Near-infrared Spectra of a Sample of Galactic Unclassified B[e] Stars^*

We obtained a low-resolution spectrum for AS 119 (IRAS 06070-0938) in 2001 March, using the Boller & Chivens Cassegrain spectrograph with a 600 l/mm grating, a slit width of 250 μm and a 512 × 512 CCD detector attached to the 2.15 m telescope at the Complejo Astronómico El Leoncito (CASLEO), San Juan, Argentina. The covered spectral range was λλ3500–4700 Å. The spectrum was corrected for atmospheric extinction. A He-Ne-Ar lamp spectrum was taken after this observation to perform the wavelength calibration. A flux standard star was used to flux calibrate the data.

Spectroscopic high-resolution observations (R = 12000) for AS 119 and MWC 819 (IRAS 06420+0122) in the region 6250–6700 Å were also carried out at CASLEO with a REOSC echelle Cassegrain spectrograph. The selected instrumental configuration was a 400 l/mm grating in cross-dispersion mode and a Tek 1024 × 1024 CCD.

Using the Coudé spectrograph attached to the Perek 2 m telescope at Ondřejov Observatory (Šlechta & Škoda 2002), we obtained optical observations for IRAS 02155+6410, MWC 728 (IRAS 03421+2935), and IRAS 07080+0605 in three different wavelength regions centered at 5700 Å, 6500 Å, and 8700 Å. We used the 830.77 l/mm grating with a SITe 2030 × 800 CCD.

All high-resolution spectra were reduced using standard IRAF⁵ tasks, such as bias subtraction, flat-field normalization, and wavelength calibration. Particularly, for the Ondřejov spectra the stability of the wavelength scale was verified by measuring the wavelength centroids of O i sky lines and two rapidly rotating stars were observed (ζ Aql, 27 Vul) for the telluric correction.

The infrared spectroscopic data of our star sample consists of K- and L-band spectra taken in 2010, 2011, 2012, 2013 and 2016 with the Gemini Near Infrared Spectrograph (GEMINI/GNIRS).

The medium-resolution K-band observations were obtained using long-slit mode with the short camera, an 111 l/mm grating and a 0.3 arcsec wide slit. This same instrumental configuration was adopted for the L-band observations for AS 119 and MWC 819. Whereas the medium-resolution L-band observations of the other three B[e] stars were carried out with the long camera, a 32 l/mm grating and a 0.1 arcsec wide slit. Both instrumental configurations yield a spectral resolution R ∼ 6 000. The spectra were centered at 2.31 μm and 2.29 μm for the K band and at 3.70 μm and 3.94 μm for the L band.

Additionally, we obtained L-band high-resolution observations (R ∼ 18000) for MWC 819 using GEMINI/GNIRS in long-slit mode, with the long red camera, an 111 l/mm grating and a 0.1 arcsec wide slit. The spectra were centered at 3.90 μm.

Several ABBA sequences were taken for each target. Flats and arcs were taken with each source/telluric standard star pair. We used IRAF software package tasks to extract and calibrate the spectra. The data reduction steps included subtraction of the AB pairs, flat-fielding, telluric correction and wavelength calibration. To perform the telluric and instrumental corrections, we used late-B-type telluric standard stars, observed near the object in both, time and sky position (air mass). We selected late-B-type standards because the only intrinsic features they exhibit are neutral hydrogen lines, which are easy to remove, allowing us to obtain a purely telluric spectrum. The latter was used to divide each program object spectrum to obtain a telluric corrected one. The wavelength calibration was performed using the telluric lines.

Wide-field Infrared Survey Explorer Space Telescope (WISE) observations of our sample stars, obtained in the W1, W2, W3, and W4 bands were retrieved from the NASA/IPAC Infrared Science Archive (IRSA). WISE mapped the sky at 3.4, 4.6, 12, and 22 μm (W1 to W4 bands) with angular resolutions of 6.1, 6.4, 6.5, and 12 arcseconds, respectively. WISE images, covering a size of 10' × 10' around each of the sample stars, were inspected to study the circumstellar surroundings.

