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A&A
Volume 695, March 2025
Article Number A227
Number of page(s) 18
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/202453359
Published online 21 March 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Wolf & Rayet (1867) reported the discovery of three stars exhibiting strong emission lines of unknown origin: HD 191765, HD 192103, and HD 192641. These stars became prototypes for the Wolf-Rayet (WR) class, and their optical appearance served as a reference for subsequent discoveries (van der Hucht 1996). It is now established that Population I WR stars represent a brief, terminal phase in the evolution of the most massive O-type stars (with initial masses >25 M⊙). They are progenitors of core-collapse supernovae (Crowther 2007) and black holes and play an important role in black hole-black hole mergers (e.g., Belczynski et al. 2010) due to the fact that a large fraction of WR stars (>40 − 50%; van der Hucht 2001; Shenar 2024) reside in binary systems. Furthermore, WR stars significantly affect the interstellar medium through high mass-loss rates and intense radiation, impacting star formation and contributing to interstellar medium chemical enrichment.

At their current evolutionary stage, Population I WR stars are highly evolved, hot (>30 kK), still massive (10–25 M⊙), and luminous (>105 L⊙) stars that have lost their outer hydrogen-rich envelopes (Crowther 2007; Shenar 2024). Their dense, fast stellar winds of ionized heavy elements define the WR phenomenon (van der Hucht 2001); they are characterized by spectra dominated by broad emission lines of highly ionized helium, nitrogen, carbon, and oxygen, along with a hot O-type continuum. WR stars are classified into three types: WN types are dominated by helium and nitrogen lines (Smith et al. 1996), WC types have prominent helium, carbon, and oxygen lines, and the rare WO types, whose spectra are similar to that of the WC type but with stronger oxygen lines (Crowther et al. 1998).

The population of WR stars in the Milky Way is estimated to be around 1200 (Rosslowe & Crowther 2015a) – 6000 (Shara et al. 2009), yet only 676 are cataloged in the online Galactic WR Catalog (GWRC; v1.29; May 2024)1 (Crowther 2015; Rosslowe & Crowther 2015b). This is due to the relatively short lifetimes of WR progenitors (a few million years), which confine them to their birth sites within dusty molecular clouds (Mauerhan et al. 2011). WR stars themselves contribute to dust production (Williams et al. 1987; Usov 1991; Lau et al. 2022), and dust extinction complicates optical detection at large distances despite their high luminosity. Infrared surveys have significantly advanced WR star detection over the past two decades, employing techniques such as near-IR imaging with interference filters and color-based selection from the Two Micron All Sky Survey (2MASS) or the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) catalogs. These methods have been used to discover several hundred new WR stars, with spectroscopic follow-up typically conducted in the near-IR range (e.g., Shara et al. 2009, 2012; Kanarek et al. 2015; Mauerhan et al. 2011; Smith et al. 2012; Rosslowe & Crowther 2018).

The WR phenomenon has also been observed among some central stars of planetary nebulae (CSPNe). These low-mass Population II WR stars are denoted as [WC], [WO], or [WN], with brackets distinguishing them from massive Population I WR stars. Of the ~130 [WR] stars cataloged in Weidmann et al. (2020), most are [WC] stars; [WN] and [WO] types are rare (Grosdidier et al. 2000, 2001; Morgan et al. 2003; Acker & Neiner 2003; Frew & Parker 2010). WR and [WR] stars of the same subclass exhibit similar spectra, and additional information, such as distance and absolute magnitude, is necessary to discriminate between them (Grosdidier et al. 2001).

This work focuses on massive WR stars. We first briefly analyze the sample of known Population I WR stars that have counterparts in the Gaia Data Release 3 (DR3) main catalog (Gaia Collaboration 2016, 2023). In t. 2, we provide a statistical overview of their number and classes. Section 3 describes our methodology for identifying strong WR candidates in Gaia DR3. We detail the selection criteria applied to the output of the Extended Stellar Parametrizer for Emission-Line Stars (ESP-ELS) algorithm. We conducted a spectroscopic follow-up of candidates, focusing on the brighter end of the distribution (G ≤ 16 mag). These observations, mainly carried out with small amateur telescopes, are presented in Sect. 4. Results for all 37 bright candidates with G ≤ 16 mag are presented in Sect. 5, and a discussion of their classification as Population I massive WR stars or Population II low-mass [WR] stars is provided in Sect. 6. Finally, Sect. 7 summarizes our findings and highlights some inconsistencies in the positions and identifications of WR stars in online databases.

2 Massive Wolf–Rayet stars in Gaia DR3 data

We cross-matched 676 known Population I WR stars from the GWRC (v1.29; May 2024) with the Gaia DR3 main catalog using a 1.5-arcsec search radius. This process identified 443 known WR stars with counterparts in Gaia DR3. Most WR stars absent from the main catalog are faint sources (J > 14 mag) that were primarily discovered through the IR detection methods previously discussed.

Approximately 60% of all known WR stars are WN types, while 40% are WC types, with the Gaia-detected sample reflecting a similar distribution. Of the four known WO stars, WR30a, WR93b, WR102, and WR142, all have counterparts in Gaia. Figure 1 illustrates typical Gaia BP/RP (= Gaia XP) spectra (De Angeli et al. 2023; Montegriffo et al. 2023) for WN, WC, and WO stars, with the main spectral features of each subclass clearly visible despite the low resolution of the data.

The XP spectra were utilized by the ESP-ELS algorithm (Creevey et al. 2023; Fouesneau et al. 2023) to identify and classify 57 511 emission-line stars (ELSs) of various types, including WR stars. The classification model was trained on seven ELS categories: WC, WN, Be stars, Herbig Ae/Be stars, T Tauri stars, active M dwarfs, and planetary nebulae (PNe).

The ESP-ELS algorithm classified 229 of the 443 known Galactic WR stars in the Gaia DR3 main catalog as WR stars, successfully assigning subclasses (WC or WN) to all but one star, WR 120-1. Unfortunately, no sampled nor continuous XP spectrum is available for WR 120-1 (WC9), precluding further investigation into the reasons for this misclassification. The WN and WC subclass distribution in the ESP-ELS classifications closely matches that of the original Galactic WR star sample, demonstrating the comparable effectiveness of the algorithm in identifying both subclasses.

Conversely, the ESP-ELS algorithm failed to classify 214 of the 443 WR stars with counterparts in the Gaia DR3 main catalog as WR stars. As shown in Fig. 2, 73 of these stars have G magnitudes exceeding the processing limit of the ESP-ELS algorithm (G = 17.65 mag), and an additional 54 lack G measurements. However, the algorithm also missed several dozen bright sources (G < 16 mag). Most pre-calibrated XP spectra for these sources, available in the Gaia main catalog, appear featureless, exhibiting unusual continua and lacking prominent carbon or nitrogen lines. As a result, the algorithm could not assign classifications to most of these stars. Only seven of these sources were assigned alternative ELS classifications, such as Herbig-Haro objects, T Tauri stars, or PNe, with nonzero probabilities (10–45%) of being WC or WN stars.

