SUBARU/HDS STUDY OF HE 1015−2050: SPECTRAL EVIDENCE OF R CORONAE BOREALIS LIGHT DECLINE

Hydrogen-deficient carbon (HdC) stars and R Coronae Borealis (RCB) type stars form a rare class of carbon-rich supergiants (Clayton 1996, 2012 for a general review). The deep minima and infrared excesses that are characteristics of RCB stars are absent in HdC stars (Warner 1967; Feast & Glass 1973; Feast et al. 1997). These stars have spectra similar to F-G supergiants but show only weak absorption features of hydrogen while molecular bands of carbon are strong (in cool RCBs and HdCs). The most promising scenario for the origin of these objects is that they are post-asymptotic giant branch (post-AGB) stars, which are low- to intermediate-mass stars at the evolutionary state from AGB to white dwarfs, particularly objects that experience a final helium flash (Iben et al. 1996). However, many other scenarios, including a merger of two white dwarfs, have also been proposed (Webbink 1984; Renzini 1990; Staff et al. 2012).

Chemical composition studies of RCB stars suggest that the presence of material in their atmosphere is exposed to s-processing. Asplund et al. (2000) find that all RCB stars show some enhancement of s-process elements relative to Fe ([Y/Fe] = 0.8; [Ba/Fe] = 0.4). Extraordinary overabundances of Y and Ba in U Aqr are also reported by Vanture et al. (1999) with estimates of [Y, Zr/Fe] ⩾ +3.0 and [Ba/Fe] = 2.1. Light neutron-capture elements like Sr, Y, and Zr are usually attributed to the weak component of the s-process, which is the neutron-capture process with relatively long timescale and small neutron exposure (Käppeler et al. 2011). Massive stars in He- or C-burning phase are suggested as the astrophysical site of the weak s-process, but there is no direct observational evidence to support this scenario.

From low-resolution spectroscopic analysis of a large number of faint high latitude carbon stars of Hamburg/ESO survey, Goswami et al. (2010) have identified HE 1015−2050 to be an HdC star. The object has an IR excess, the observed magnitudes detected by NASA's Wide-field Infrared Survey Explorer are 14.663 at 3.4 and 4.6 μm bands, 12.887 at 12 μm, and 9.195 at 22 μm band. The object is also monitored by the Catalina Real-Time Transient Survey; light-curve data obtained between 2006 February and 2012 May indicate that the object exhibits no variability at the level of measurement uncertainties (Andrew Drake 2012, private communication). In this Letter, we discuss its high-resolution spectrum and confirm its hydrogen deficiency. Further, we find that the spectrum displays features that are reminiscent of RCBs at early decline or near recovery to maximum. The spectrum is characterized by strong features of Sr and Y. There are no obvious stronger features of heavier elements (i.e., Mo, Ba), indicating that this object can be a source of weak s-process.

The high-resolution spectroscopic observations of HE 1015−2050 was carried out with the High Dispersion Spectrograph (HDS) of the 8.2 m Subaru Telescope (Noguchi et al. 2002) on 2012 January 13. The spectrum was taken with a 30 minute exposure at a resolving power of R ∼ 50, 000. The observed bandpass ran from about 4010 Å to 6800 Å with a gap of about 75 Å from 5355 Å to 5430 Å due to the physical spacing of the CCD detectors. The low-resolution (R ∼ 1300) spectrum obtained with 2 m HCT at IAO, Hanle is taken from Goswami et al. (2010). The data were reduced, in the standard fashion, using IRAF³ spectroscopic data reduction package.

A selection of about 30 absorption lines are used to determine the radial velocity of the object HE 1015−2050. Velocities are derived from the central cores of lines. The mean Heliocentric radial velocity is estimated to be ∼18 ± 1.5 km s⁻¹. The radial velocity survey of RCB stars by Lawson & Kilkenny (1996) records a typical peak-to-peak variations of 10–20 km s⁻¹. The velocities estimated from the emission features of H_α, Na i D₂, D₁, and Sc ii λ5684.19 are, respectively, 17.1, 19.3, 19.6, and 21.8 km s⁻¹, and are similar to the velocity derived from the photospheric absorption lines. There is, however, evidence that the narrow emission lines are blueshifted relative to the pre-decline absorption lines (e.g., R CrB; Cottrell et al. 1990).

