Gaia DR2 and EDR3 data and evolutionary status of post-AGB stars with high radial velocities

Abstract

Using the Gaia Data Release 2 (DR2) and Early Data Release 3 (EDR3) data and list of post-AGB candidates, we investigate the parallax, proper motion, and binarity for 20 post-AGB stars and candidates that have high radial velocities. From their Gaia distances, their luminosities and kinematics are derived. The evolutionary status of these stars is discussed from their location on the post-AGB evolutionary tracks. Nine stars are confirmed to be post-AGB stars that have initial main-sequence masses of around one or two solar masses. From their kinematics information, two objects among them are identified to belong clearly to the halo population, suggesting low mass. We discuss the origin and evolutionary status of other objects in the sample of this work with high radial velocities.

1 Introduction

Post-asymptotic giant branch (AGB) stars are transition objects evolving from the tip of the AGB horizontally towards the left in the Hertzsprung–Russell (H-R) diagram into early stages of young planetary nebulae (PNe). The post-AGB evolutionary stage is short-lived, depending on the core mass (Iben & Renzini 1983; Schoenberner 1983). During the transition from the tip of the AGB to the early stages of young PNe phase they appear as M-, K-, G-, F-, A-, and OB-type post-AGB supergiants for a short period (Parthasarathy & Pottasch 1986, 1989; Pottasch & Parthasarathy 1988; Parthasarathy 1993a, 1993b). They mimic the spectra of supergiants because of their extended thin atmospheres around the white-dwarf like C-O core (after severe mass-loss and the termination of the AGB phase of evolution). Before the advent of the Infrared Astronomical Satellite (IRAS), very few post-AGB supergiant candidates were known. Analysis of IRAS data has revealed many cool to hot post-AGB supergiants (Preite-Martinez 1988; Kwok et al. 1989). The list of post-AGB stars detected from the analysis of IRAS data by several investigators can be found in the paper of Vickers et al. (2015) and references therein. Progress in understanding post-AGB stars can be found in review papers (e.g., van Winckel 2003; Kamath & Van Winckel 2022).

Multi-wavelength studies of significant samples of post-AGB candidates were carried out by several investigators during the past 35 years, which enables us to understand their chemical composition, circumstellar shells, s-process nucleosynthesis, and late stages of evolution of low-mass stars (e.g., De Smedt et al. 2012, 2016; Kamath et al. 2022; Parthasarathy 2022, and references therein). However, for a better understanding of these stars, their distances, radial velocities, and accurate proper-motion measurements are required. With the advent of Gaia (Data Release 2 (DR2) and Early Data Release 3 (EDR3); Gaia Collaboration 2018; Lindegren et al. 2018), accurate parallaxes (distances), radial velocities, and proper-motion measurements of a large sample of post-AGB stars became available.

In an earlier paper (Parthasarathy et al. 2020), we studied the Gaia DR2 data and the evolutionary status of eight high-velocity, hot post-AGB stars. Finding high-velocity objects among post-AGB stars is useful to constrain the final stage of stellar evolution of low-metallicity, low-mass stars in the halo population. Such stars are very rare and are old low-mass stars in advanced stages of evolution. Some of them may belong to the Galactic halo. In this paper, we present an analysis of Gaia DR2 and EDR3 data of stars listed as post-AGB stars or candidates in literature.

2 Data and analysis

We investigate the list of stars given in the paper of Vickers et al. (2015) as likely or possible post-AGB stars, and searched the Gaia DR2 and EDR3 for post-AGB stars with radial velocities and with accurate parallaxes. The sample of Vickers et al. (2015) contains the list of almost all the known post-AGB stars. Here we define post-AGB stars as those which are in the transition region between the tip of the AGB and very early stages of PNe. These objects are often termed as proto-planetary nebulae.

We select 20 objects that have absolute values of radial velocities larger than 45 km s⁻¹ (|RV| > 45 km s⁻¹). The galactic longitudes and latitudes, parallaxes, radial velocities, G (Gaia G-band magnitude), V, (B − V), and spectral types are given in tables 1 and 2. Spectral types and V, B magnitudes are taken from SIMBAD. Among the 20 stars, 12 are high galactic latitude stars, 11 stars have high negative radial velocities, and nine have high positive radial velocities.