Photometric data for the sample stars was gathered from several catalogs: Johnson photometry, 2MASS, Infrared Astronomical Satellite (IRAS), Midcourse Space Experiment (MSX), WISE, and AKARI. In addition, data for one of the sample stars was obtained from ASAS-3 Photometric V-band Catalog (Pojmanski 2002).

IRAS 02155+6410. Clarke et al. (2005) included IRAS 02155+6410 into a group of objects that showed normal H − K colors, but large K − [8] infrared excess. According to these IR colors, they suggested that this kind of objects might not be heavily dust embedded, and are probably post-AGB stars with optically thin cool dust shells, H ii regions, or galactic cirrus contaminated fields. Neither SiO maser emission (Jiang et al. 1999) nor CO associated clouds were detected (Kerton & Brunt 2003). Miroshnichenko et al. (2007) reported IRAS 02155+6410 as an object with a spectral energy distribution (SED) similar to those objects showing the B[e] phenomenon in the IRAS wavelength range. From the SED, these authors derived a T_eff = 9 000 K and assigned an A3 spectral type to its optical counterpart.

Our Hα absorption line profile looks similar to the one shown by Miroshnichenko et al. (2007). This line shows a typical Lorentzian type profile in absorption with an incipient emission core (Figure 1, left panel). We did not find [O i] λλ6300, 6364 Å forbidden emission lines. From N II and Si II absorption lines, we estimated a mean value of the projected rotational velocity V sin i = 100 ± 34 km s⁻¹. This value was obtained using a similar method than the one used in Chauville et al. (2001), by performing the Fourier transform to an empirical line profile given by

$$\begin{eqnarray}&&\psi (\lambda )={\exp }\{-{[\alpha {(\lambda -{\lambda }_{o})}^{\beta }+\gamma ]}^{-1}\}\end{eqnarray} \tag{ 1 }$$

and obtained by fitting the parameters α, λ_o, β, and γ.

The K-band spectrum shows a strong Brγ line in pure absorption (Figure 1, center panel), and no presence of forbidden emission lines or CO molecular bands (Figure 2, top panel). There are some features in the region 2.35–2.38 μm that are probably residuals of telluric lines.

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The star shows a featureless spectrum in the L-band (Figure 2, bottom panel). It is noticeable by the absence of the Brα line, which might be filled by emission, as Hα and Brγ are in strong absorption.

A dusty envelope surrounding the star can be seen in the WISE multicolor image at W1 (3.4 μm), W3 (12 μm), and W4 (22 μm) bands (Figure 3).

IRAS 03421+2935 = MWC 728. Merrill & Burwell (1949) first detected this object as an emission-line source. Its IRAS colors are similar to those of OH/IR stars, however no maser lines at 1612/1667 MHz were detected (te Lintel Hekkert et al. 1991; Chengalur et al. 1993). The optical spectra published by Miroshnichenko et al. (2007) show double-peaked Balmer lines, very strong [O i] lines in emission and absence of Fe ii emission lines. These authors also detected absorption lines of Li i λ6708 Å, and Ca i λ6717 Å that are indicative of a late-type companion. A further optical and near-IR spectroscopic study carried out by Miroshnichenko et al. (2015) confirmed the binary nature of MWC 728, which is composed of a B5 V primary star (T_eff = 14 000 K) and a G8 III companion (T_eff = 5 000 K). They determined an orbital period of 27.5 days. This star was also studied by Aret et al. (2016), who included it in a survey of B[e] stars with [O i] emission lines to search for [Ca ii] emission lines. Particularly, these lines arise in complementary high-density environments (such as the inner-disk regions around B[e] supergiants), but no indication of emission lines from [Ca ii] or the Ca ii IR triplet was found in this object. Among our observations we included the optical spectrum which was shown partly by Aret et al. (2016).