During deep declines of RCBs (e.g., R CrB and RY Sgr) the forming dust cloud extinguishes the photospheric light and a rich narrow-line emission spectrum appears consisting of neutral and singly ionized metals (Payne-Gaposchkin 1963; Alexander et al. 1972; Clayton 1996). Most of these lines referred as E1 (Alexander et al. 1972) are short lived that fade within two or three weeks and are replaced by a broad-line spectrum. Some of the early-decline narrow emission lines remain strong for an extended period of time. These lines referred as E2 are mostly low excitation lines, and primarily multiples of Sc ii and Ti ii. The Sc ii λ4246 line is one of the strongest lines in the E2 spectrum of RCB stars in decline (Rao et al. 1999). The spectrum of HE 1015−2050 shows narrow-line emission features as well as broad absorption features, leading us to believe that the spectrum was acquired when the object was obscured by forming dust. We put forward our arguments supporting this idea in the following sections.

4.1. Spectral Characteristics: Hydrogen Deficiency

In Figure 1, we show a comparison of the spectrum of HE 1015−2050 with the spectrum of HD 26, a hydrogen normal CH star, in the wavelength region 4310−4330 Å. The features due to G band of CH, seen in deep absorption in HD 26, are distinctly shallow in HE 1015−2050. We have also compared the H_α feature in HE 1015−2050 with its counterpart in HD 26 (Figure 1, right panel). While H_α appears as a strong absorption feature in the spectrum of HD 26, in the spectrum of HE 1015−2050 we note an emission feature at the absorption core. The velocity estimated from the H_α emission feature is ∼ +17.1 ± 1.5 km s⁻¹, very similar to the velocity derived from the photospheric absorption lines.

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Marginal detection or non-detectability of H_β and H_γ features could also be evidence for hydrogen deficiency. Weak and shallow features of H_β (Figure 5) and H_γ in the spectrum of HE 1015−2050 are seen with marginally detected emissions at the absorption cores. The Balmer lines, which are typically very weak in RCB stars, are seen going into emission in a few cases (i.e., V 854 Cen; Rao & Lambert 1993; Clayton et al. 1993). Strong Balmer lines and features of CH bands are seen in the spectrum of V 854 Cen (Kilkenny & Marang 1989; Lawson & Cottrell 1989).

4.2. Spectral Characteristics: Carbon Molecular Bands

4.3. Spectral Characteristics: Emission Features

Na i D₂ and D₁ features. In Figure 2, we show the Na i D features of HE 1015−2050. In the low-resolution (λ/δλ ∼ 1300) spectrum, Na i D₂ and D₁ features are not resolved and appear in absorption (Figure 2, lower panel). These features are clearly resolved in the high-resolution (λ/δλ ∼ 50, 000) spectrum and appear in emission (Figure 2, upper panel). Emission features of Na i D₁ and D₂ are reportedly seen in decline spectra of RCBs; at maximum these features appear in absorption. The appearance of Na i D₂ and D₁ in emission in the high-resolution spectrum of HE 1015−2050 is a clear indication that the spectrum is acquired at its early decline or near recovery to maximum. In the R CrB decline spectra, these two features in emission show a large variation throughout the decline (Rao et al. 1999).

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Forbidden oxygen lines. Sharp emission features of the forbidden lines of [O i] 5577, 6300 and 6363 Å are identified in the spectrum of HE 1015−2050. The estimated flux ratio ([F(6300) +F(6363)]/F(5577)) is ∼0.29. Rao et al. (1999) have reported a value of ∼18 for this flux ratio throughout the 1995–1996 decline of R CrB.