The parallaxes taken from Gaia DR2 and EDR3 are given in table 2. The distance derived from the parallaxes are also listed in the table. Four stars have a large relative parallax uncertainty (⁠|$>\!20\%$|⁠) in Gaia EDR3. For these stars, only the lower limit of the distance is presented. The table also gives the distances estimated by Bailer-Jones et al. (2021) using a prior constructed from a three-dimensional model of the Galaxy. Excluding the above four objects, the two estimates of the distance for each object agree within |$5\%$|⁠. We adopt the distances simply obtained from the parallaxes in the present work.

The normalized unit weight error (RUWE) values are also given in the table. The RUWE values also indicate the reliability of the parallaxes. Whereas RUWE values are sensitive to the photocentric motions by unresolved companions (Lindegren et al. 2021; Stassun & Torres 2021), they could be affected by other factors including nebulosity of proto-planetary nebulae. The RUWE values of nine stars, including the above four objects with large uncertainties of parallaxes, are larger than 1.4. Among them, four objects are likely to be post-AGB stars according to previous studies on stellar properties including infrared excess and metal depletion. See below for more details on these stars. The remaining five stars have large uncertainties in parallaxes and/or very large RUWE values, and, hence, are not regarded as candidates of post-AGB stars in this paper.

The other 11 stars in our sample have RUWE values smaller than 1.4. Among them, BD +33 2642 is suggested to belong to a binary system (Van Winckel et al. 2014). For the remaining 10 objects, there is no signature of binarity from the Gaia astrometry.

The kinematic information that is calculated based on the Gaia EDR3 astrometry is presented in table 3, excluding the four objects with large uncertainties of parallaxes.

E(B − V) values are obtained from dust maps in literature or by comparing expected intrinsic colors with observed ones (table 1). We use three-dimensional dust extinction maps from Chen et al. (2019) and Green et al. (2018) and a two-dimensional dust extinction map from Schlegel, Finkbeiner, and Davis (1998). For a star to have an E(B − V) estimate from three-dimensional maps, it needs to have a precise parallax measurement (relative uncertainty smaller than |$20\%$|⁠) and be within the sky coverage of the maps. Since Chen et al. (2019) focus on the low Galactic latitude field (|b| < 10°), we prioritize values from Chen et al. (2019) over Green et al. (2018) for objects with |b| < 10°. We note that Green et al. (2018) only covers the sky with declination larger than −30°, and hence we could not derive E(B − V) from three-dimensional maps for HD 16745 and HD 178443 despite precise parallax measurements available for these objects. The extinction coefficients from Green et al. (2018) and Chen et al. (2019) are converted to E(B − V) using values provided in Green et al. (2018), Schlafly and Finkbeiner (2011), and Casagrande et al. (2019). In addition to the interstellar extinctions considered in these dust maps, some objects could be affected by circumstellar dust extinction given the evolutionary status of the objects. Thirteen objects are indeed IRAS sources and their (B − V) colours are likely affected by circumstellar reddening. For instance, E(B − V) of HD 56126 (IRAS 07134+1005) estimated from the spectral type is 0.56, whereas E(B − V) from the dust map is quite small (0.08 or less). For these stars we used the observed (B − V) values from SIMBAD and intrinsic (B − V)₀ values estimated from their spectral types using table 15.7 of Allen’s Astrophysical Quantities (Cox 2000), with interpolation to derive E(B − V) values. The E(B − V) values derived in this way are prioritized over the values from dust maps.

On the other hand, the E(B − V) values of IRAS 07140−2321, IRAS 07227−1320, and IRAS 14325−2321 estimated from the spectral types are significantly smaller than those from the dust map. This suggests that the estimate of the reddening from the dust map or spectral types could be uncertain for these objects. For these three stars, we adopt E(B − V) from the dust map.

The typical error for E(B − V) from the dust map is 0.02–0.03 for high Galactic latitude objects. We adopt 0.05 as the uncertainty of reddening of these objects to determine the luminosity. This is consistent with the error of E(B − V) given in Vickers et al. (2015) for our sample (0.043 on average). For objects with large reddening, in particular objects that could be affected by circumstellar reddening, we assume the error of E(B − V) to be 0.2, including the uncertainty of the subclass of spectral types that results in difference of E(B − V). The E(B − V) and the error adopted are given in table 1. Taking the errors into account, the E(B − V) values estimated in this study agree well with those obtained for HD 56126 (0.43) and IRAS 14325−6428 (1.07) by Kamath et al. (2022). Although our values are slightly larger than those for IRAS 05208−2035 (0.01) and HD 46703 (0.23) found by Oomen et al. (2018), and for IRAS 08187−1905 (0.07) and HD 161796 (0.13) by Kamath et al. (2022), the discrepancy is smaller than 0.1 if the error ranges are taken into account.

