fitted background emission as a function of the columns of the image and subtracted it from the entire region. We extracted a one-dimensional spectrum from a two-dimensional spectrum in the object frame. The extracted spectrum was binned by the slit width. The Doppler shift due to the orbital motion of the Earth was corrected. We combined the spectra acquired each night. Finally, we normalized the continuum level of the spectrum.
Image processing of the HIRES data was performed through a pipeline called Mauna Kea Echelle Extraction in addition to IRAF. Each component of the binary system was not resolved either in the MALLS data or in the HIRES data.
Date (JST) | JD (−2,450,000) | Integ. Time (s) | V-mag | Telescope |
---|---|---|---|---|
2008-12-03 | 4804 | 60 | 8.7 | Keck (PI: Dahm) |
2008-12-04 | 4805 | 60 | 8.7 | Keck (PI: Dahm) |
2008-12-17 | 4818 | 60 | 8.5 | Keck (PI: Hireseng) |
2010-11-14 | 5515 | 200 | 8.5 | Keck (PI: Herbig) |
2011-11-18 | 5884 | 180 | 10.1 | Keck (PI: Reipurth) |
2015-10-13 | 7309 | 1800 | 9.95 | Nayuta |
2015-10-31 | 7327 | 1800 | 9.82 | Nayuta |
2015-11-29 | 7356 | 1800 | 9.61 | Nayuta |
2015-12-01 | 7358 | 1800 | 9.61 | Nayuta |
2016-01-10 | 7398 | 1800 | 8.14 | Nayuta |
2016-01-12 | 7400 | 1800 | 8.14 | Nayuta |
2016-01-13 | 7401 | 1800 | 8.19 | Nayuta |
2016-01-16 | 7404 | 1800 | 8.19 | Nayuta |
2016-03-03 | 7451 | 1800 | 8.53 | Nayuta |
2016-03-11 | 7459 | 1800 | 8.68 | Nayuta |
2016-04-05 | 7484 | 1800 | 9.20 | Nayuta |
2016-10-13 | 7675 | 1800 | 8.71 | Nayuta |
2016-10-20 | 7682 | 1800 | 8.88 | Nayuta |
2016-11-03 | 7696 | 1800 | 9.36 | Nayuta |
2016-11-08 | 7701 | 1800 | 9.48 | Nayuta |
2016-11-11 | 7704 | 1800 | 9.50 | Nayuta |
2016-11-17 | 7710 | 1800 | 9.74 | Nayuta |
2016-11-21 | 7714 | 1800 | 9.72 | Nayuta |
2018-01-23 | 8142 | 1800 | 10.34 | Nayuta |
2019-01-15 | 8499 | 1800 | 10.01 | Nayuta |
2019-01-18 | 8502 | 1800 | 9.93 | Nayuta |
Date (JST) | He I | Hα | [O I] | Fe II 6435 | Fe II 6456 | Fe II 6516 |
---|---|---|---|---|---|---|
2008-12-03 | 0.96 | − 63.2 + 0.4 − 0.5 | − 1.60 + 0.21 − 0.19 | −2.03 | −2.00 | −2.46 |
2008-12-04 | 0.85 | − 76.5 ± 0.4 | − 1.37 + 0.14 − 0.19 | −2.06 | −2.08 | −2.57 |
2008-12-17 | − 38.7 ± 0.3 | − 1.18 + 0.05 − 0.08 | −1.36 | −1.24 | −1.48 | |
2010-11-14 | 1.27 | − 38.5 + 0.3 − 0.4 | − 1.51 ± 0.13 | −1.85 | −2.11 | −2.19 |
2011-12-18 | 0.22 | − 85.9 + 0.6 − 0.7 | − 3.38 + 0.23 − 0.25 | −1.42 | −1.58 | −1.71 |
2015-10-13 | 0.20 | − 156.3 + 0.9 − 1.0 | − 3.03 + 0.27 − 0.29 | − 3.44 + 0.14 − 0.15 | − 3.27 ± 0.09 | − 3.48 + 0.11 − 0.12 |
2015-10-14 | 0.21 | − 167.7 ± 0.9 | − 3.33 + 0.28 − 0.29 | − 3.56 + 0.13 − 0.15 | − 3.42 ± 0.09 | − 3.43 ± 0.11 |
2015-10-31 | 0.24 | − 132.2 ± 1.0 | − 3.10 + 0.31 − 0.32 | − 2.90 + 0.15 − 0.16 | − 2.48 + 0.10 − 0.11 | − 2.89 + 0.12 − 0.13 |
2015-11-29 | 0.14 | − 146.8 + 1.0 − 1.1 | − 2.52 + 0.30 − 0.32 | − 3.46 ± 0.17 | − 3.31 ± 0.11 | − 3.27 ± 0.13 |