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Our spectra show a Hα line in strong emission with a two-peaked asymmetric profile (V/R < 1, see Figure 1) that resembles the line spectrum from 2014 November 10, shown by Miroshnichenko et al. (2015, see their Figure 10). Furthermore, the Hα emission is overimposed on a broad photospheric absorption component as shown in Figure 4 (top panel). Several hydrogen Paschen lines are seen in the region between 8500 and 8900 Å and show broad absorption profiles with a central emission that remains below the continuum level (Figure 4, bottom right panel). The He i λ5876 Å, and He i λ6678 Å lines show inverse P-Cygni profiles (Figure 4). We also identified absorption lines of N ii λ6284 Å, and Fe ii λ5780 Å. Lines from the spectrum of the cool companion are also present in absorption, such as Li i λ6708 Å, Fe i λ6401 Å, Fe i λ6594 Å, and Fe i λ6614 Å, as well as the Na i doublet interstellar lines λλ5890, 5896 Å. In this spectral region, forbidden emission lines of [O i] λλ6300, 6364 Å are prominent. Using the N II line profile, we estimated a mean value V sin i = 123 ± 41 km s⁻¹ for the primary star.

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The K-band spectrum shows the Brγ line in emission (Figure 1), and CO molecular bands in weak absorption. In Figure 2 (top) we observe ¹²CO bandheads of 2–0, 3–1 and 4–2 molecular transitions. In addition, an incipient ¹³CO absorption is present. We also identified several absorption lines of neutral atoms of Ca i, Na i, Mg i, Ti i, Si i, and Fe i.

In the L band, we observe Pfγ and Brα lines in strong emission and the line Hu₁₄₋₆ at 4.021 μm, in weak emission (Figure 2, bottom panel).

IRAS 06070-0938 = AS 119. This star was reported as an object with large infrared excess by Dong & Hu (1991) and considered as an unclB[e] star by Lamers et al. (1998). Based on optical and near-infrared spectra and on multicolor photometry, Miroshnichenko et al. (2006) suggested that AS 119 belongs to the FS CMa group and might be a possible binary in a phase of rapid mass transfer. In contrast to this, Lee & Chen (2009) using data from 2MASS classified it as a T Tauri star.

Cidale et al. (2001) determined the fundamental parameters of this star using the BCD spectrophotometric system (Barbier & Chalonge 1941; Chalonge & Divan 1973). They classified it as a B giant but the measured observations were not flux calibrated so that the parameters may not be reliable.

For the present work, we obtained a new low-resolution spectrum around the Balmer discontinuity (Figure 5) and measured the parameters based on a flux-calibrated spectrum which resulted in λ₁ = 74 Å, D = 0.16 (for details, see Aidelman et al. 2018). Then, using the new BCD effective temperature calibration curves (Zorec et al. 2009), we derived an improved set of fundamental parameters: T_eff = 23 200 ± 1 570 K, log g = 4.3 ± 0.1, M_V = − 2.3 ± 0.2 mag, and the spectral type B3 V.

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In Figure 5, we can observe the highest members of Balmer series in absorption. Hδ line appears as if it is filled in by emission, and Hγ shows a broad absorption profile with a sharp emission overimposed on the red wing. Comparing this spectrum with the one taken in 1997 (see Figure 2 in Cidale et al. 2001), we notice more Balmer lines toward the Balmer's limit and a decrement in the line strengths of Hγ and Hδ. Numerous lines of Fe ii and [Fe ii] as well as [S ii] λ4069 Å are seen in emission. Fe ii and [Fe ii] lines are also more intense than in 1997. Absorption lines of Ca ii λλ3933, 3968 Å, He i λ4471 Å, and Mg ii λ4481 Å are also present.

The optical high-resolution spectrum taken at CASLEO shows single-peaked emission in [O I] and [N II] and a two-peaked Hα line profile in strong emission, with a V/R < 1 (Figure 1, left panel). The Hα profile looks similar in shape and intensity to the one shown by Miroshnichenko et al. (2006). The Na i D-lines show a deep narrow component with a radial velocity of 30 km s⁻¹, similar to the velocity of other photospheric lines (30–40 km s⁻¹), two broad weak redshifted components around 160 km s⁻¹, and a weak emission component blueward Na i λ5890 Å at −50 km s⁻¹ (Figure 6). A weak He i λ5876 Å line seems to be present with a velocity of 40 km s⁻¹.