The absolute V magnitudes are calculated from the apparent magnitudes and distances given in tables 1 and 2, respectively. The luminosity is calculated from the absolute magnitude and the bolometric corrections taken from Cox (2000). The values of bolometric corrections in Flower (1996) are 0.1–0.25 mag larger than those of Cox (2000), resulting in differences in log (L/L_⊙) of less than 0.1 dex. The changes of E(B − V) of 0.05 and 0.2 result in the difference of log (L/L_⊙) of 0.08 and 0.25, respectively. The T_eff values and spectral types of most stars are available from the literature. These values are given in table 2.

Kinematics are calculated using the Gaia EDR3 parallaxes and proper motion measurements. We adopt 8.21 kpc as the distance between the Sun and the Galactic center (McMillan 2017) and 0.021 kpc as the vertical offset of the Sun (Bennett & Bovy 2019). Solar motion is adopted from Schönrich, Binney, and Dehnen (2010) for radial and vertical velocities (11.1 km s⁻¹ and 7.25 km s⁻¹, respectively) and is calculated as 245.34 km s⁻¹ using proper motion measurements by Reid and Brunthaler (2004). The orbital energy is calculated assuming the Milky Way potential from McMillan (2017). These results are given in table 3. Kinematics of the post-AGB star candidates in Galactocentric frame are presented in figure 1.

We note that the sample selection of high-velocity post-AGB stars would not be affected if they belong to low-mass binaries, because the radial velocity variations expected for low-mass binaries are not as large as the radial velocities of the stars studied in this paper (tables 1 and 2).

3 Notes on the 20 high-velocity post-AGB candidates

Figure 2 shows the luminosity of the objects as a function of effective temperature with the post-AGB evolution tracks (Miller Bertolami 2016). The four objects with large uncertainties in the parallaxes (see section 2) are excluded. This figure indicates that typical luminosity range of low-mass post-AGB stars is 3 < log (L/L_⊙) < 4.

We find that nine objects have luminosities within this range, among which five have reliable Gaia parallaxes (the RUWE values are smaller than 1.4). BD −12 4970, which has very high luminosity [log (L/L_⊙) = 5.5], is also regarded as a post-AGB star. Among the 10 remaining stars with lower luminosity than the post-AGB star range given above, five with small RUWE have lower luminosity, log (L/L_⊙) < 3, and the other five have large RUWE values.

Here we report some detailed information for individual objects separately with the above grouping. It should be noted that the luminosities of binary stars are still uncertain due to the uncertainty of parallaxes. Although most of them are found in the groups with large RUWE values in this section, some known binary stars are also included in the groups with small RUWE values. Information on the binarity is given in the following notes for individual objects where available.

3.1 Post-AGB stars with small RUWE values

• HD 56126 (IRAS 07134+1005)

  This is a high Galactic latitude, and high-velocity, metal-poor, F-type post-AGB star with 21 μm emission feature. It is overabundant in carbon and s-process elements (Parthasarathy et al. 1992). De Smedt et al. (2016) report detailed abundances including [Fe/H] = −0.91 and large excesses of s-process elements (e.g., [Ba/Fe] = 1.82). More recently, Kamath et al. (2022) list this object as a single post-AGB star with s-process enrichment.

• IRAS 07140−2321 (V421 CMa)

  Gielen et al. (2011) derived T_eff = 7000 K, log g = 1.5, and [Fe/H] = −0.8.

• HD 116745 (CD −46 8644, Fehrenbach’s star, ROA 24)

  This is a high galactic latitude and high-velocity metal-poor halo post-AGB star (Gonzalez & Wallerstein 1992). Gonzalez and Wallerstein (1992) found it to be overabundant in carbon and s-process elements. They derived T_eff = 6950 K, log g = 1.15 and [Fe/H] = −1.77. HD 116745 is a member of the globular cluster Omega Cen.

