Massive Stars in the SDSS-IV/APOGEE SURVEY. I. OB Stars

Table 2 contains the results from the measurements of the equivalent width (EW), and full width at half maximum (FWHM) of the hydrogen (H i Br11 and Br13) and helium (He ii 7–12 and 7–13) lines observed in the spectra of the APOGEE-OB sample. We followed standard procedures to obtain these measurements that we briefly describe as follows. In order to obtain the line-profile parameters, we used several spectroscopic analysis routines available in the IRAF¹⁶ SPLOT package. For each spectral line, we performed model fittings using both Voigt and pure Gaussian profiles, with the associated continuum computed from regions far from the observed line wings (in general, several tens of angstroms from the estimated line centers), with the final uncertainty in the EW and FWHM values ranging from about 10% for spectra with the highest S/N (usually above 500) to 15%–20% for the spectra presenting lower S/N values in the range of 250–500.

We notice that the He ii 7–12 λ16923 line (when present) is sometimes seen close to the edge of the APOGEE spectral band (at about 16954 Å), which at this level may complicate the estimation of the continuum (at the red side of the line) by the algorithms of the IRAF tasks. In such cases, we proceeded to measure the line parameters carefully "by hand," using the IRAF task tools in the iterative mode. When this procedure was not possible, the results were simply discarded. Fortunately, only for a few cases (corresponding to early O-stars with no He ii 7–12 values quoted in Table 2), we were unable to properly measure that spectral line.

In general terms, the spectra of stars earlier than O9 display, besides the H i Brackett lines, the 7–12 and 7–13 He ii lines, making it possible to estimate luminosity classes and spectral types from the associated EW and FWHM measured values. Figure 4 shows the EW[Br11+Br13] versus mean FWHM[Br11+Br13] diagram in which the O-stars of luminosity classes i–ii and iii are represented, respectively, by red and orange squares, while the O-stars of luminosity classes iv–v are shown as blue circles. On the other hand, B-stars are represented by yellow (class i–ii–iii) and green (class iv–v) triangles. We can see that there is a clear separation between B- and O-type stars, the former spread in a region of the diagram well above the O-star sample, as these occupy the bottom part of the diagram.

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In fact, we find that four groups of stars occupy well-defined areas of the EW[Br11+Br13] versus mean FWHM [Br11+Br13] plot. These are delimited by colored dotted lines in Figure 4. Region (a) (red) is occupied by O-type stars of luminosity classes i–ii–iii with the extreme super-giants concentrated in its upper part, Region (b) (blue) by O-type stars mainly of the sub-giant and dwarf classes, and Region (c) (green) by B-type stars of luminosity classes iv–v, and Region (d) (yellow) is dominated by giant to super-giant B-type stars. From the diagram, we infer that (i) there is a clear separation between O-stars of classes i–ii from classes iv–v, with the class iii members sharing most of the space of the first group, with a few remaining exemplars found among the last. (ii) The few O- and B-type stars seen in the interfaces between regions (a)–(d), and regions (b)–(c) belong to the O9.5 to B0-B1 spectral types.

In Figure 5, we show the EW versus Spectral Type (SpType) diagrams made from the EW-Br11 and EW-Br13 values listed in Table 2. From the observed distributions of sources on both diagrams, it is possible to separate the stars into three distinct groups:

• (i)  
  A first set is formed exclusively by O stars ("O-stars only"), corresponding to sources with APOGEE spectra where the H Brackett lines have EW values satisfying the criteria EW(Br11) < 1.0 Å and EW(Br13) < 0.75 Å. We notice that in the case of the earliest O-type stars (like the two O5v seen in Figure 4(b)) the measured Br13 EW values probably go beyond the mentioned limit. Nevertheless, it is possible to discriminate early O-type stars from the B-type ones based also on the presence of the He ii 7–12 and 7–13 line transitions.
• (ii)  
  A second group ("[O + B0-B1] stars") presenting EW measured values in the range 1.0 < EW(Br11) < 2.1 Å, and 0.75 < EW(Br13) < 1.5 Å, which is formed by OB stars of spectral types in the range of O7-O8 to B0-B1.
• (iii)  
  Finally, a third group presenting EW values satisfying the criteria EW(Br11) > 2.0 Å and EW(Br13) > 1.5 Å. This last group contains only B-type stars ("B-stars-only").