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The only feature that clearly appears in the K-band spectrum is the Brγ line in emission (see Figure 1). In the region around 2.3 μm, a weak CO molecular absorption feature could be present. However, the S/N of the spectrum does not allow us to confirm it (Figure 2, top panel).

The L-band spectrum shows Brα, Pfγ, and several lines of the Humphreys series in emission. The Brα line is the strongest one and shows a single-peaked profile, with an intensity of 1.7 above the continuum level (see Figure 1). The other hydrogen line profiles are wide and weak as can be seen in Figure 2 (bottom panel). In the same figure, we notice the presence of a weak molecular band in absorption located at 4.0830 μm. This feature could be attributed to the ²⁸Si¹⁶O (v = 4 − 2) bandhead, although the other two bands of the same isotope at 4.0437 μm (v = 3 − 1) and at 4.0042 μm (v = 2 − 0) are not evident. The former, if present, might be blended with the Brα line (see Figure 7).

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IRAS 06420+0122 = MWC 819. This is a B[e] star (The et al. 1994) whose evolutionary state is still not clearly established. Lamers et al. (1998) put it into the unclB[e] group. It was misclassified as a planetary nebula (Kohoutek 2001), and Miroshnichenko (2007) proposed it as a FS CMa candidate. Marston & McCollum (2008) presented narrowband Hα imaging of its environment showing lobes on one side of the star. Pereira et al. (2008) studied its optical spectrum and, according to the line strength of [N ii] λ5754 Å relative to [O i] λ6300 Å, concluded that it is either a sgB[e] or a protoplanetary nebula. Using a set of [Fe ii] lines they derived an excitation temperature T_ex = 12 000 K.

In Figure 1 we show our observation of the Hα line profile which reaches an intensity of 100 over the continuum level and has two peaks with V/R < 1. As our spectrum in the optical and red regions is underexposed, except for the extremely strong Hα, [O I] and [N II] emission, we could not reliably identify any other lines.

The main feature in the K-band spectrum is a strong single-peaked Brγ line in emission (even stronger than the Brα line, see Figure 1). The Pfund lines are visible in weak emission up to n ∼ 34 (Figure 2, top panel). There are some weak lines of [Fe ii] between 2.22 μm and 2.30 μm. Emission lines at λ2.258 μm and λ2.308 μm could be identified as Fe ii and [Ni ii], respectively. We detected neither He i λ2.185 μm nor CO bandheads around 2.3 μm.

The main features in the L-band are emission lines of Humphreys series and also Brα and Pfγ lines, the latter being narrow and very strong (Figure 2, bottom panel). It is noticeable the strong Fe ii λ3.938 μm emission line. The L-band spectrum displays faint emission features, which might be tentatively identified with the bandhead transition v = 2 − 0 of the carbon monosulfide (CS) molecule near 3.87 μm (see Figure 8). The second bandhead near 3.908 μm (v = 3 − 1) is not evident in the present observation. Anyway if it were present, it would be difficult to detect as it is blended with the Hu₁₅₋₆ line.

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To identify the CS molecule we computed synthetic bandhead models. For this, we modified our existing CO disk code (Kraus et al. 2000) by implementing the CS line lists and Einstein transition probabilities given by Chandra et al. (1995). From the shape of the first bandhead we estimated a CS rotation velocity projected to the line of sight of <20 km s⁻¹. The temperature and column density of the best-fitting model are T_(CS) = 800 K and N_(CS) = 10²⁰cm⁻².

To confirm the presence of CS and to better constrain the physical parameters, we obtained new infrared high-resolution (R = 18000) observations in 2016, which also revealed a weak bandhead near 3.87 μm (Figure 9) although we could neither fit the whole molecular pattern nor provide another identification of this emission feature. This could be caused by the strong pollution of the telluric lines longward the position of the expected first CS bandhead, or blends with other features that we were not able to identify. Moreover, as there is a large time interval (5 years) between both observations, the molecular ring or shell structure detected in 2011 could have expanded, thus the emission could have been diluted. A possible candidate for the blending is the iron monoxide molecule FeO. The P and Q branches of the FeO spectrum lay under the feature identified as the first CS bandhead while the R branch has smaller features at 3.85 μm. The possible presence of FeO may suggest some unusual chemistry caused by Brα pumping (Dalgarno et al. 1997).