• IRAS 17436+5003 (HD 161796)

  This is a high galactic latitude and high-velocity F-type post-AGB supergiant (Parthasarathy & Pottasch 1986). Luck, Bond, and Lambert (1990) derived T_eff = 6500 K, log g = 0.70, and [Fe/H] = −0.32. This object is listed by Kamath et al. (2022) as a single post-AGB star without s-process enrichment.

• BD −12 4970 (LS IV −12 13)

  This is a high-velocity, hot (B0.5Ia) post-AGB candidate. It is not an IRAS source. High-resolution spectroscopic study of this star is important.

• BD +33 2642

  This is a high Galactic latitude and high-velocity, metal-poor, hot post-AGB star. This object is also classified into a proto-planetary nebula. However, its nebula is not bright, and it is not an IRAS source. Napiwotzki, Heber, and Koeppen (1994) studied this star and derived T_eff = 20000 K, log g = 2.9, and [Fe/H] = −2.0. Its chemical composition indicates depletion of refractory elements. The [O/H] = −0.8 indicates it is intrinsically metal-poor. This star belongs to a binary system with a low-mass faint companion. The high-resolution spectrum shows no spectral features of the secondary. The orbital period determined by Van Winckel et al. (2014) is 1105 d. The binarity of this object would not affect the RUWE value, which is smaller than 1.4 (1.295).

3.2 Post-AGB stars and candidates with large RUWE values

• HD 46703 (IRAS 06338+5333)

  This star is a high Galactic latitude, and high-velocity, metal-poor, F-type pop II post-AGB star (Luck & Bond 1984). Luck and Bond (1984) derived T_eff = 6000 K, log g = 0.4, and [Fe/H] = −1.57. Parthasarathy and Pottasch (1986) were the first to find that it is a weak IRAS source with far-IR colors and a flux distribution similar to that of high Galactic latitude post-AGB star HD 161796 (Parthasarathy & Pottasch 1986). Hrivnak et al. (2008) also report [M/H] = −0.6 for this object, discussing depletion and binarity. This belongs to a binary system. Oomen et al. (2018) report an orbital period of 597 d for this object. The RUWE value of this star is 1.622, which is clearly higher than 1.4, as expected from the binarity.

• IRAS 08187−1905 (HD 70379, V552 Pup)

  This is a high galactic latitude and high-velocity F6 post-AGB supergiant (Reddy & Parthasarathy 1996). Reddy and Parthasarathy (1996) derived the chemical composition of this star from an analysis of high-resolution spectra. They derived T_eff = 6500 K, log g = 1.0, and [Fe/H] = −0.5. This star is listed by Kamath et al. (2022) as a single post-AGB star without s-process enrichment. The RUWE value of this object is 1.695, which is even higher than that of HD 46703. Although this would not indicate that this object belongs to a binary system, further investigation on the binarity will be useful.

• IRAS 14325−6428

  This is a high-velocity F5I star with IRAS colours and flux distribution similar to post-AGB stars and PNe. De Smedt et al. (2016) report [Fe/H] = −0.56 with excesses of s-process elements. This star is also regarded by Kamath et al. (2022) as a single post-AGB star without s-process enrichment. The RUWE value of this object is quite large (2.181). Whereas further study to investigate the binarity would be useful, this star can be treated as a post-AGB star with excesses of s-process elements according to literature.

• HD 137569 (IRASF 15240+1452)

  This is a high Galactic latitude and high-velocity post-AGB star. No Fe lines are detected in its spectrum by Martin (2004, 2006), who classified it as a metal-poor, hot post-horizontal branch (post-HB) star.

3.3 Objects with low luminosity with small RUWE

• IRAS 07227−1320

  This star is listed as a possible post-AGB star by Vickers et al. (2015). Its spectral type is M1I. No chemical composition study is available. The luminosity of this object [log (L_*/L_⊙) = 2.58] is lower than post-AGB stars in our sample.

• BD +32 2754

  This star is also listed as a possible post-AGB star by Vickers et al. (2015). It is a high Galactic latitude and high-velocity F-type star. There is no chemical composition analysis of this star. It may belong to the Galactic halo. The luminosity of this object [log (L_*/L_⊙) = 1.10] is clearly lower than post-AGB stars.

• HD 178443 (LSE 182)

  This is not an IRAS source. It is a high Galactic latitude and high-velocity (343.5 km s⁻¹) star. McWilliam et al. (1995) derived T_eff = 5180 K, log g = 1.65, and [Fe/H] = −2.07. They classify it as a red-HB star. It is a Galactic halo star (see next section). The luminosity of this object [log (L_*/L_⊙) = 1.99] is lower than post-AGB stars in our sample.