Atomic lines: emission features of Sc, Mg, Ti, Fe, Ba. We have made a weak detection of the line Sc ii λ4246.8 in emission in the spectrum of HE 1015−2050. This line is one of the strongest lines in the "E2" spectrum of RCB stars in decline (e.g., Rao et al. 1999). The Sc ii line at 5657.88 Å also appears in emission. As shown in Figure 3, an emission feature at the absorption core of Mg i at 5167.3 Å is seen in its spectrum. A few other emission features such as Mg i 5183.6 Å, Ti i 5173.7 Å,  Ti ii 5185.9 Å,  and Fe ii 5197.5 Å are also noticed in the spectrum (Figure 3). As shown in Figure 4, the lines of Sc ii λ5684.2 and Ba i λ6498.7 are clearly seen in emission. The emission lines of Sc ii λ5684, Fe ii λ6247, Ba ii λ6497, Fe ii λ5362, Fe i λλ5405, 5371, 5447, Ti ii λλ5490, 5492, Si i λ4102, Zr ii λ4096 are reportedly seen in the 1995 October 18 decline spectrum of R CrB with red absorption components (Rao et al. 1999). Some unidentified emission features are known to exist in RCB decline spectra (Whitney et al. 1992; Asplund 1995); the spectrum of HE 1015−2050 also displays a number of emission features that remain unidentified.

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4.4. Spectral Characteristics: Absorption Spectrum

Helium. Among the four lines from the triplet series of neutral helium seen in R CrB's decline spectrum (Rao et al. 1999), except for 5876 Å, the other three, 3889 Å,  7065 Å,  and 10830 Å, are out of the wavelength region considered here. However, He i lines of D3 5876 Å are not detected either in emission or absorption in the spectrum of HE 1015−2050. This line is observed in broad emission in R CrB's decline spectrum (Rao et al. 1999). It is possible that this line has also been filled by emission but has not risen above the continuum. Other He i triplets at 4471 Å and 4713 Å and singlet lines of He i at 4922 Å and 5015 Å are not detected in the spectrum of HE 1015−2050.

Lithium. Features due to Li are not detected in the spectrum of this object. Only four RCB stars (e.g., UW Cen, R CrB, RZ Nor, and SU Tau) are known to be Li-rich with Li abundance ranging from 2.6 to 3.5 (Asplund et al. 2000). Pollard et al. (1994) reported the presence of Li in two of the LMC R CrB stars. Vanture et al. (1999) mention a tentative identification of the Li i 6708 Å line in U Aqr. Z UMi, a cool RCB star, also shows lithium (Kipper & Klochkova 2006). In Figure 5, the location of Li i at 6708 Å in the spectrum of HE 1015−2050 is shown, compared with its counterpart in Z Umi, where this feature is seen strongly. The spectrum of Z Umi is taken from Goswami et al. (1997).

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Atomic lines: Na, Mg, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni. Many neutral and singly ionized lines of these elements are detected in the spectrum of HE 1015−2050. Some of these lines show filled-in emission. For instance, Ca i line at 4226.7 Å and Fe i line at 4222.2 Å detected in absorption show filled-in emission at the absorption cores, whereas the Na i line at 5688.2 Å and the Ca i line at 4094.9 Å also detected in absorption do not show any filled-in emission. Several neutral and singly ionized lines of Fe i are also detected. Lines due to V, Cr, Mn, Co, and Ni are also seen in the spectrum in blends with contributions from other atomic lines. The Cr i line at 5674.1 Å is clearly detected in absorption. A few absorption features due to Fe i and Ti i are shown in Figure 4 along with a Mg i absorption feature at 5172.7 Å.

