6 Spectral features in the near-infrared spectra of stars

 

The 2.36 $\mu$m-4.05 $\mu$m region is a valuable spectral probe for both hot and cool stars. HI lines (Bracket, Pfund and Humphreys series), HeI and HeII lines, atomic lines and molecular lines (CO, H₂O, NH, OH, SiO, HCN, C₂H₂, ...) are sensitive to temperature, gravity and/or the nature of the outer layers of the stellar atmosphere (outflows, etc.). In this section we give a qualitative discussion of the spectral features seen in the sources in our sample. We illustrate this with the spectra of representative sources in the atlas. For the spectral classes where several post-helium spectra are available we have selected the spectra with the best signal-to-noise.

 

 

Table 5: Overview of the spectra in the atlas of stars with spectral types earlier than A0.

Spectral type	V	IV	III	II	I , Ia, Ib & Iab	other
O4					1
O6	1
O6.5	1
O7.5					1
O9	2				1
O9.5	1				3
O9.7					1
B0	3	1
B0.5	1		1		1
B1	2		1		2
B1.5		1	1		1
B2	3	3	1			1
B2.5	3				1
B3	1	1	2
B5	1	1	1	1	2
B6	2		1
B7		2			1
B8	2		1		2
B9	2	1	1		1	1
B						7
WC						6
WN						8
PN						3
novae						2
supernovae						1

Tables 5 and 6 show an overview of the total number of spectra in the atlas per spectral type.

 

 

Table 6: Overview of the atlas spectra of stars with spectral types later than A0.

Spectral type	V	IV	III	II	I, Ia, Ib & Iab	other
A0	2	1	1	1
A1					1
A2			1		3
A3	1				2
A5	2		1			1
A7	3
A8	1		1			1
A9	2	1
F0	1	1	1	1
F2	2	1			3	1
F3	1	3	2		1
F4	1
F5	1		1	1	3
F6		1		1	2
F7	4			1	2
F8	1	1				4
G0	2		1		2
G1		1
G2	3	1	1		1
G3	1	1			1
G4	1				1
G5	1				2
G6	1	1			1
G7		1			1
G8	1	1	4		1
G9			2
K0	1	1	3	1	1
K1	1		1		1
K1.5			1
K2			1		1
K3			2	1	1
K4	2		2		1
K5	1		4		1
K7	1		1		1
M0			1
M1	1		1		2
M2			1		2
M2.5		1
M3			2	1	2
M4			1	1		1
M5			5	2		2
M6			5
M7			4			1
M7.5						1
M8			2			2
M9						1
S						2
C				8		8

6.1 OB stars

The spectra in the observed wavelength region of the normal OB-type giants and dwarfs show hydrogen lines in absorption of the Bracket (Br$\alpha$ [4.0523 $\mu$m], Br$\beta$ [2.6259]), Pfund (Pf$\gamma$ [3.7406 $\mu$m] - Pf22 [2.4036 $\mu$m]) and Humphreys series (Hu14 [4.0209 $\mu$m] and higher). Some O supergiants show these hydrogen lines in emission and also show helium ionic lines (HeII(7-6) [3.0917 $\mu$m]). The Be stars in the sample exhibit their hydrogen lines in emission. These lines originate from the gas in the circumstellar disk.

A detailed quantitative discussion of the spectra of OB-stars can be found in Lenorzer et al. (2002).

The spectra of Wolf-Rayet stars are characterised by various broad emission lines originating in the hot dense stellar winds that drive the extremely high mass loss of these stars. We see emission lines of HeI, HeII, CIII, CIV and the forbidden [CaIV] line at 3.21 $\mu$m. (See also van der Hucht et al. 1996; Willis et al. 1997; Morris et al. 1999) A discussion of all ISO-SWS observations of WR stars is in preparation by Morris et al.

Figure 9 shows the spectra of HD 190429 [O4If+], HD 30614 [O9.5] and HD 68273 [WC8] with the most prominent hydrogen and helium lines indicated.