• PHL 1580

  This is a high Galactic latitude and high-velocity, hot post-AGB star. McCausland et al. (1992) derived T_eff = 24000 K, log g = 3.6, and [Fe/H] = −0.6. They find it to be carbon-deficient. This star may have left the AGB before the third dredge-up. The luminosity of this object [log (L_*/L_⊙) = 1.12] is clearly lower than post-AGB stars.

• LS III +52 5 This is a high-velocity (−232.8 km s⁻¹) and high proper motion star. In the Luminous Stars catalogue its spectral type is given as OB- (Hardorp et al. 1964). It is not an IRAS source. Detailed spectroscopic study of this star is important. The luminosity of this object [log (L_*/L_⊙) = 1.22] is clearly lower than post-AGB stars.

3.4 Others with uncertain luminosity and large RUWE

• IRAS 02143+5852

  This is a high radial velocity F7Iae star. The Hα line is in emission. T_eff is estimated from its spectral type to be 6000 K. Fujii, Nakada, and Parthasarathy (2002) undertook BVRIJHK photometry. Omont et al. (1993) classified it as a carbon-rich post-AGB star. The error in parallax is large (tables 1 and 2).

• IRAS 05089+0459

  This is a high galactic latitude and high-velocity M3I post-AGB candidate. Iyengar and Parthasarathy (1997) made near-IR photometric observations (R = 12.68, I = 11.62). There is no chemical composition analysis of this star. The error in Gaia EDR3 parallax is high (table 2).

• IRAS 05208−2035 (BD −20 1073, AY Lep)

  This is a high galactic latitude and high-velocity post-AGB candidate. Gielen et al. (2011) derived T_eff = 4000 K, log g = 0.5, and [Fe/H] = 0.0. The observed B − V colour indicates that it may be a G-type star. The spectral type is not available in SIMBAD. On the other hand, [Fe/H] = −0.7 and a small overabundance of s-process elements are derived by Rao, Giridhar, and Lambert (2012). Oomen et al. (2018) derive the orbital period of this binary system to be 23 d. Although the luminosity of this star is still uncertain, it is likely to be a binary post-AGB star.

• IRAS 15210−6554

  From Gaia DR2 data, we find this to be a high-velocity star. Its spectral type is K2I and Galactic latitude b is |$-{7{_{.}^{\circ}}7}$|⁠. Based on the IRAS colours and flux distribution, it is classified as a post-AGB star. This star does not have an accurate Gaia DR2 parallax.

• IRAS 18075−0924

  This is a high-velocity star. The Gaia DR2 parallax is not accurate. Spectroscopic and photometric study of this star is needed. Based on IRAS colours and flux distribution, it is classified as a post-AGB candidate.

4 Discussion and concluding remarks

4.1 Populations of post-AGB stars with high radial velocities

Nine objects in our sample, HD 46703, HD 56126, IRAS 07140−2321, IRAS 08187−1905, HD 116745, IRAS 14325−6428, HD 137569, BD +33 2642, and IRAS 17436+5003, are identified as post-AGB stars with high radial velocities (tables 1 and 2). The very luminous object BD −12 4970 is discussed separately. Their computed absolute luminosities and comparisons with post-AGB evolutionary track (figure 2) indicates that their initial main-sequence mass is less than two solar masses. Among them, only two stars, HD 116745 and BD +33 2642, clearly belong to the galactic halo population (table 3, figure 1). IRAS 07140−2321 has the largest L_z and E (table 3). The other six post-AGB stars are not separated from disk stars in figures 1 and 2, although they have relatively high radial velocity. This indicates that the criterion of the radial velocity (|V_Helio| > 45 km s⁻¹) is not sufficient to select halo post-AGB stars effectively. The radial velocities of the clear examples of halo objects identified by this work, HD 116745 and BD +33 2642, are V_Helio = 240 and −94 km s⁻¹, respectively. It should be noted that the above criterion is adopted in this work as we do not miss halo objects from the sample of Vickers et al. (2015).

IRAS 07140−2321 is a unique object that has high total energy of orbital motion and high z-component of angular momentum. The star seems to belong to the disk population rather than the halo from the prograde rotation with small v_R and v_z. The distance and the high total energy suggests that it is an outer disk object.