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Interestingly, in case of the third group, it is possible to see that the strength of the hydrogen Br11 and Br13 lines diminish monotonically from later to earlier spectral types. Indeed, the observed linear relation is quite good, mainly for B stars earlier than ≈B3-B3.5. In Figure 6, we show the EW[Br11+Br13] versus SpType diagram for giants to super-giant stars (left), together with that for sub-giant and dwarf stars (right). Both diagrams were made using H line parameters taken from Table 2. Considering the observed trend in both diagrams, it is possible to conclude that there is a well-defined linear relation between EW[Br11+Br13] and SpType in the case of early- to mid-B stars. Based on linear fittings to the data in each diagram, we have the following empirical linear relations:

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(a) Giants and super-giants (B0-B3)

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{class}\ {\rm{I}},{\rm{II}},{\rm{III}}]=0.56\times \mathrm{EW}[\mathrm{Br}11+\mathrm{Br}13]\\ &&\qquad \qquad \qquad \qquad \qquad \quad +\,8.99\end{eqnarray} \tag{ 1 }$$

(b) sub-giants and dwarfs (B0-B5)

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{class}\ {\rm{IV}},{\rm{V}}]=0.64\times \mathrm{EW}[\mathrm{Br}11+\mathrm{Br}13]+8.47.\end{eqnarray} \tag{ 2 }$$

The numerical results from these equations relate to the spectral types in accordance with the correspondence shown in Figure 5, i.e., 10 corresponds to B0, 11 to B1, ..., 15 to B5.

The two He ii lines seen in the APOGEE spectra of O-type stars are generated by the 7–12 and 7–13 transitions, and from the line measurements presented in Table 2 we can see that the former vanishes in O-stars belonging to luminosity classes iv–v, and spectral types later than O9-O9.5 (e.g., sources #18, 29 and 91–O9v and sources #25, 78 and 85–O9.5iv–v). The only notable exception is source #20, an O9v star that does show a weak He ii 7–12 line at λ16923, and no He ii absorption line at λ15723, that disappears for all O-stars of spectral types later than O8.5. On the other hand, the cases of giants and super-giants are quite different, and from Table 2 (as well as from the spectra shown in Figure 3), we note that both He ii lines are present, sometimes even in the case of the O9.5–O9.7 later types.

In Figure 7, the SpType versus EW [He ii] diagrams for O stars (He ii (7–12 (left) and 7–13 (right)), constructed from the EW line measurements of Table 2 are shown. In these diagrams, stars of luminosity classes i–ii–iii, and luminosity classes iv–v (from now group 1 and group 2) are represented, respectively, by red squares and blue circles. We can see that O-type stars earlier than O8 appear to be separate in both diagrams, with the sources splitting into two distinct sequences. The first formed by O5 to O9 dwarfs and sub-giants, follows steeper sequences than those of group 2 sources. Linear regressions on the data for groups 1 and 2 yield the following empirical linear relations between spectral types and He ii equivalent line widths:

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(a) Dwarfs and sub-giants

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{He}\,{\rm{II}}\ \ (7-12)]=-5.05\times \mathrm{EW}[7-12]+9.32,\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{He}\,{\rm{II}}\ \ (7-13)]=-6.40\times \mathrm{EW}[7-13]+9.10.\end{eqnarray} \tag{ 4 }$$

(b) Giants and super-giants

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{He}\,{\rm{II}}\ \ (7-12)]=-3.50\times \mathrm{EW}[7-12]+9.59,\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\mathrm{SpType}\ \ [\mathrm{He}\,{\rm{II}}\ \ (7-13)]=-3.63\times \mathrm{EW}[7-13]+9.48.\end{eqnarray} \tag{ 6 }$$

Again, the numbers relate to the spectral types following the correspondence shown in Figure 7, e.g., 5 corresponds to O5, 6 to O6, ..., 9 to O9, and 10 to B0.

Optical spectra for sources APOGEE-OB-01, 02, and 04 were obtained with the SPM Echelle spectrograph and a Marconi-4 2048 × 2048 CCD detector mounted on the 2.1 m Telescope of the Observatorio Astronómico Nacional on Sierra San Pedro Mártir in Baja California, México. We used a slit width of 200 μm (about 275) and a cross dispersing grating of 300 lines/millimeter with a 3600 Å cutoff filter, together with a 13'' mask separating contiguous orders. The resultant spectral resolution is R ≈ 18,000 within orders 30–61, corresponding to the 3600 < λ/Å < 7000 range. For star #1, we obtained three 15 minute integrations, while for stars APOGEE-OB-02 and 04, two 15 minute integrations were sufficient. Each observation was preceded and followed by a 150 s Th–Ar–Ne lamp exposure to allow for wavelength calibration at each possible flexure.

The reduction of the SPM Echelle data was done using standard techniques by using the IRAF ONEDSPEC, TWODSPEC, and APEXTRACT packages. The one-dimensional spectra of the science targets were extracted from the two-dimensional frames by summing pixels in the data range and subtracting off the background value for each order column, which was measured as the median of contiguous non-target pixels. Cosmic rays and other anomalous signal detections were suppressed from each of the extracted spectra by removing pixels that deviate 5σ or more from the mean within a 100 pixel wide box that steps through the spectrum. The bad pixels were replaced through a linear interpolation of the removed data range, and the wavelength calibration was performed using the ThArNe lamp spectra. The final optical Echelle spectra for stars APOGEE-OB-01, 02, and 04 were then normalized through the fitting of the continuum emission in the associated wavelength ranges.