Figure 9: Spectral features in the spectra of O and WR stars. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

In Fig. 10 we display the most prominent spectral lines in HD 93030 [B0Vp], HD 193924 [B2IV], HD 58350 [B5Ia], HD 196867 [B9IV] and HD 56139 [B2 IV-Ve].

Figure 10: Spectral features in the spectra of B and Be stars. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

6.2 AF stars

The near-infrared spectrum of A trough F-type stars is dominated by the HI lines of the Bracket (n=4), Pfund (n=5) and Humphreys (n=6) series. We normally see those lines in absorption, except in some stars with a shell (e.g. Herbig Ae/Be stars) where the hydrogen emission originating in the shell can fill in some of the photospheric absorption (e.g. HD 190073).

Figure 11 shows a selection of spectra of A-F giants and supergiants while Fig. 12 displays a selection of A-F dwarfs.

Figure 11: Spectral features in the spectra of A and F giants and supergiants. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

Figure 12: Spectral features in the spectra of A and F dwarfs. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

6.3 G stars

In the spectra of G-type stars we see important contributions from various atomic lines. Towards later type G-stars the strength of the hydrogen absorption lines weakens and we start to see molecular species. The CO first overtone band appears between 2.36-2.45 $\mu$m.

In Fig. 13 we present a selection of the spectra of G dwarfs. Figure 14 shows a selection of the spectra of G supergiants.

Figure 13: Spectral features in the spectra of G dwarfs. Only the band-heads of the CO molecular band are indicated, although many fainter lines over the band are blended and contribute to a substantial depression of the continuum opacity. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

Figure 14: Spectral features in the spectra of G supergiants. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra.

6.4 KM stars

In cool stars (K,M) the spectrum shows atomic lines, but is dominated by molecular bands. We see the CO first overtone (2.36-2.45 $\mu$m), the OH band (3.02-3.4 $\mu$m), the H₂O band (2.36-3.8 $\mu$m) and the onset of the SiO first overtone (beyond 4.00 $\mu$m ) The strength of the bands as we see them does not only depend on the fundamental stellar parameters. The emission from the circumstellar dust can fill in and weaken the photospheric absorption bands considerably. In M giants the circumstellar gas can contribute to an additional absorption or emission (Tsuji et al. 1997). Figure 15 depicts a number of cool giants ranging from K0 to M8 in spectral type.

Figure 15: Spectral features in the spectra of K and M stars. The flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra. Only the strongest lines of the OH, CO and SiO molecular bands are indicated. A multitude of fainter lines in these bands and in the H2O band are not resolved and contribute to a substantial depression of the continuum.

6.5 Carbon stars

Secchi (1868) was the first to identify the class of carbon stars among red giants based on the presence of C₂ lines in the optical spectrum. Wallerstein & Knapp (1998) reviews what we have learned about carbon stars since then. The optical spectra also show lines of CH, CN, heavy elements like Tc, ¹²C and ¹³C. Various theories exist on the mixing mechanisms that bring those elements to the surface. Many carbon giants on the Asymptotic Giant Branch (AGB) are variable and show a high mass loss. The circumstellar dust shell obscures their photospheres at visible wavelengths and causes an excess at mid-infrared and far-infrared wavelengths.

At near-infrared wavelengths the circumstellar dust is optically thin and we see a broad absorption feature centred around 3.05 $\mu$m. This band is due to the stretching modes of C₂H₂ and HCN. The strength and shape of the bands associated with several vibrational transitions of HCN (2.5 $\mu$m, 3.6 $\mu$m, 3.85 $\mu$m) and C₂H₂ (2.6 $\mu$m, 3.8 $\mu$m) varies from source to source. We also recognise the CO first overtone band around 2.5 $\mu$m, the CH fundamental band (3-4 $\mu$m) and the CS first overtone band around 4 $\mu$m (Aoki et al. 1998). Figure 16 shows three carbon-rich objects with the important molecular bands indicated.

Figure 16: Spectral features in the spectra of carbon stars: the flux scale is in Jy, normalised to 1 at 3.8 $\mu$m and offset for the different spectra. Only the strongest lines of the CO, CS and CH molecular bands have been indicated.