2015-12-01 | 0.13 | − 150.6 ± 1.1 | − 3.13 ± 0.36 | − 3.48 ± 0.18 | − 3.30 ± 0.12 | − 3.46 + 0.14 − 0.15 |
2016-01-10 | 1.74 | − 44.6 + 1.5 − 1.4 | − 0.54 + 0.24 − 0.33 | − 1.58 + 0.20 − 0.22 | − 1.35 + 0.12 − 0.13 | − 1.33 ± 0.13 |
2016-01-12 | 1.36 | − 50.0 ± 0.6 | − 0.67 + 0.15 − 0.17 | − 1.51 + 0.08 − 0.09 | − 1.34 ± 0.05 | − 1.40 ± 0.06 |
2016-01-13 | 1.32 | − 52.4 ± 0.7 | − 0.88 + 0.19 − 0.22 | − 1.51 + 0.10 − 0.11 | − 1.35 ± 0.06 | − 1.50 ± 0.07 |
2016-01-16 | 1.09 | − 49.2 + 0.6 − 0.7 | − 0.79 + 0.15 − 0.18 | − 1.37 + 0.09 − 0.10 | − 1.17 ± 0.05 | − 1.26 ± 0.06 |
2016-03-03 | 1.40 | − 66.6 + 0.6 − 0.7 | − 1.52 + 0.19 − 0.20 | − 1.85 + 0.09 − 0.10 | − 1.36 + 0.05 − 0.06 | − 1.68 + 0.06 − 0.08 |
2016-03-11 | 0.86 | − 84.2 + 3.2 − 3.1 | − 1.67 + 0.73 − 0.88 | − 2.37 + 0.41 − 0.45 | − 1.58 + 0.27 − 0.30 | − 2.15 + 0.28 − 0.35 |
2016-04-05 | 0.64 | − 111.2 ± 1.0 | − 1.94 + 0.30 − 0.33 | − 2.81 ± 0.17 | − 2.45 ± 0.11 | − 2.75 + 0.11 − 0.14 |
2016-10-13 | 1.00 | − 59.1 ± 1.1 | − 1.78 + 0.32 − 0.35 | − 1.78 + 0.17 − 0.18 | − 1.43 + 0.08 − 0.09 | − 1.54 + 0.12 − 0.13 |
2016-10-20 | 1.33 | − 80.8 ± 2.4 | − 1.97 + 0.66 − 0.75 | − 2.15 + 0.35 − 0.38 | − 1.36 + 0.13 − 0.15 | − 1.84 + 0.23 − 0.29 |
2016-11-03 | 0.16 | − 121.6 ± 1.0 | − 2.36 + 0.30 − 0.32 | − 3.35 ± 0.16 | − 3.15 ± 0.11 | − 3.11 + 0.12 − 0.13 |
2016-11-08 | 0.13 | − 136.5 + 1.1 − 1.0 | − 2.83 + 0.31 − 0.32 | − 3.54 ± 0.16 | − 3.35 ± 0.10 | − 3.43 ± 0.12 |
2016-11-11 | 0.12 | − 133.0 ± 0.9 | − 2.90 + 0.28 − 0.29 | − 3.43 ± 0.15 | − 3.16 ± 0.10 | − 3.28 ± 0.11 |
2016-11-17 | 0.13 | − 142.4 + 1.0 − 0.9 | − 3.66 ± 0.29 | − 3.53 ± 0.15 | − 3.37 ± 0.10 | − 3.44 ± 0.11 |
2016-11-21 | 0.06 | − 136.0 + 1.1 − 1.0 | − 3.53 + 0.32 − 0.33 | − 3.42 + 0.16 − 0.17 | − 3.11 + 0.11 − 0.12 | − 2.85 ± 0.12 |
2018-01-23 | 0 | − 119.9 ± 2.5 | − 3.36 + 0.62 − 0.71 | − 1.26 + 0.24 − 0.33 | − 0.70 + 0.15 − 0.20 | − 1.12 + 0.20 − 0.24 |
2019-01-15 | 0.22 | − 130.1 ± 0.9 | − 3.14 + 0.26 − 0.27 | − 2.95 + 0.10 − 0.11 | − 2.83 + 0.07 − 0.08 | − 3.06 ± 0.10 |
2019-01-18 | 0.07 | − 139.1 + 1.1 − 1.0 | − 4.03 ± 0.32 | − 3.61 ± 0.16 | − 3.28 ± 0.11 | − 3.37 + 0.12 − 0.13 |
The He I absorption line has been detected in many Herbig Ae/Be stars. A Herbig Ae/Be star, V645 Cyg, has blueshifted absorption features in the He I absorption line [
In our observations, the He I line showed a weak emission feature during the quiescent phase and a blueshifted broad absorption feature during the outburst phase (
feature originates from the Herbig Be star, we considered it to be the source of the outbursts.
It is well-known that the [O I] emission traces a jet. [O I] narrow-band images of Z CMa were obtained with an angular resolution of 0.03 arcsec [
In the optical spectra of Z CMa, the [O I] emission line showed a double peak feature with peaks at 6291.0 Å and 6300.6 Å (
we concluded that the [O I] emission line was veiled when the outbursts occurred.