Interestingly, three of the four known WO stars were classified as PNe by the ESP-ELS algorithm, with WR30a being the only WO star unassigned an ELS classification. This result is surprising, as WO stars do not typically exhibit prominent PN spectral features, such as strong emission in the 5000 Åregion caused by the blending of the [O III] λλλ959, 5007 doublet or Ηβ emission. One might have expected these WO stars to be classified as WC stars since the two subclasses share strong emission features in the C IV λ5804 and C III λ4650 regions. However, the wavelength ranges used to train the ESP-ELS algorithm do not include the C IV λ5804 line (see the Gaia DR3 online documentation; Ulla et al. 2022; van Leeuwen et al. 2022). This likely contributed to the misclassification, as the near-UV and near-IR lines characteristic of WO stars may have been confused with similar emissions from PNe.

thumbnail Fig. 1

Typical Gaia XP spectra of WR stars. The main helium, nitrogen, carbon, and oxygen lines are marked.
thumbnail Fig. 2

Distribution of the G magnitudes of known Galactic WR stars. Stars included in the Gaia ESP-ELS analysis are shown in blue, and objects not analyzed by the algorithm (for various reasons; see the main text for details) in yellow. The dashed black line represents the processing limit of the algorithm (G < 17.65 mag).

3 Search for new WR stars

Our search for new WR stars began with the serendipitous discovery of a WR star during a search for new symbiotic systems using Gaia DR3 data. The method developed for identifying symbiotic stars has proven highly effective, leading to the discovery of dozens of new symbiotic systems (Merc et al., in prep.). As a byproduct, one of our symbiotic candidates turned out to be a new WR star, denoted WR-C-14 in this work (see Table B.1). Encouraged by this discovery, we refined our selection criteria (outlined below) to specifically target new WR stars.

3.1 Selection criteria

To identify the most promising candidates for spectroscopic follow-up, we extracted all sources classified as WC or WN stars by the ESP-ELS algorithm from Gaia DR3. This resulted in a sample of 565 WR candidates: 136 classified as WC and 429 as WN. Additionally, two more WN candidates were mentioned in Fouesneau et al. (2023) but were apparently not included in the final Gaia DR3 table.

As an initial step, we excluded 229 known Galactic WR stars previously discussed. Next, we filtered out 188 sources with near-IR colors atypical for WR stars. Figure 3 illustrates the position of all ESP-ELS WR sources in a near-IR color-color diagram constructed using 2MASS observations (Skrutskie et al. 2006), alongside the typical WR near-IR region (Mauerhan et al. 2011). These non-WR sources cluster in specific regions of the diagram, are discussed further in Sect. 3.3. The diagram also includes known WR stars not identified by the ESP-ELS algorithm and [WR] CSPNe from Weidmann et al. (2020). Known WR stars undetected by the algorithm exhibit higher average J - H and H - K values, likely due to greater dust extinction. Conversely, the [WR] CSPNe occupy distinct regions, mostly outside the typical WR near-IR domain.

To ensure sample quality, we also incorporated Gaia Ηα pseudo-equivalent width (pEW Ηα) measurements, which are effective for identifying true Ηα emitters. For WR stars, the emission detected by Gaia in most cases arises not from hydrogen but from broad helium or carbon emission lines coinciding with the Ηα region. Leveraging this overlap, we retained only sources with negative pEW Ηα values (indicative of emission) and a relative error below 30%. This filtering removed 8 sources with positive pEW Ηα and 20 with high relative errors. These criteria are motivated by the distribution of pEW Ηα and its relative error for known WR stars, detected and undetected as WR stars by the ESP-ELS algorithm, as well as [WR] CSPNe, as shown in Fig. 4.

thumbnail Fig. 3

Position of WR stars and candidates in the 2MASS colorcolor diagram. Solid black lines define the near-IR region of WR stars (Mauerhan et al. 2011). Upper panel: sources identified as WR stars by the Gaia ESP-ELS algorithm (filled symbols) divided into several groups based on the analysis presented in this paper (see the main text): previously known Galactic WC (GWC+; light blue triangles) and WN stars (GWN+; dark blue triangles), known WR stars in the Magellanic Clouds (MWR; purple triangles), our selected candidates (Cand; red star symbols), and the remaining objects with known (Ookn; green circles) and unknown nature in the SIMBAD database (Ooun; yellow circles). Known WR and [WR] stars not classified by the algorithm are shown as empty markers: known Galactic WC (GWC-; light blue triangles) and WN (GWN-; dark blue triangles) stars and [WR] CSPNe ([PN]; pink circles). Bottom panel: position of the sources identified as WR stars by the Gaia ESP-ELS algorithm, color-coded by the quality flag. The lower the value of the flag, the higher the probability that the source is a WR star.

3.2 Cross-match with SIMBAD

Finally, we cross-checked the remaining candidates with the Set of Identifications, Measurements, and Bibliography for Astronomical Data (SIMBAD) database (Wenger et al. 2000), removing all objects of confirmed nature. This included 51 WR stars in the Magellanic Clouds (not included in the Galactic WR database used for crossmatching), two cataclysmic variables, one Herbig-Haro object, one high-mass X-ray binary, a star named HD 45166 (identified as a WR star in SIMBAD but absent from the GWRC – given the peculiar nature it is often classified as a quasi-WR and is probably a post-merger of two lower-mass helium stars; see Shenar et al. 2023), and five additional sources discussed in more details below: one WR candidate (IPHAS J195935.55+283830.3), one massive WR star (WR31-1), and three [WR] PNe.

IPHAS J195935.55+283830.3 is a WR candidate with an optical spectrum available in Corradi et al. (2010). Its near-IR colors are consistent with those of WR stars, making it a compelling candidate. The authors also reported a faint, asymmetric, and very extended nebula near the star, with the target positioned close to the bright, bow-shaped eastern edge. This nebula was proposed as a possible WR ring nebula. In 2019, our group obtained a low-resolution spectrum of this bright arc, now available in the HASH PN database2 (Bojicic et al. 2017). The spectrum shows shock-excitation features, including strong [S II] λλ6716-31, moderately strong [N II] λλ6548-83, and [O i] 46300, along with high excitation [O III] λλ4959, 5007 emission stronger than Hβ. These features are consistent with a WR ring nebula formed by stellar winds but could also be attributed to an ancient supernova remnant (SNR).

WR31-1 (THA 35-II-153) was initially missed during our crossmatch with the online GWRC because the database provides incorrect coordinates for this object. The correct position is listed in SIMBAD.

The three [WR] CSPNe excluded from our final list are PMR 1, NGC 6751, and RaMul 2. PMR 1 and NGC 6751 are listed in the CSPNe catalog (Weidmann et al. 2020), while RaMul 2 was recently discovered, spectroscopically confirmed by amateur astronomers (Le Dȗ et al. 2022), and further analyzed by Werner et al. (2024).

The CSPNe catalog by Weidmann et al. (2020) includes approximately 130 [WR] stars. Of these, only PMR 1 and NGC 6751 were classified as WN stars by the ESP-ELS algorithm (RaMul 2 is not part of the catalog). Notably, more than 75% of the [WR] CSPNe in the catalog lack an assigned ELS class, while the remaining 25% are classified as PNe due to contamination from nebular lines. Pre-calibrated XP spectra for 27 [WR] CSPNe are available in the Gaia main catalog.