Atomic lines: Sr, Y, Mo, Ba, Ce. The spectrum of HE 1015−2050 is found to exhibit strong features of Strontium. Sr ii λ4077.7 is detected as a strong absorption feature that seems to show filled-in emission. Sr ii line at 4215.5 Å is also detected as a strong feature in absorption. This feature is likely to have been affected by contributions due to strong blue-degraded (0,1) CN 4216 band. These two Sr features are shown in Figure 5. Although detected clearly, Ba ii at 4554 Å (Figure 5) and 6496 Å (Figure 4) are obviously not stronger than Sr lines. While the Ba ii line at 4554.036 Å is observed with an equivalent width of 131 mÅ  we have made a marginal detection of the Ba ii feature at 6496.9 Å in absorption. Lines due to Y ii and Ce ii are also detected in the spectrum in blends with contribution from other atomic lines. We have made a weak detection of the Mo i line at 5689.2 Å in absorption (Figure 5).

4.5. Circumstellar Environment: Polarimetric Evidence

The high-resolution spectrum of the HdC star HE 1015−2050, acquired on 2012 January 13, shows features that are reminiscent of RCBs at early decline or near recovery to maximum. The decline of optical light in RCBs is attributed to the formation of clouds of carbon dust (O'Keefe 1939). The presence of circumstellar material is supported by our polarimetric observation.

The appearance of a narrow-line emission spectrum consisting of lines of neutral and singly ionized metals, and a few broad lines including the Ca ii H and K, the Na i D lines is a common characteristics of RCBs in decline (Payne-Gaposchkin 1963; Alexander et al. 1972; Clayton 1996; Rao et al. 1999). With emission lines of Na i D₁, Na i D₂, Sc ii, Mg i, etc., the spectrum of HE 1015−2050 seems to show characteristics of an RCB decline spectrum.

The non-detection of Li features may be used as a clue for the production mechanism of this object. Among the two most promising scenarios for the origin of RCBs, Li enhancements are consistent with the FF models as in the case of Sakurai's object (Lambert 1986; Asplund et al. 1998). However, a high carbon isotopic ratios as well as enhancement of ¹⁸O with no Li production are expected from WD merger scenario. From these, the latter is a more likely production scenario for this object; however, knowledge of the carbon and oxygen isotopic ratios would be necessary to support this.

The spectrum shows strong features of Sr but no obvious strong features of heavier elements (i.e., Mo, Ba). It has been suggested that the rapid proton-capture (rp) process that occurs during the merger of a white dwarf and a neutron star is expected to produce a large excess of Mo (Z = 42) compared to Sr and Y, with no excess of barium and other heavy elements (Cannon 1993). It is therefore unlikely that the rp-process is responsible for the surface composition of this object. As Thorne-Ztkow objects are suggested to reveal the products of rp-process in their surface composition, HE 1015−2050 does not form a good candidate for a Thorne-Ztkow object.

Strong spectral features of Sr and weak detection of Mo and Ba indicate that its atmosphere is enriched with material resulting from weak s-process. Several possibilities exist for giving rise to such atmospheres. The object may have been formed out of material that is already enriched with s-process material. Alternatively, the object may have been in a binary system with a companion that had undergone unusual s-process enrichment and had transferred mass to this object. However, the binary status of the object cannot be confirmed at present, and also, except for DY Cen (Rao et al. 2012), none of the RCB and HdC stars is known to be binary (Clayton 1996). Another possibility concerns the fact that the spectrum does not show enhancement in heavy s-process elements, and hence, it seems necessary for the s-process to produce only light s-process material. This requires a low neutron density. It was shown by Smith (2005) that assuming all other parameters same, higher neutron densities produce larger amounts of heavier elements than of lighter ones. As suggested for U Aqr (Bond et al. 1979), HE 1015−2050 could also be an He-C core of an evolved star of near-solar initial mass that ejected its H-rich envelope at the He core flesh. A single neutron exposure occurred at the flash, resulting in a brief neutron irradiation producing more of the s-process elements. Detailed chemical composition studies based on high-resolution spectra taken at maximum would be worthwhile to unearth the production mechanism.

We thank the referee, Geoff Clayton, for his many useful suggestions. This work made use of the SIMBAD astronomical database, operated at CDS, Strasbourg, France, and the NASA ADS, USA.