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In Figure 10 we show the spatial distribution of tenuous dust around the star. The image was built in logarithmic scale, combining WISE W2 (4.6 μm), W3 (12 μm), and W4 (22 μm) bands.

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IRAS 07080+0605. This is a poorly studied star in the Northern Milky Way that has a strong Hα line in emission (Kohoutek & Wehmeyer 1999). Miroshnichenko et al. (2007) presented portions of a high-resolution spectrum that reveals broad H absorptions and double-peaked emissions in Hα, Hβ and [O i] lines with a peak separation of ∼50 km s⁻¹. They noted the extremely strong IR excess from 2MASS and IRAS fluxes. These authors classified the underlying star with an A spectral type probably surrounded by circumstellar material originated from rapid mass transfer in a binary system.

Our Hα line observation displays a strong and complex double-peaked profile (V/R < 1 with a peak separation of ∼90 km s⁻¹) superimposed over a broad photospheric component (Figure 1). Single-peaked emission lines of [O i] λλ6300, 6364 Å, [N ii] λλ6548, 6584 Å, and [Ni ii] λ6667 Å are also prominent. Permitted lines of Si ii λλ6347, 6371 Å, and Fe ii λ6456 Å are in absorption (Figure 11). From Fe II and Si II absorption lines we estimated a mean value of the projected rotational velocity, V sin i, of 65 ± 2 km s⁻¹.

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The K-band spectrum shows an inverse P-Cygni type profile in Brγ line (Figure 1). We do not detect any additional features that could reveal the presence of a late-type companion. However, it is interesting to remark the presence of the characteristic ro-vibrational v = 0–2 R- and P-branch CO absorption features between 2.29–2.32 μm and 2.35–2.37 μm, respectively (Figure 2, top panel). We confirm that this intriguing feature does not correspond with residual telluric lines. The mean radial velocity of the CO lines is around 8 km s⁻¹ similar to that of the forbidden lines present in the optical region.

The star displays a featureless L-band spectrum. As Brα line is not detected, we suggest it could be filled in with emission.

Using ASAS-3 Photometric V-band Catalog (Pojmanski 2002), we plot the visual light curve for IRAS 07080+0605 (see Figure 12, top panel) using two different data sets that cover seven years, from 2002 to 2009. We kept only those data labeled with quality A. The star shows light variations with an amplitude of around 0.6 mag. Using the Lomb-Scargle algorithm (Scargle 1982), we derived an approximate period P = 265 days (see Figure 12, bottom panel).

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To characterize the circumstellar environment, we used a simple model of an envelope made of gas and dust, and analyzed its effect on the spectral energy distribution (SED) of a normal star with a spectral type similar to that of the sample stars. By comparing the theoretical results with the observations, we can derive the global physical properties of the gas and dust envelopes.

We consider the circumstellar material distributed in a gaseous region located close to the star (≈3 R_*) and a dusty region, far from the star (≳100 R_*) (Zorec 1998; Muratore et al. 2010). We took the star flux models from Kurucz (1979).

The proposed model of the gaseous circumstellar envelope is based on those presented in Cidale & Ringuelet (1989) and Moujtahid et al. (1999). We calculate the emergent flux of a system formed by a star plus a spherical envelope by considering that the envelope can be reduced to an equivalent shell. We can then apply a plane-parallel solution for the transfer equation. The observed flux at a distance d obtained in this way can be expressed as

$$\begin{eqnarray}&&{f}_{\lambda }^{* +G}=\displaystyle \frac{{R}_{* }^{2}}{{d}^{2}}\,{F}_{\lambda }^{* }\,{\alpha }_{\lambda }\left(\displaystyle \frac{{R}_{* }}{{R}_{G}},{\tau }_{\lambda }^{G}\right)+\displaystyle \frac{{R}_{G}^{2}}{{d}^{2}}\,{S}_{\lambda }({T}_{G})\,{\beta }_{\lambda }\left(\displaystyle \frac{{R}_{* }}{{R}_{G}},{\tau }_{\lambda }^{G}\right),\end{eqnarray} \tag{ 2 }$$