Figure 5 presents representative XP spectra for two [WR] CSPNe: NGC 40 ([WC8]) and BD+30 3639 ([WC9]). The characteristic features of the [WC] subclass are clearly identifiable in both spectra. However, NGC 40 is classified as a PN by the ESP-ELS algorithm, while BD+30 3639 remains unclassified.

thumbnail Fig. 4

Gaia pEW Hα measurements. Upper panel: position of sources identified as WR stars by the Gaia ESP-ELS algorithm (filled symbols), along with known Galactic WR stars not classified by the algorithm and [WR] CSPNe (empty symbols). The symbols have the same meaning as in Fig. 3. Bottom panel: position of the sources identified as WR stars by the Gaia ESP-ELS algorithm, color-coded by the quality flag.
thumbnail Fig. 5

Gaia XP spectra of the [WR] PNe NGC 40 and BD+30 3639. Both stars show typical [WC] features in their spectra. NGC 40 is classified as a PN by the ESP-ELS algorithm, while BD+30 3639 has no assigned ELS class.

3.3 Sources with non-WR near-IR colors

We examined the 188 sources rejected due to their non-WR near-IR colors in more detail. A crossmatch with the SIM-BAD database revealed 16 known Magellanic WR stars and one additional Galactic WR star, WR111-6. This source was missed during the initial crossmatch with the online GWRC because the database provides incorrect coordinates (taken from the discovery paper of Shara et al. 2012, interestingly, they did not provide a finding chart for this particular object). It should correspond to the coordinates of Gaia DR3 4095702533139975168.

Among the remaining sources, only 9 are known objects in SIMBAD, classified as general stars, young stellar objects, or cataclysmic variables. This leaves 162 objects of unknown nature, primarily clustering in the lower-left region of the near-IR color-color diagram (Fig. 3). These objects are generally faint, with approximately 80% having magnitudes G > 16 mag, and all but five are classified as WN candidates by the ESP-ELS algorithm. Sampled XP spectra of only four of these sources are available. Three of them exhibit strange continuum shapes, but one source, Gaia DR3 4655312994920412416, shows typical spectral features of a WN star. Very close to this source, a star HD 268856 that SIMBAD identifies as Brey 11, a known Magellanic WR star, is located. Our inspection of the XP spectra of these two sources indicates that SIMBAD misidentifies the object, probably because of the incorrect coordinates reported in the Large Magellanic Cloud (LMC) WR catalog from Breysacher et al. (1999). Brey 11 corresponds instead to Gaia DR3 4655312994920412416, not HD 268856 (see their Gaia XP spectra in Fig. 6). The latest LMC WR catalog of Neugent et al. (2018) provides correct coordinates of Brey 11 in accordance with Sanduleak (1970) but incorrectly associates the star with HD 268856.

Additional Gaia XP spectra are available in continuous form. We analyzed, with the use of the GaiaXPy package3, all 137 available spectra for yet unclassified sources with non-WR near-IR colors, including the four that also have sampled XP spectra. Most of these spectra exhibit unusual continua and lack clear signatures of WR stars. Only seven sources, apart from Brey 11, could be identified as probable ELSs, while a few others show uncertain Ha emission. Among these, Gaia DR3 5965760637051514496 displays typical WC features. Although disqualified due to its near-IR colors (located near the WR region boundary), it remains a strong WR candidate and warrants further investigation.

Figure 4 offers further insights into the clump of rejected sources. Most of these sources have very high relative errors in their pEW Ha measurements, suggesting unreliable XP spectra (in line with our analysis of continuous XP spectra) that may have misled the ESP-ELS algorithm. Two sources of unknown nature in SIMBAD, Gaia DR3 5545207781674551424 and Gaia DR3 4206279695997383040, stand out due to their low relative errors (<10%) in the pEW Ha measurements, despite being relatively faint (G ~ 16.5 mag). The first source shows possible Ha emission but lacks other characteristic WR features, while the second does not have an XP spectrum available for analysis.

The bottom panels of Figs. 3 and 4 display the positions of ESP-ELS-detected sources, color-coded by their ELS Class flag. This parameter indicates the likelihood of a source being a WR star, with lower values representing higher probabilities. The clump of SIMBAD-unknown objects with non-WR near-IR colors, small pEW Ha, and high relative error on the measurement are assigned the lowest WR probabilities.

The bar chart in Fig. 7 illustrates the distribution of ELS Class flag categories for the 565 sources identified as WR stars by the ESP-ELS algorithm. The lowest flag categories (0 or 1) predominantly include confirmed WR stars, while the highest categories (3 or 4) contain mostly non-WR objects. However, using the ELS Class flag as a strict selection criterion at this stage would have been overly restrictive. Notably, 14 promising candidates and 8 confirmed WR stars fall into categories 3 or 4. These 22 sources are not exceptionally faint, with G magnitudes ranging from 10 to 17 mag.

thumbnail Fig. 6

Gaia XP spectra of Gaia DR3 4655312994920412416 and HD 268856. The correct counterpart of the Magellanic WR star Brey 11 is Gaia DR3 4655312994920412416.
thumbnail Fig. 7

Number of sources in each ELS quality flag category. All 565 sources classified as WR stars by the Gaia ESP-ELS algorithm are shown. Symbols have the same meaning as in Fig. 3.

3.4 Selected list of candidates

Applying the selection criteria and filtering discussed in the previous section resulted in a list of 59 strong WR candidates, presented in Table B.1. These include 37 relatively bright sources with G ≤ 16 mag and 22 fainter candidates with G > 16 mag. The magnitude division was chosen to ensure decent S/Ns for spectral confirmation using the small-aperture telescopes available to us, guided by our prior experience with spectral confirmation of PNe (Le Dȗ et al. 2022) and observations of symbiotic stars (e.g., Merc et al. 2022, 2023, 2024; Petit et al. 2023). Observations of the fainter candidates will be conducted using larger telescopes and presented in a subsequent paper.

We note that three of our candidates, WR-C-32, WR-C-40, and WR-C-54, were recently confirmed as new WR stars by Marin et al. (2024). These objects remain in our list as they were identified independently by our team, and we obtained observations for two of them before their confirmation was reported in the literature.

Additionally, some of the candidates in our list have prior classifications in the literature. WR-C-01 was identified as an Ha-excess source by Fratta et al. (2021), while WR-C-02 is listed as an ELS (Merrill & Burwell 1950; Stephenson & Sanduleak 1971) and an eclipsing binary with a 13.6-day period (LaCourse et al. 2015). WR-C-34 was previously classified as an asymptotic giant branch star candidate (Robitaille et al. 2008), WR-C-36 is identified as a PN in (Kanarek et al. 2015), and WR-C-37 was listed as a young stellar object candidate by Dunham et al. (2015). Additionally, some targets were also categorized as long-period variables (see Table B.1).

4 Spectroscopic follow-up

We conducted spectroscopic observations of all bright candidates (G ≤ 16 mag) using facilities of the Southern Spectroscopic Project Observatory Team (2SPOT) consortium and through access to the Calern Observatory, affiliated with the Observatoire de la Cote d’Azur (OCA). Observations were carried out from August to December 2024 (see Table B.2).

The 2SPOT consortium4 was established in 2019 by five French amateur astronomers and operates two remotely controlled telescopes in Chile. These include a 0.3 m Ritchey-Chrétien telescope equipped with a medium-resolution Eshel spectrograph (R = 10 000) and a 0.3-m F/4 Newtonian telescope equipped with a low-resolution Alpy600 spectrograph (R = 600). For this study, we utilized the Alpy600 spectrograph with a 23 μm slit. The telescopes are located at Deep Sky Chile (DSC), near Cerro Tololo Observatory.