BD −12 4970 (LS IV −12 13) is a hot, high-velocity star with accurate parallax. Its computed absolute luminosity indicates that its initial main-sequence mass may be 4.0 solar masses. The kinematics of this object suggest that it belongs to the disk population.

Among the objects studied in Parthasarathy, Matsuno, and Aoki (2020), three objects (LS 3593, LSE 148, and HD 214539) have clear kinematic features of halo objects (figure 1), whereas those of three other stars are not distinguished from disk stars. Another object, LS 5107, has high total energy of the orbital motion and high z-component of angular momentum, as found for IRAS 07140−2321 in the current sample. As LSE 148 is a less-luminous object, the clear halo post-AGB stars identified by the study are LS 3593 and HD 214539.

4.2 Comments for other objects

The two less-luminous stars HD 178443 and PHL 1580 also belong to the halo population (figure 1). The high-velocity, hot metal-poor star PHL 1580 with accurate parallax is found to have very low luminosity (table 2) compared to post-AGB stars. It may be a hot sub-dwarf star. LSE 182 (HD 178443) is a high-velocity metal-poor star in the Galactic halo and could be a red HB star (McWilliam et al. 1995).

BD +32 2754 also has low absolute luminosity. It may be a sub-dwarf. IRAS 07227−1320 with the spectral type M1 may be a cool post-AGB star. Further study is needed to understand its chemical composition and evolutionary stage. The computed absolute luminosity of post-AGB star HD 161796 (table 2) and its location in figure 2 indicates that its initial main-sequence mass may be in the range around two solar masses.

Kamath, Wood, and Van Winckel (2015) found dusty post-red giant branch (post-RGB) stars in the Large Magellanic Cloud and the Small Magellanic Cloud. They found that these stars have mid-IR excesses and stellar parameters (T_eff, log g, [Fe/H]) similar to those of post-AGB stars, but their luminosities are less than 2500 L_⊙. The stars’ lower luminosities indicate they have lower masses and radii. Some of the stars in our sample also have luminosities less than 2500 L_⊙ (table 2, figure 2) and they may be post-RGB stars similar to those found by Kamath, Wood, and Van Winckel (2015). The very low luminosity stars like PHL 1580 mentioned above are a puzzle. They may be post-HB stars or evolving towards AGB-manque star stage.

Recently, Bond (2020) found BD +14 3061 to be a luminous, metal-poor, yellow post-AGB supergiant star in the Galactic halo. He found it to be a very high-velocity star moving in a retrograde Galactic orbit. It is not an IRAS source. The Galactic halo post-AGB stars have relatively low core mass. They evolve slowly and, by the time they evolve to G- and F-type post-AGB stages, their circumstellar dust shells get dispersed into the interstellar medium. They never become PNe. The Galactic halo post-AGB supergiants are very rare. Discovering them is a challenging task. Bond (2020) derived absolute visual magnitude M_V of this star from Gaia DR2 parallax to be M_V = −3.44. Since its bolometric correction is close to zero (i.e., M_V = M_bol), Bond (2020) proposed that these Galactic halo A and F supergiants are useful as standard candles, as they are luminous and have the same absolute luminosity. Some of the galactic halo post-AGB stars in our sample seem to be similar to BD +14 3061. An extensive survey is needed to detect more Galactic halo post-AGB supergiants.

5 Summary

This paper investigates the list of post-AGB star candidates of Vickers et al. (2015), selecting objects with high radial velocities. We identify two clear examples of high-velocity, low-mass post-AGB stars and a few candidates from the evolutionary status and kinematic information derived from the Gaia DR2 and EDR3. Through the studies of this paper and of a previous one (Parthasarathy et al. 2020), four clear halo post-AGB stars are identified (HD 116745, BD +33 2642, LS 3525, and HD 214539). We also find that the list of Vickers et al. (2015) include objects which are not classified into post-AGB stars, taking a new estimate of luminosity based on parallax measurements made with Gaia. Further studies of the sample of Vickers et al. (2015) with spectroscopy to determine radial velocities will be useful to obtain statistics of post-AGB stars as well as information on individual objects.

Acknowledgements

MP was supported by the NAOJ Visiting Fellow Program of the Research Coordination Committee, National Astronomical Observatory of Japan (NAOJ), National Institutes of Natural Sciences (NINS).

References