with ${\tau }_{\lambda }^{G}$, optical depth of the gaseous shell; R_*, stellar radius; R_G, gaseous shell effective radius; ${F}_{\lambda }^{* }$, photospheric stellar flux; ${S}_{\lambda }({T}_{G})$, source function; and T_G, electron temperature of the gas. The factor α_λ describes the attenuation of the star flux in the gas shell and the term S_λβ_λ describes the emissivity of the gas. α_λ and β_λ can be numerically calculated.

The circumstellar dust region is treated using the same scheme proposed for the gaseous region. We characterize the dust by a total optical depth, ${\tau }_{\lambda }^{D}$, a temperature, T_D, and a source function, S_λ = B_λ(T_D). The model allows to add multiple dust components. We represent the interstellar extinction by ${\tau }_{\lambda }^{{ISM}}$. Both, ${\tau }_{\lambda }^{D}$ and ${\tau }_{\lambda }^{{ISM}}$ are related to the absorption A(λ) through the expression τ = 0.4ln(10) A(λ). Using the law given by Cardelli et al. (1989), $A(\lambda )=[{R}_{v}a(1/\lambda )+b(1/\lambda )]E(B-V)$, where R_v is the total to selective extinction, and $E(B-V)$ is the color excess. We adopt ${R}_{v}^{{ISM}}=3.1$ for the interstellar dust, while for the circumstellar dust we try different values of R_v^D greater than 3.1.

Finally, we obtain an expression for the observed flux at a distance d, for: star + gaseous shell + dust shell + interstellar extinction given by

$$\begin{eqnarray}\begin{array}{rcl}{f}_{\lambda }^{* +G+D+{ISM}} & = & {f}_{\lambda }^{* +G}{e}^{-{\tau }_{\lambda }^{D}}+\displaystyle \frac{{R}_{D}^{2}}{{d}^{2}}\,{S}_{\lambda }({T}_{D})\\ & & \times [1-2{E}_{3}(2{\tau }_{\lambda }^{D})]\,{e}^{-{\tau }_{\lambda }^{{ISM}}}.\end{array}\end{eqnarray} \tag{ 3 }$$

To eliminate the dependence on the distance, d, we normalize the flux to that at a reference wavelength λ_ref. Using expression (3), we obtain the theoretical SED, from 0.1 to 100 μm, for different sets of the free parameters of our model: R_G, T_G, ${\tau }_{\lambda }^{G}$, R_D, T_D, R_v^D, ${E}_{D}(B-V)$, and E_ISM(B − V). We adopt from the literature the spectral type and luminosity class for the sample stars when available, and construct the observed energy distribution using data from different photometric catalogs. In Figure 13, we present the best fit to the observed SEDs for the sample stars and in Table 6 we list the adopted model parameters.

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Table 6.  Dust and Gas Envelope Parameters Obtained with the Best-fitting Model to the Observed SEDs of the Sample Stars

IRAS ID	Alternative	Teff	log g	Tgas	Rgas	${\tau }_{{\rm{V}}}^{\mathrm{gas}}$	Ndust	Tdust1	Rdust1	Tdust2	Rdust2	Tdust3	Rdust3	Av
	Name	[K]		[K]	[R*]			[K]	[R*]	[K]	[R*]	[K]	[R*]	[mag]
02155+6410	⋯	11000	4.0	7000	2.0	0.3	2	370	1850	140	17000	⋯	⋯	1.5
03421+2935	MWC 728	14000	4.5	12000	1.2	0.5	2	900	320	150	8000	⋯	⋯	1.0
		5000	3.0	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯
06070-0938	AS 119	23000	4.0	11000	5.0	0.9	3	850	900	400	3600	100	60000	4.3
06420+0122	MWC 819	26000	3.0	13000	4.0	0.8	3	700	1400	300	7500	90	83400	5.4
07080 + 6410	⋯	13000	4.5	11000	1.2	0.9	3	1000	350	600	700	100	40000	2.5