It is claimed that the blue peak did not exist until 2000 but grew significantly in 2002 [
The mass loss rate was estimated from the [O I] luminosity. We considered 1150 pc as the distance to Z CMa. We assumed constant V-R color and constant continuum flux in the R-band wavelengths, so that the ratio of the band width of the V-band filter to the [O I] equivalent width corresponded to the ratio of the V-band luminosity to the [O I] luminosity. We used the formula of [
log 10 M ˙ w = − 4.3 + log 10 L 6300 L ⊙ , (1)
where L6300 is the [O I] luminosity and Le is the solar luminosity. The mass loss rate was found to be almost constant at 10−5.5 Me∙yr−1.
The fact that the [O I] emission line was unchanged in the luminosity and veiled by continuum light during the outbursts suggests that the origin of the jet is the FU Orionis object. Otherwise, the jet is insensitive to the rapid change in the mass accretion.
Z CMa shows a strong Hα emission line. It is suggested that the Hα emission line of Z CMa emanates from jet and that its high velocity component comes from the expanded atmosphere of the Herbig Be star [
The observed equivalent widths of the Hα line in the outburst phase seemed to be consistent with the model. R2 is 0.91 between the model and the observed equivalent widths for V < 9.6 mag. The Hα emission line was veiled to the continuum light when the outburst occurred. Based on the variation in the equivalent widths of the He I absorption line, it was determined that the Herbig Be star was the source of the outbursts. The fact that the Hα emission line was veiled with the continuum light suggests that the Hα line originates from a place unrelated to the outbursts, i.e., from the FU Orionis object. In the quiescent phase, the equivalent widths of the Hα emission line did not match the model. We discuss this equivalent width variation in the Hα line in the quiescent phase in the following sub-section.
Several emission lines of Fe II were detected (
direct integration.
In the previous sections, the equivalent widths of the emission lines of [O I], Hα, and Fe II, as well as those of the He absorption feature, were investigated. In the outburst phase, the equivalent widths of the He I absorption line increased,
whereas those of the [O I], Hα, and Fe II emission lines decreased. We claimed that the Herbig Be star contributed to the optical continuum flux in the outburst phase. The Hα and Fe II emission lines were heavily veiled by the continuum light of the Herbig Be star in the outburst phase, and we think that these lines originated from the FU Orionis object. The [O I] line originated from a low-density region away from the central star such as a jet emanating from the FU Orionis object. Nature of the outburst is still under debate; it is not clear whether the outburst was originated as a consequence of a real outburst associated to the Herbig Be star, or because of changes in the dust cocoon such as the formation of a new hole. Broad-band photometry and polarimetric observations will reveal geometrical change of the cocoon, composition of the dust, and grain size distribution. If the amount of visible light transmitted through the cocoon changes, the amount of the visual extinction changes. Because the adjacent continuum flux as well as the line flux would change, an equivalent width of a line does not change. It is clear that the equivalent widths of many emission and
absorption lines changed during the outbursts. These phenomena are not accounted by the geometrical change of the dust cocoon. We claim that, at least, a part of the outbursts are attributed to the real outburst of the Herbig Be star.
The equivalent widths of the emission and absorption lines also changed in the quiescent phase. The He I absorption feature was barely detected. The [O I] emission line was veiled by the continuum light. The intensities of the Hα and Fe II emission lines increased as the star became brighter. Previous studies suggested that the FU Orionis object dominates the V-band luminosity during the quiescent phase [
We conducted optical spectroscopic monitoring observations of a young binary Z CMa. Medium-resolution spectra were obtained on 21 nights between 2015 and 2019. During the observation period, the object experienced four brightening phases with an amplitude of 2 magnitudes in the V-band. The intensity of the He I absorption line increased with an increase in the luminosity of the object. Because the He I line is a characteristic feature of an early-type star, we concluded that large-amplitude light variations were associated with the Herbig Be star. The equivalent widths of the [O I] line decreased as the continuum flux increased. The mass loss rate estimated from the equivalent widths of the [O I] line remained almost constant at 10−5 Me∙yr−1. The intensities of the Hα and Fe II emission lines did not change in the outburst phase. However, these lines became stronger as the luminosity of the object increased in the quiescent phase. We propose that the light variations in the outburst phase are caused by the Herbig Be star and those in the quiescent phase are due to the FU Orionis object. Because the FU Orionis object shows emission lines, we claim that its light variation is caused by mass accretion.
This work was supported by JSPS KAKENHI Grant Number JP17K05390.
The authors declare no conflicts of interest regarding the publication of this paper.
Akimoto, H. and Itoh, Y. (2021) Optical Spectroscopic Monitoring Observations of a Young Binary Z CMa. International Journal of Astronomy and Astrophysics, 11, 406-421. https://doi.org/10.4236/ijaa.2021.113019