For northern hemisphere candidates, we used our observatories in Kermerrien and Cornillon, France. These setups are similar to the Chile-based ones but use smaller 0.2 m F/5 Newtonian telescopes. Observations were recorded with ATIK 414 EX cooled cameras with 1392 × 1040 pixel sensors with a pixel size of 6.45 μm. This configuration yields a dispersion of approximately ~3.0 Å/pixel, and covers a spectral range of 3800–7800 Å.

We were also granted access to the C2PU’s 1-m F/6.8 Cassegrain Epsilon telescope at the Calern Observatory. Observations were conducted using a LISA spectrograph with a 50 μm slit (R = 500) and an ATIK 414 EX cooled camera. This configuration yields a dispersion of ~2.5 Å/pixel and a spectral range of 4000–7500 Å.

Observations typically used sub-frames of 1200 s at bin 1×1, with total exposure times ranging from 40 minutes to several hours, depending on the magnitude of the target. Data reduction was performed using the Integrated Spectrographic Innovative Software (ISIS)5. All spectra were corrected for dark frames, bias, and flat fields using standard techniques. The instrumental response was determined using reference spectra of flux standard stars obtained under the same observing conditions as the targets. Wavelength calibration was performed using an argon-neon lamp. No telluric correction was applied to the final spectra.

5 Results and spectral classification

We observed all 37 of our bright candidates (G ≤ 16 mag), confirming the majority as WR stars. Of these, 33 are newly identified WR stars, while one source, WR-C-26, was already known as WR94. However, we uncovered a long-standing misidentification of WR94 in the literature: the GWRC and SIMBAD incorrectly associate WR94 (also referred to as Hen 3-1420; Henize 1976) with HD 158860 (=CD-33 12168). Consequently, several statistical studies included the incorrect counterpart of WR94. Our observations confirm that WR94 is actually CD33 12168B, a star located less than 10 arcseconds from HD 158860 (see the field in Fig. 8), in line with the original report of Henize (1976). The confusion likely stems from Table VII in van der Hucht et al. (1981), where the “B” in the identifier CD-33 12168B was omitted, despite the correct position being shown in their finding chart. We positioned both stars within the slit of our spectrograph and confirmed unequivocally that WR94 corresponds to CD-33 12168B. The spectra of HD 158860 and CD-33 12168B (WR94) are shown in Figs. 9 and A.3 (see WR-C-26).

Our main results are summarized in Table B.2 and discussed in detail in subsequent sections. We spectroscopically confirmed 17 WC stars and 16 WN stars, with subclasses in the WC5-9 and WN3-8 ranges, respectively. The observed spectral classifications align with the WC or WN ELS classes assigned by the ESP-ELS algorithm.

Our diagnostic relied on identifying and analyzing key emission lines within the spectra and on the relative strength of those close enough in wavelength to be compared qualitatively (e.g., C III λ5696 and C iv λ5806 for WC types, or N iii λ4640 and He II λ4686 lines for WN types). We primarily followed the classification scheme presented in Table 2 of van der Hucht (2001), as in a large fraction of the spectra, the fainter lines were too difficult to measure to apply the quantitative schemes of Smith et al. (1990) or Crowther et al. (1998) for WC stars, and Smith et al. (1996) for WN stars.

thumbnail Fig. 8

Field of WR94 from the VPHAS+ survey. Marked are HD 158860 and CD-33 12168B (correct counterpart of WR94).
thumbnail Fig. 9

Our spectrum of HD 158860, incorrectly associated with WR94 in SIMBAD, in the GWRC, and subsequently in several published works.

5.1 Newly confirmed WC stars

The classification of WC stars (Torres & Massey 1987; Crowther et al. 1998; van der Hucht 2001) relies on the relative strength of the two closely spaced C iii λ5696 and C iv λ5806 lines, which are less affected by dust extinction. The C iii λ5696 line tends to be stronger than C iv λ5806 in late subclasses, while C iv λ5806 dominates in early subclasses. The full width at half maximum (FWHM) of these lines increases from late to early WC types. The C II λ4267 line is also useful for distinguishing between late subclasses WC8 and WC9. Unfortunately, this line is strongly affected by dust extinction and is undetectable or very weak in our data.

The spectra plotted in Fig. A.1 exhibit very similar characteristics. The C III λ5696 emission is relatively strong compared to C IV λ5806. The C II λ4267 line is not detected in any spectrum, likely due to dust extinction as the C II λ7231 line appears very strong in the red part of each spectrum. The intensity of C III λ4650 also suffers from high reddening. O V λ5592 is either absent or very weak. For most spectra, the FWHM of the C iii λ5696 line ranges from 17 to 30 Å, except for WR-C-31, which has a larger FWHM of 60 Å. The flat-topped profile of this line in WR-C-31 indicates that this star belongs to the WC8 subclass (Torres & Massey 1987).WR-C-46 and WR-C-16 also potentially belong to the WC8 subclass according to their C IV λ5806 / C III λ5696 ratio. All other stars belong to the late WC9 subclass, according to the classification scheme discussed above. WR-C-37 (WC9) was previously identified as a young stellar object candidate in Dunham et al. (2015), but our spectrum does not support this conclusion.

The spectra of the stars shown in Fig. A.2 are dominated by C iv λ5806 emission, with the C iii λ5696 line stronger or comparable to O V λ5592. These spectral features rule out very late and early subclasses such as WC8, WC9, or WC4. WR-C-05, WR-C-10, and WR-C-58 exhibit very strong C iv λ5806 emission compared to C iii λ5696, and their O V λ5592 line is comparable to or even slightly stronger than C iii λ5696 when corrected for dust extinction. For these reasons, we classified these stars as WC5. The multiple absorption lines in the WR-C-10 spectrum suggest the possible presence of a companion. WR-C-47 likely belongs to the WC6 subclass, as the prominence of C iv λ5806 is less marked than in the previously discussed spectra, and its C III λ5696 emission is more intense than O V λ5592. The other stars, with C III emission much stronger than O V, belong to the WC7 subclass.

5.2 Newly confirmed WN stars

The classification of WN stars is primarily based on the comparison of the nitrogen lines of N IV λ4057, N V λ4604, and N III λ4640 (Hamann et al. 1995; Smith et al. 1996; van der Hucht 2001) in the blue part of the spectrum. Our candidates suffer from high interstellar reddening, which often makes the detection of N V or N iv lines uncertain, thereby complicating the classification. We note that He ii λ5411 and C iv λ5806 become stronger from late WN7 to early WN4 subclasses, while He I λ5875 tends to be very weak or completely absent in subclasses earlier than WN4. The spectra of WN stars for which a subclass can be determined are shown in Fig. A.3.

WR-C-01 likely belongs to the WN4 subclass since our spectrum shows N V λ4604 and no N III λ4640. WR-C-11 shows a He II λ4686 line stronger than N III λ4640 and a relatively intense He II λ5411. WR-C-11 likely belongs to the WN5 subclass. WR-C-42 shares similar characteristics but with a stronger He I λ5875, indicating that this star rather belongs to the WN6 subclass.

WR-C-26 corresponds to WR94 (see Sect. 5), a known WN5o star according to the GWRC (v1.29). However, we classify this star as a potential WN6 since our spectrum shows the presence of a faint N V λ4604, remaining much weaker than the N III λ4640.

WR-C-54 (LS III +44 21) is a recently confirmed WN6+O6.5 binary system (Marin et al. 2024). The spectrum of WR-C-28 is very similar to WR-C-54, showing He II λ4686 stronger than N III λ4640, absence of C IV λ5806 and He I λ5875, and many absorption lines. WR-C-28 may also be a binary system with a WN6 companion.