In the case of star APOGEE-OB-03, we used ESO archive data. The observation was made in 2003 January 16 in the framework of the ESO program 70.D-0110(A) (PI: Poretti), using the FEROS Echelle spectrograph (R ≈ 48,000) coupled to the ESO-1.5 m telescope. For the reduction of this spectrogram, we followed the same procedure as in the case of the San Pedro Martir Echelle spectrograms described above, normalizing the final spectrum by the fitted continuum in the appropriate wavelength ranges.

In Figure 9, we present the normalized San Pedro Martir ECHELLE and ESO-FEROS blue optical spectra (4000A–4700A) of the four stars. They are shown in decreasing order of effective temperature (from the top to the bottom). The He ii lines λ4200 and λ4542 clearly seen in the first three spectra, while they vanish in case of star APOGEE-OB-04. On the other hand, the He i line λ4471 presents the opposite behavior, becoming stronger from the earlier to the later types.

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We measured the EW of the spectral lines in these blue spectra and estimated independent spectral types for the four stars. In Table 5, we present the EW values for the following absorption lines: (i) eight He lines corresponding to the He i–ii λ4026, He i λ4121, He ii λ4200, He i λ4387, He i λ4471, He ii λ4542, He ii λ4686, and He ii λ4713 transitions, and (ii) three Si lines corresponding to the Si iv λ4089, Si iv λ4116, and Si iii λ4552 transitions. In Table 6, we list the computed EW ratio values used in the optical spectral classification procedure (Conti & Alschuler 1971; Conti & Frost 1977; Mathys 1988; Walborn & Fitzpatrick 1990; Sota et al. 2011) of the APOGEE-OB-01 to APOGEE-OB-04 stars.

As can be seen in Figure 9, APOGEE-OB-01 (BD+58 142) is the earliest-type star of the four. In the past, based on photographic magnitudes, it was classified as a B0 star by Kopylov (1953). Despite being a bright (V = 9.71) and very blue (U − B = −0.62) northern source (Mermilliod 2006), to date, it did not receive any further spectroscopic studies. Its optical spectrum presents very broad and strong He ii λλ 4542–4686 lines. The relative strengths of the λλ 4471–4542 He lines suggests a spectral type probably later than O7 (for which the two lines should appear with the same intensities). On the other hand, from the λλ 4144–4200 He lines, it is possible to estimate a spectral type earlier than O9 (for which the two mentioned lines should appear with the same intensities). From the EW ratios [4542]/[4387], [4200]/[4144], [4089]/[4026] shown in Table 6, and the criteria of Sota et al. (2011; their Table 4), we conclude that the spectral type of this star is O8 and, based on its very strong absorption He ii 4686 line, its luminosity class is v.

APOGEE-OB-02 (LS V +34 21) was previously classified as a B0 star by Jose et al. (2008), who performed an extensive UBVI CCD photometric study of the young open cluster Stock 8. As in the case of APOGEE-OB-01, the ratio of the He ii λλ 4542–4686 lines indicates a spectral type later than O7. On the other hand, from the relative intensities of the λλ 4144–4200 He lines, it is safe to state that it has a spectral type earlier than O9. From the EW ratios [4542/][4387], [4200]/[4144], and [4089]/[4026] shown in Table 6, and the criteria presented in Sota et al. (2011) we assign an O8.5 spectral type. The He lines in the optical spectrum of star #2 are narrower than those in the star #1 and, considering that its He ii λ4686 line appears less intense (or strong), we conclude that its luminosity class is probably iii (Sota et al. 2011).

APOGEE-OB-03 (HD 51756) was previously classified (based on photoeletric photometry) as a B0.5 iv star by Morgan et al. (1955). Later, based on low-resolution optical spectra it was reclassified as a B1 v star by Abt (2008).

From the ESO-FEROS optical spectrum of APOGEE-OB-03 shown in Figure 9, we can see that it looks quite different from those of stars APOGEE-OB-01 and APOGEE-OB-02, with very intense and narrow λ4026, λ4144, and λ4471 He i lines. From the computed [4542]/[4387], [4200]/[4144], [4552],[4542], [4089]/[4026], and [4686]/[4713] ratio values in Table 6, and the criteria presented by Sota et al. (2011) (their Tables 4 and 6), we conclude that APOGEE-OB-03 is an O9.7 iii star.

The last optical spectrum in Figure 9 is that for APOGEE-OB-04 (HD 253983). Based on UBV CCD photometry, it was previously classified as a B1v star by Abt & Corbally (2000). Its San Pedro Martir Echelle spectrum is quite similar to that for star APOGEE-OB-03, with the exception that the He ii λλ 4200-4542 lines are absent. Based on the criteria defined in Table 6 of Sota et al. (2011), together with its very small [4542]/[4387] and [4200]/[4144] ratios, and its very large [4552]/[4542] ratio values, we conclude that B0 is the most probable spectral type for star #4. In the case of the B-type stars, it is not possible to use the criteria presented by Sota et al. (2011) to estimate luminosity classes. Instead, we used the results by Didelon (1982) for B-stars, and from the observed helium and hydrogen EW values we are able to assign luminosity class iv–v to this star.