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Except for IRAS 02155+6410, all stars display intense [O i] line emission which is typically formed in a circumstellar or circumbinary disk around B[e] stars. Moreover, the spectra of AS 119, MWC 819, and IRAS 07080+0605 show emission of [N ii], and the latter star also of [Ni ii]. These forbidden emission lines are optically thin. Modeling their profiles provides full information of the kinematics within their line-formation region. In all stars, the [O i] lines display symmetric, single-peaked (in our medium-resolution spectra) profiles that are redshifted with respect to the laboratory wavelength. These redshifts are 6 ± 1 km s⁻¹ for IRAS 07080+0605, 26 ± 1 km s⁻¹ for AS 119, 73 ± 1 km s⁻¹ for MWC 819, and 30 ± 1 km s⁻¹ for MWC 728, and we correct all forbidden emission lines with these radial velocities to center the line profiles.

Only for MWC 819 can the shapes of the forbidden lines be reproduced with a pure Gaussian profile. In the spectra of all other stars, the profiles require an additional, non-Gaussian component, for which we assume Keplerian rotation, to be in line with the suggestion that B[e] stars are surrounded by an equatorial disk. For simplicity, we apply a rotating ring model for the gas kinematics, add (if needed) a Gaussian component, and convolve the resulting line profile with a Gaussian component of 23 km s⁻¹, respectively. 25 km s⁻¹ representing the resolution of the Ondřejov, respectively, CASLEO spectrograph.

We find that the [O i] and [N ii] lines of MWC 819 can be reproduced with just a single Gaussian profile with a velocity of 16.5 ± 2 km s⁻¹. If a rotation component is present in these profiles, its velocity should be smaller than ∼5 km s⁻¹. In contrast, the [O i] lines of MWC 728 can be fitted reasonably well with just a single ring with rotation velocity (projected to the line of-sight) of 29.5 ± 0.5 km s⁻¹. For AS 119, the [O i] lines are composite, meaning that both a rotation (25 ± 1 km s⁻¹) and a Gaussian (25 ± 2 km s⁻¹) component are required to reproduce the profile shape, whereas the profile of the [N ii] line is much narrower and can be fitted with a single Gaussian velocity of 16 ± 2 km s⁻¹. As for the lines in MWC 819, we cannot exclude a rotation component contributing to the profile of [N ii] but, if present, it should be small (<5 km s⁻¹). The model parameters for our fits are summarized in Table 7 for the objects AS 119, MWC 819, and MWC 728, and model profiles are shown in Figure 14 in comparison to the observations.

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Table 7.  Velocities needed for Fitting the Line Profiles of the Forbidden Lines in AS 119, MWC 819, and MWC 728

Line		AS 119			MWC 819			MWC 728
		${v}_{\mathrm{rot},\mathrm{los}}$	vgauss		${v}_{\mathrm{rot},\mathrm{los}}$	${v}_{\mathrm{gauss}}$		${v}_{\mathrm{rot},\mathrm{los}}$	vgauss
[O i] 6300		25 ± 1	25 ± 2		<5	16.5 ± 2		29.5 ± 0.5	⋯
[O i] 6364		25 ± 1	25 ± 2		<5	16.5 ± 2		29.5 ± 0.5	⋯
[N ii] 6583		<5	16 ± 2		<5	16.5 ± 2		⋯	⋯

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The [O i] line profiles of IRAS 07080+0605 contain just a single rotation velocity component (projected to the line of sight) of 26 ± 0.5 km s⁻¹. However, the lines of [N ii] and [Ni ii] are less trivial. Their profiles display an extended blue wing causing very asymmetric profile shapes. Hence, a single rotation profile is not suitable to fit these asymmetries. Instead, additional blueshifted components are required to account for the blue wing. For simplicity, we assume that the complete line profile is a superposition of emission from individual gas rings, each with its own blueshifted radial velocity component. Then, the profile shapes of the [N ii] and [Ni ii] lines can be fitted with a combination of four and three rings, respectively. The combination of rotating rings and their individual blueshift is listed in Table 8, and the final fits to the observed profiles are shown in Figure 14. Interestingly, we find that the same rotation component of 65 km s⁻¹ in both the [N ii] and [Ni ii] profiles, as well as the ring with 26 km s⁻¹ as seen in the [O i] lines is present in both. The slight difference in needed radial velocity for the ring at 65 km s⁻¹ in modeling the [Ni ii] might not be real, because this line is the weakest hence noisiest in our spectrum.