The spectrum of WR-C-55 is marked by the strong and broad emission line of He II λ4686, a very weak N V λ4604 emission and the absence of N iii λ4640 and He I λ5875. WR-C-55 belongs to the WN3 subclass. The spectrum of WR-C-57 shares similar characteristics but with faint N iii λ4640 and He I λ5875 in emission and an unusually strong C iv λ5806 line. This indicates that WR-C-57 is a transition-type WN/C star (see Conti et al. 1989 for more information on WN/C class).

The remaining stars most likely belong to late WN subclasses. WR-C-29 and WR-C-36 have He ii λ4686 slightly stronger than N iii λ4640 and weak He I λ5875, suggesting they belong to the WN7 subclass. The Ββ line of WR-C-36 is stronger than He II λ5411, indicating the star is hydrogen-rich. Kanarek et al. (2015) previously considered a WN7h classification for this star based on near-IR observations (referred to as 1430-AB0 in their study), but they classified this source as a PN at the end due to the absence of P Cygni absorption lines typically associated with WN7h stars. However, we do not support this conclusion, as WR-C-36 is too luminous to be a PN (although slightly fainter than expected for the WN7h star; see Sects. 6.1 and 6.2, and Tables B.4 and B.5) and also shows no evidence of PN nebular emission lines in its spectrum (Fig. A.3) and PN features in the mid-IR domain (see Sect. 6.4). Furthermore, the object is not included in the HASH PN Database (Bojičić et al. 2017).

The He II 44686 and N III λ4640 lines of WR-C-23 and WR-C-35 have comparable intensities. Their He I λ5875 line is also relatively strong, suggesting that these stars belong to the WN8 subclass.

The spectra shown in Fig. A.4 have a S/N ratio too low to precisely determine the subclasses of the stars, but their spectral features strongly suggest they are WN stars. WR-C-59 is likely a weak-lined WN3 star, with a possible detection of N V λ4604. WR-C-44 and WR-C-50 exhibit weak He II λ4686 emission and no N III λ4640, suggesting these stars are likely in the WN5-6 subclass range. It is also likely the case of WR-C-41, since N III λ4640 appears stronger than N V λ4604. In the spectrum of WR-C-39, N III λ4640 is detected but remains slightly weaker than He ii λ4686, indicating that this star may belong to the WN7-8 subclass range.

5.3 Objects of non-WR nature

The spectra of three objects in our sample, which do not exhibit characteristics of WR stars, are grouped in Fig. A.5. These spectra display narrow H I and He I emission lines (limited to Ha in the case of WR-C-03) against a flat continuum. Notably, WR-C-02 is a previously identified eclipsing binary system, where the emission lines are distinctly marked by a prominent P Cygni profile.

6 Distinguishing between WR and [WR] stars

Here we analyze the absolute magnitudes, vertical z distances from the Galactic mid-plane, and IR properties of the newly confirmed WR stars and candidates to distinguish massive Population I WR stars from Population II low-mass [WR] stars, which can exhibit similar spectroscopic features.

thumbnail Fig. 10

Comparison of b and v magnitudes provided by the GWRC and derived from Gaia XP spectra.

6.1 Absolute Mv magnitudes of newly confirmed WR stars and candidates

The assessment of absolute magnitudes of WR stars depends on parameters like reddening and distance, which are not always precisely known. Without accurate values for these factors, the analysis can suffer from significant uncertainties. Our aim was to estimate the absolute magnitudes of our candidates in the v (Smith 1968) and Ks bands and to compare the results with those of known WR stars.

To calculate absolute magnitudes in the v band, we followed the methodology outlined in Rate & Crowther (2020). This approach utilizes the (b − v) measurement, where b and v are monochromatic magnitudes at 4270 and 5160 A, respectively. These wavelengths were chosen to minimize contamination from emission lines (Smith 1968), which can strongly influence reddening estimates. The (b − v) colors in our analysis were derived from the XP spectra of Gaia using the GaiaXPy library. Notably, (b − v) can also be obtained from our data even when the spectra are not calibrated in absolute fluxes. We compare the results at the end of the section.

To evaluate the reliability of magnitudes derived from Gaia XP spectra, we compared the b and v magnitudes of known WR stars, computed from XP spectra6, with magnitudes provided by the GWRC based on ground-based observations. Figure 10 includes data for approximately 180 known WR stars, for which b and/or v magnitudes were both available in the GWRC and derivable from XP spectra. The comparison shows excellent consistency between the datasets, supporting the use of XP spectra for assessing Mv with reasonable confidence.

The color excess E(b − v) was calculated using typical intrinsic (b − v)0 colors for WR stars, as listed in Table 4 of Rate & Crowther (2020). These intrinsic colors depend on the WR subclass and binarity. For single stars, (b − v)0 ranges from −0.18 to −0.37 mag. For bright sample candidates with observed spectra, we assigned (b − v)0 based on the subclasses determined in Sect. 5, assuming they are single WR stars. For faint sample candidates not yet observed (both WC and WN according to the ELS classification), we adopted a representative average value of (b − v)0 = −0.3 ± 0.1 mag. For the three candidates confirmed as non-WR stars, we arbitrarily adopted the same average value. The extinction Av was computed assuming a standard Rv = 4.12 (see Turner 1982). The resulting Mv was calculated using distances from the Bailer-Jones et al. (2021) catalog (see Table B.3).

Uncertainties in Mv include errors in the b, v fluxes, intrinsic colors (b − v)0 , and distances. Flux uncertainties in b and v were derived from the Gaia XP spectra, while uncertainties in (b − v)0 were taken from Table 4 of Rate & Crowther (2020). For distances, we used the 16th and 84th percentiles of the geometric distance estimates from Bailer-Jones et al. (2021).

The absolute magnitudes Mv of the candidates are summarized in Table B.4. For candidates with unreliable Gaia data or missing XP spectra (both sampled and continuous forms), we did not compute magnitudes. As a result, these 20 targets have no entries in Table B.4. Additionally, no distance measurement is available for WR-C-49, so Mv could not be determined7. Figure 11 compares the absolute magnitudes Mv of known WR stars and our candidates. In the figure, we excluded 9 candidates with low S/N XP spectra, which resulted in Mv uncertainties exceeding 4 mag (see Table B.4).

The mean values M¯v${\bar M_v}$ obtained for known Galactic WR stars was found to be −4.6 ± 1.4 mag, with sample standard deviation as uncertainty. Population II [WR] stars are significantly fainter, given their luminosities are at least an order of magnitude lower than those of Population I WR stars. According to Table 5 of Rate & Crowther (2020), M¯v${\bar M_v}$ for WN stars ranges from −3.6 to −7.0 mag, and for WC stars, it ranges from −3.9 to −4.6 mag. Table 4 of Rate & Crowther (2020) gives a mean value of −4.3 ± 1.8 mag, consistent with our results obtained using Gaia XP spectra.

Most candidates plotted within the range (see Fig. 11) likely belong to Population I WR stars. All candidates with MV > −3 mag and G > 14 mag were spectroscopically observed, and all but one exhibit clear WR spectral features. WR-C-03, with G = 15.69 mag and Mv = −1.6 ± 0.7 mag, significantly deviates from the WR sample (and the uncertainty of its absolute magnitude is low). Our observations confirm that WR-C-03 is not a WR star (see Fig. A.5).

Table B.4 also lists absolute magnitudes calculated using (b − v) colors derived from our spectra. For stars with G < 14 mag, the results agree well with those obtained solely from XP spectra. For fainter stars, the low S/N in the blue part of our spectra precludes this analysis.

thumbnail Fig. 11

Absolute magnitude, Mv, of known WR stars and our candidates (see Fig. 3 for the symbol meanings). The candidates that we confirmed as non-WR are shown as empty red stars. Magnitudes are derived from the Gaia XP spectra. The dashed black lines denote the lower and upper limits of the mean value we derived for known Galactic WR stars (i.e., the sample made of GWC+, GWN+, GWC-, and GWN-).

6.2 Absolute MKs magnitudes

In addition to Mv, we also evaluated MKs${M_{{K_s}}}$, following the approach described by Rosslowe & Crowther (2015b) and Rate & Crowther (2020). The apparent near-IR magnitudes are taken from the 2MASS catalog. From these data and intrinsic colors of WR stars of various subtypes (see Table 4 of Rosslowe & Crowther 2015b), we calculated color excesses E(J − Ks) and E(H − Ks). If the subtype of a particular candidate is known from our observations (Sect. 5), we adopted the intrinsic color typical for that subtype. For the faint, yet unobserved candidates (and for three non-WR stars), we used the average color of the WC or WN stars assigned based on the ELS class of the candidate. If that was the case, it is indicated in Table B.5.

From the color excesses, we calculated two values of Ks band extinction using Eqs. (1) and (2) of Rosslowe & Crowther (2015b), in the same way as presented in the aforementioned paper. Finally, we calculated MKs magnitudes by adopting distances from Table B.3. The two extinction values were averaged for the calculation. The uncertainties in the absolute magnitudes include the uncertainties in the input 2MASS magnitudes, in the extinction laws, in the distances, and in the average extinction value adopted.

We were able to calculate MKs${M_{{K_s}}}$ for all candidates except WR-C-49, which does not have any distance estimate available. The magnitudes for all new WR stars classified in this work (Table B.5) are consistent with those of known WR stars, as shown in Table 5 and Figs. 1 and 2 of Rosslowe & Crowther (2015b). Magnitudes of WR-C-18, WR-C-24, WR-C-32, and WR-C-43, in the range of −7.8 to −8.8 mag, indicate that these might belong to WC9d types (with additional circumstellar dust).

From the faint sample, 9 targets classified as WC stars by Gaia ELS show magnitudes within the typical range of WC stars. The magnitude of WR-C-30 suggests the presence of circumstellar dust. From 13 objects with WN ELS class, several fall outside the typical range of ~(−2.5 to −7.0) mag (Rosslowe & Crowther 2015b), including WR-C-4, WR-C-9, WR-C-12, WR-C-17, and WR-C-53. None of these targets have Mv magnitudes listed in Table B.4. The XP spectra for these objects are not sufficiently conclusive to rule out the potential WR nature of these candidates. Consequently, we defer further discussion of the faint sample to a subsequent paper, where we will present spectroscopic follow-up observations of these sources.

6.3 Vertical distances from the Galactic disk

Young massive stars are typically confined to the Galactic thin disk (see, e.g., Fig. 6 in Rosslowe & Crowther 2015b), whereas significantly older CSPNe are often found at larger vertical distances from the disk. We therefore calculated the z distances for all candidates using the astropy package and adopting distances from Bailer-Jones et al. (2021), except for WR-C-49, for which no distance is available. The results are summarized in Table B.3.

Among the newly confirmed WR stars, the majority lie within 200 pc of z = 0, though several are located at larger distances: WR-C-05 at −270−81+70$ - 270_{ - 81}^{ + 70}$ pc, WR-C-11 at −338−69+51$ - 338_{ - 69}^{ + 51}$ pc, WR-C-16 at −201−64+46$ - 201_{ - 64}^{ + 46}$ pc, WR-C-37 at 402−79+97$402_{ - 79}^{ + 97}$ pc, WR-C-41 at −290−84+57$ - 290_{ - 84}^{ + 57}$ pc, WR-C-57 at 281−40+55$281_{ - 40}^{ + 55}$ pc, and WR-C-59 at 533−83+103$533_{ - 83}^{ + 103}$ pc. WR-C-59, in particular, might belong to the WR stars with the largest z distances. However, caution is necessary when interpreting these results, as most of these stars have low ϖ/σϖ. ≈ 1–4 (S/N of the parallax), making their distance estimates potentially unreliable. Notably, the parallax of WR-C-11 in Gaia DR3 is negative.

Several faint, unobserved candidates also have |z| > 200 pc. Noteworthy is WR-C-17, with z=881−187+228$z = 881_{ - 187}^{ + 228}$ pc. This object was noted in the previous section as having an MKS value fainter than is typical for WR stars.

thumbnail Fig. 12

Typical morphology of WR and [WR] nebulosities. The images were constructed from WISE observations. The red, green, and blue channels correspond to W4 (24 μm), W2 (4.6 μm), and W1 (3.4 μm) filters, respectively. Left panel: RaMul2 [WR] PN, whose colors and morphology are typical of a young and compact PN. The field of view is 6′ × 5′. Right panel: WR102 ring nebula, showing a complex morphology made of several bubbles. The field of view is 10′ × 10′.

6.4 Mid-IR properties

The properties of [WR] PNe in the mid-IR provide useful observational information that can further support the classification of Population I or II WR stars. PNe are characterized by intense emission in the mid-IR region (Frew & Parker 2010), generated by the dust ejected by the PN progenitor and heated by its remnant core. This emission is particularly strong for compact and young PNe, and it is easily observable as a compact, perfectly circular-shaped emission in the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) W4 band (24 μm; see Fig. 12). This is a key observational feature often employed in the search for new PNe (e.g., Le Dȗ et al. 2022).

We checked each PN associated with a [WR] central star in the catalog of Weidmann et al. (2020). All of them show the aforementioned typical mid-IR emission. This is not surprising, as the evolutionary scheme of a [WC] central star is assumed to go from late [WC], through early [WC] and PG1159 stars, to a white dwarf (Grosdidier et al. 2000). Given that [WR] PNe are at the beginning of their PN phase, they are compact and dense.

Some Population I WR stars are associated with a ring nebula made of ejected material. Only a few massive WR stars exhibit this feature. WR ring nebulae also emit in mid-IR but produce various kinds of bubbles, arcs, clumps, or amorphous shapes. Three broad morphological types are defined in Toalá et al. (2015). Their mid-IR observational properties are very different from those of compact PNe (see Fig. 12), but they can be confused with evolved or well-resolved PNe.

The WISE colors of our candidates do not correspond to those of compact PNe, and none of them show prominent emission in the WISE W4 band. This supports the conclusion that our candidates are not [WR] CSPNe and most likely belong to massive Population I WR stars.