Table 8.  Set of Rings Around IRAS 07080+0605 Detected in the Various Line Profiles. Listed Are their Rotation Velocities Projected to the Line of Sight (${v}_{\mathrm{rot},\mathrm{los}}$) and their Radial Velocities (v_rad). All Velocities Are in Units [km s⁻¹]

Line		Ring 1	Ring 2	Ring 3	Ring 4	Ring 5
[N ii] 6583	${v}_{\mathrm{rot},\mathrm{los}}$	110 ± 2	65 ± 2	45 ± 1	26 ± 0.5	⋯
	vrad	−42 ± 1	−25 ± 1	−10 ± 1	0 ± 0.5	⋯
[Ni ii] 6667	${v}_{\mathrm{rot},\mathrm{los}}$	⋯	65 ± 2	⋯	26 ± 0.5	<10
	vrad	⋯	−20 ± 1	⋯	0 ± 0.5	0 ± 0.5
[O i] 6300	${v}_{\mathrm{rot},\mathrm{los}}$	⋯	⋯	⋯	26 ± 0.5	⋯
	vrad	⋯	⋯	⋯	0 ± 0.5	⋯
[O i] 6364	${v}_{\mathrm{rot},\mathrm{los}}$	⋯	⋯	⋯	26 ± 0.5	⋯
	vrad	⋯	⋯	⋯	0 ± 0.5	⋯

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Our results provide certainly not a unique solution to the profile fits hence circumstellar kinematics, and it is also not known whether IRAS 07080+0605 has a companion. Nevertheless, our findings of possible multiple rings centered at various velocities is in agreement with what was reported for other B[e] stars in binary systems (see, e.g., Maravelias et al. 2018). Alternatively, the blue wings of the [N ii] and [Ni ii] lines might suggest high-velocity outflows of ionized gas, maybe in the form of a jet or knots, as is seen in the B[e] star MWC 137 (Mehner et al. 2016; Kraus et al. 2017).

Infrared flux ratios of H i recombination lines can be used as a diagnostic tool to constrain the geometry of the ionized circumstellar material around early-type peculiar stars such as B[e] or Be stars (Lenorzer et al. 2002a). Particularly, the H i emission-line strengths are sensitive to the density of the emitting gas: high densities result in optically thick lines for which line strength is only dependent on the emitting surface while low-density gas produces optically thin lines. In this way it is possible to separate disk and wind structures. Lenorzer et al. (2002a) constructed a log (Hu₁₄₋₆/Brα) versus log (Hu₁₄₋₆/Pfγ) line ratio diagram that allow to determine the characteristics of the envelope according to the star location on the diagram. Stars located close to the region corresponding to the Menzel case-B recombination theory (Baker & Menzel 1938) (shaded region in Figure 15) have optically thin envelopes while stars located toward the top-right region of the diagram have optically thick circumstellar disks.

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Only three stars of our sample show Pfγ, Hu₁₄₋₆ and Brα lines in emission: IRAS 06070-0938 (AS 119), IRAS 03421+2935 (MWC 728), and IRAS 06420+0122 (MWC 819). To measure the line ratios, we flux-calibrated each spectrum and corrected the equivalent width of Brα and Pfγ lines for photopheric absorption of the underlying star based on the relation proposed by Lenorzer et al. (2002b). The obtained values are listed in Table 9. In Figure 15 we show the location of these stars in the Lenorzer et al. (2002a) diagram. MWC 819 is close to the optically thin isothermal wind region, while AS 119 and MWC 728 are near the region of B[e] stars with thin circumstellar disks.