Two of our candidates, WR-C-27 and WR-C-35, show mid-IR emission in their vicinity (see Figs. 13 and 14). WR-C-27 exhibits an arc-shaped emission in the WISE W4 band, but there is no obvious optical counterpart in the SuperCOSMOS Ha Survey (SHS; Parker et al. 2005) or the VST Photometric Ha Survey of the Southern Galactic Plane and Bulge (VPHAS+; Drew et al. 2014). The case of WR-C-35 is more interesting: the mid-IR emission has a bright optical counterpart clearly visible in the SHS survey. The nebula is filamentary in shape, with an approximate apparent size of 16x13 arcmin (see Fig. 14). Until recently, this nebula was identified as a SNR named G11.1-1.0, but it was removed from the 2024 version of the SNR catalog (Green 2024) after several authors instead identified this object as an HII region (Gao et al. 2019; Dokara et al. 2021). WR-C-35 is located near the center of the nebula, which may actually be a new WR ring nebula. Further analysis that is outside the scope of this paper is required to confirm whether this nebula is indeed associated with the WR star.

thumbnail Fig. 13

Arc-shaped mid-IR emission northwest of WR-C-27. Left panel: WISE color image constructed in the same way as the images in Fig. 12. Right panel: Herschel Photodetector Array Camera and Spectrometer (PACS) color image. The red, green, and blue channels correspond to observations at 160 μm, 100 μm, and 70 μm, respectively.
thumbnail Fig. 14

Possible WR ring nebula of WR-C-35. Upper row: WISE color image (left) and Herschel PACS color image (right). Bottom row: starless Ha image processed from SHS (left) and the VPHAS+ image from u, g, r, i, and Ha bands (right).

7 Conclusion

By applying the ESP-ELS algorithm to Gaia DR3 data, specifically low-resolution XP spectra, 565 sources were identified as potential WR candidates. Approximately half of these were already known Galactic or Magellanic WR stars. According to our investigation, about 40% of the sample are likely contaminants, with faint magnitudes and unreliable pEW Ha measurements. Our selection criteria, primarily based on 2MASS colors and pEW Ha measurements, produced a refined list of 59 strong candidates (~10% of the original sample).

In this study, we focused our spectroscopic observations on the 37 candidates brighter than G = 16 mag. Spectra were acquired using small telescopes in Chile and France, as well as the 1 m C2PU’s Epsilon telescope at the Calern Observatory. We spectroscopically confirmed 33 new Galactic WR stars, of which 16 are WN-type stars and 17 are WC-type stars. This total includes three WR stars independently discovered by Marin et al. (2024). Of the observed candidates, only three were found not to be WR stars.

Additionally, we report the detection of a possible new WR ring nebula surrounding WR-C-35, which we confirmed to be a WN8 star. This WR star lies near the center of a complex filamentary optical nebula, typical of material shaped by fast, hot stellar winds. Previously identified as an SNR under the designation G11.1-1.0, the nebula was recently reclassified as an HII region by Gao et al. (2019) and Dokara et al. (2021).

We also identified discrepancies in the coordinates of some objects in both the online GWRC and the SIMBAD database. The correct coordinates for WR31-1 (THA 35-II-153) are given by SIMBAD, while the GWRC lists incorrect values. WR111-6 has inaccurate coordinates in both sources; they should correspond to Gaia DR3 4095702533139975168. The GWRC incorrectly identifies HD 158860 as WR94 instead of CD33 1268B. SIMBAD misidentifies the Magellanic WR star Brey 11 as HD 268856; the correct identification is Gaia DR3 4655312994920412416.

Recent discoveries, including those by Marin et al. (2024) and our own work, demonstrate that undiscovered Galactic WR stars remain to be found. The Gaia low-resolution XP spectra provide a valuable option for this effort. Our starting sample was limited to the ESP-ELS algorithm outputs, which only include a subset of candidate Ha emitters. Future work will aim to expand the analysis to a broader sample, applying the insights gained from this study. In a similar way, we are currently conducting a search for new symbiotic systems (Merc et al., in prep.).

In a subsequent paper of this series, we will present follow-up results for our fainter WR candidates observed with larger-diameter telescopes. Additionally, we will analyze the photometric data available for our candidates to investigate variability and potential binarity.

Data availability

The optical spectra of the observed candidates are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/695/A227

Acknowledgements

We are thankful to an anonymous referee for carefully reading the manuscript and for the comments and suggestions that improved it. We warmly thank Jean-Pierre Rivet from the Calern Observatory for granting us access to C2PU’s Epsilon telescope. We also thank Marcel Drechsler for processing the starless version image of the possible WR-C-35 ring nebula from SHS. The research of J.M. was supported by the Czech Science Foundation (GACR) project no. 24-106080 and by the Spanish Ministry of Science and Innovation with grant no. PID2023-146453NB-100 (PLAtoSOnG). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has also made use of the Python package GaiaXPy, developed and maintained by members of the Gaia Data Processing and Analysis Consortium (DPAC), and in particular, Coordination Unit 5 (CU5), and the Data Processing Centre located at the Institute of Astronomy, Cambridge, UK (DPCI). This publication has made use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has used SIMBAD database (Wenger et al. 2000), Vizier catalog access tool (Ochsenbein et al. 2000), Aladin Sky Atlas (Bonnarel et al. 2000), and the cross-match service (Boch et al. 2012) provided by CDS, Strasbourg Observatory, France. This research has made use of the SVO Filter Profile Service “Carlos Rodrigo”, funded by MCIN/AEI/10.13039/501100011033/ through grant PID2020-112949GB-I00.

Appendix A Optical spectra of our candidates

thumbnail Fig. A.1

Spectra of newly discovered WC8 and WC9 stars. Spectra are dominated by C iii emission. WR-C-31, WR-C-16, and WR-C-46 are the only WC8 stars in the sample; the other stars belong to the WC9 subclass. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
thumbnail Fig. A.2

Spectra of newly discovered WC5, WC6, and WC7 stars. Spectra are dominated by C IV emission. According to the relative intensity of their O V and C iii lines, WR-C-05, WR-C-10, and WR-C-58 belong to the WC5 subclass, WR-C-47 to WC6, and all other stars to WC7. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
thumbnail Fig. A.3

Spectra of newly discovered WN stars. WR-C-57 is classified as WN/C for its unusually strong C IV λ5804 line, WR-C-55 belongs to WN3 subclass, WR-C-01 to WN4, WR-C-11 to WN5, WR-C-26 (already known as WR94; see the main text), WR-C-28, WR-C-42, WR-C-54 are classified WN6-type WR stars, WR-C-29, WR-C-36 belong to WN7 subclass and finally WR-C-23 is a WN8-type WR star. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
thumbnail Fig. A.4

Possible WN stars for which we have only low-S/N spectra available. WR-C-59 likely belongs to the WN3 subclass, WR-C-44, WR-C-41, and WR-C-50 to the range of subclasses WN5-6, and WR-C-39 to the range of subclasses WN7-8. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
thumbnail Fig. A.5

Observed candidates with non-WR spectra.The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.

Appendix B Tables

Table B.1

Selected WR star candidates.

Table B.2

Spectroscopic observations of WR stars candidates, together with their classification.

Table B.3

Gaia DR3 (Gaia Collaboration 2023) parallax (ϖ), goodness-of-fit (GOF), and RUWE of target stars. The geometric distances from Bailer-Jones et al. (2021) are listed and adopted for the calculations in this work. The calculated vertical distances from the Galactic disk are also included in the Table. The last column contains the subclass of each target.

Table B.4

Absolute Mv magnitudes of selected WR candidates. The distances from Bailer-Jones et al. (2021) were used in all the calculations (see Table B.3).

Table B.5

Absolute MKs${{\rm{M}}_{{K_s}}}$ magnitudes of selected WR candidates. The distances from Bailer-Jones et al. (2021) were used in all the calculations (Table B.3). See more details in the text.

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6

The WR filters were approximated using filters with similar properties from the Spanish Virtual Observatory filter profile service.

7

The galactic latitude of the source (b = -27.5°; see Table B.1) suggests that this star is a rather poor WR candidate.

All Tables

Table B.1

Selected WR star candidates.

In the text
Table B.2

Spectroscopic observations of WR stars candidates, together with their classification.

In the text
Table B.3

Gaia DR3 (Gaia Collaboration 2023) parallax (ϖ), goodness-of-fit (GOF), and RUWE of target stars. The geometric distances from Bailer-Jones et al. (2021) are listed and adopted for the calculations in this work. The calculated vertical distances from the Galactic disk are also included in the Table. The last column contains the subclass of each target.

In the text
Table B.4

Absolute Mv magnitudes of selected WR candidates. The distances from Bailer-Jones et al. (2021) were used in all the calculations (see Table B.3).

In the text
Table B.5

Absolute MKs${{\rm{M}}_{{K_s}}}$ magnitudes of selected WR candidates. The distances from Bailer-Jones et al. (2021) were used in all the calculations (Table B.3). See more details in the text.

In the text

All Figures

thumbnail Fig. 1

Typical Gaia XP spectra of WR stars. The main helium, nitrogen, carbon, and oxygen lines are marked.
In the text
thumbnail Fig. 2

Distribution of the G magnitudes of known Galactic WR stars. Stars included in the Gaia ESP-ELS analysis are shown in blue, and objects not analyzed by the algorithm (for various reasons; see the main text for details) in yellow. The dashed black line represents the processing limit of the algorithm (G < 17.65 mag).
In the text
thumbnail Fig. 3

Position of WR stars and candidates in the 2MASS colorcolor diagram. Solid black lines define the near-IR region of WR stars (Mauerhan et al. 2011). Upper panel: sources identified as WR stars by the Gaia ESP-ELS algorithm (filled symbols) divided into several groups based on the analysis presented in this paper (see the main text): previously known Galactic WC (GWC+; light blue triangles) and WN stars (GWN+; dark blue triangles), known WR stars in the Magellanic Clouds (MWR; purple triangles), our selected candidates (Cand; red star symbols), and the remaining objects with known (Ookn; green circles) and unknown nature in the SIMBAD database (Ooun; yellow circles). Known WR and [WR] stars not classified by the algorithm are shown as empty markers: known Galactic WC (GWC-; light blue triangles) and WN (GWN-; dark blue triangles) stars and [WR] CSPNe ([PN]; pink circles). Bottom panel: position of the sources identified as WR stars by the Gaia ESP-ELS algorithm, color-coded by the quality flag. The lower the value of the flag, the higher the probability that the source is a WR star.
In the text
thumbnail Fig. 4

Gaia pEW Hα measurements. Upper panel: position of sources identified as WR stars by the Gaia ESP-ELS algorithm (filled symbols), along with known Galactic WR stars not classified by the algorithm and [WR] CSPNe (empty symbols). The symbols have the same meaning as in Fig. 3. Bottom panel: position of the sources identified as WR stars by the Gaia ESP-ELS algorithm, color-coded by the quality flag.
In the text
thumbnail Fig. 5

Gaia XP spectra of the [WR] PNe NGC 40 and BD+30 3639. Both stars show typical [WC] features in their spectra. NGC 40 is classified as a PN by the ESP-ELS algorithm, while BD+30 3639 has no assigned ELS class.
In the text
thumbnail Fig. 6

Gaia XP spectra of Gaia DR3 4655312994920412416 and HD 268856. The correct counterpart of the Magellanic WR star Brey 11 is Gaia DR3 4655312994920412416.
In the text
thumbnail Fig. 7

Number of sources in each ELS quality flag category. All 565 sources classified as WR stars by the Gaia ESP-ELS algorithm are shown. Symbols have the same meaning as in Fig. 3.
In the text
thumbnail Fig. 8

Field of WR94 from the VPHAS+ survey. Marked are HD 158860 and CD-33 12168B (correct counterpart of WR94).
In the text
thumbnail Fig. 9

Our spectrum of HD 158860, incorrectly associated with WR94 in SIMBAD, in the GWRC, and subsequently in several published works.
In the text
thumbnail Fig. 10

Comparison of b and v magnitudes provided by the GWRC and derived from Gaia XP spectra.
In the text
thumbnail Fig. 11

Absolute magnitude, Mv, of known WR stars and our candidates (see Fig. 3 for the symbol meanings). The candidates that we confirmed as non-WR are shown as empty red stars. Magnitudes are derived from the Gaia XP spectra. The dashed black lines denote the lower and upper limits of the mean value we derived for known Galactic WR stars (i.e., the sample made of GWC+, GWN+, GWC-, and GWN-).
In the text
thumbnail Fig. 12

Typical morphology of WR and [WR] nebulosities. The images were constructed from WISE observations. The red, green, and blue channels correspond to W4 (24 μm), W2 (4.6 μm), and W1 (3.4 μm) filters, respectively. Left panel: RaMul2 [WR] PN, whose colors and morphology are typical of a young and compact PN. The field of view is 6′ × 5′. Right panel: WR102 ring nebula, showing a complex morphology made of several bubbles. The field of view is 10′ × 10′.
In the text
thumbnail Fig. 13

Arc-shaped mid-IR emission northwest of WR-C-27. Left panel: WISE color image constructed in the same way as the images in Fig. 12. Right panel: Herschel Photodetector Array Camera and Spectrometer (PACS) color image. The red, green, and blue channels correspond to observations at 160 μm, 100 μm, and 70 μm, respectively.
In the text
thumbnail Fig. 14

Possible WR ring nebula of WR-C-35. Upper row: WISE color image (left) and Herschel PACS color image (right). Bottom row: starless Ha image processed from SHS (left) and the VPHAS+ image from u, g, r, i, and Ha bands (right).
In the text
thumbnail Fig. A.1

Spectra of newly discovered WC8 and WC9 stars. Spectra are dominated by C iii emission. WR-C-31, WR-C-16, and WR-C-46 are the only WC8 stars in the sample; the other stars belong to the WC9 subclass. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
In the text
thumbnail Fig. A.2

Spectra of newly discovered WC5, WC6, and WC7 stars. Spectra are dominated by C IV emission. According to the relative intensity of their O V and C iii lines, WR-C-05, WR-C-10, and WR-C-58 belong to the WC5 subclass, WR-C-47 to WC6, and all other stars to WC7. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
In the text
thumbnail Fig. A.3

Spectra of newly discovered WN stars. WR-C-57 is classified as WN/C for its unusually strong C IV λ5804 line, WR-C-55 belongs to WN3 subclass, WR-C-01 to WN4, WR-C-11 to WN5, WR-C-26 (already known as WR94; see the main text), WR-C-28, WR-C-42, WR-C-54 are classified WN6-type WR stars, WR-C-29, WR-C-36 belong to WN7 subclass and finally WR-C-23 is a WN8-type WR star. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
In the text
thumbnail Fig. A.4

Possible WN stars for which we have only low-S/N spectra available. WR-C-59 likely belongs to the WN3 subclass, WR-C-44, WR-C-41, and WR-C-50 to the range of subclasses WN5-6, and WR-C-39 to the range of subclasses WN7-8. The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
In the text
thumbnail Fig. A.5

Observed candidates with non-WR spectra.The dashed black lines indicate the zero intensity scale of each spectrum. The position of the telluric absorption bands is marked with the symbol ⊕.
In the text

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