Hot Subdwarf Stars Identified in Gaia DR2 with Spectra of LAMOST DR6 and DR7. I. Single-lined Spectra

Hot subdwarf stars (i.e., sdO/B) are evolved low-mass stars around 0.5 ${M}_{\odot }$. They show similar spectra to main-sequence (MS) stars of O/B type, but at lower luminosity and with broader spectral features. These hot stars occupy the extreme blue region of horizontal branch (HB) in the Hertzsprung–Russell (HR) diagram and burn He in their cores, therefore, they are also known as extreme horizontal branch (EHB) stars (Heber 1986).

The formation and evolution of hot subdwarf stars are still not clear. To end up on the EHB, the progenitors of these hot stars have to lose nearly the whole envelope mass by the end of their red giant branch stages. Therefore, binary evolution is thought to be the main formation mechanism for hot subdwarf stars (Han et al. 2002, 2003). On the observational side, about half of the hot subdwarfs are found in close binaries (Maxted et al. 2001; Napiwotzki et al. 2004; Copperwheat et al. 2011), and their companions could be brown dwarfs, MS stars, white dwarfs (WDs), and even neutron stars or black holes (Kawka et al. 2015; Kupfer et al. 2015). Studies on hot subdwarf stars therefore can shed light on the details of binary evolution processes, such as Roche lobe overflow, common envelope ejection, and the merger of two He WDs (Han et al. 2002, 2003; Zhang & Jeffery 2012; Chen et al. 2013; Zhang et al. 2017; Vos et al. 2019). Moreover, close hot subdwarf binaries with compact companions (e.g., WDs, neutron stars, or black holes) are potential verification sources (Kupfer et al. 2018) for the future space-based gravitational wave detectors, such as LISA (Amaro-Seoane et al. 2017) and TianQin (Luo et al. 2016a). Close hot subdwarf + massive WD binaries are possible progenitor systems for SNe Ia, in which the hot subdwarf companion may survive the explosion as a hypervelocity remnant (Wang et al. 2009; Vennes et al. 2017; Li et al. 2018; Raddi et al. 2019).

Pulsating sdO/B stars allow the accurate determination of mass and internal structure by using asteroseismic methods, which provide excellent tests for the formation and evolution models (Kawaler et al. 2010; Charpinet et al. 2011; Baran et al. 2012; Battich et al. 2018; Zong et al. 2018). The diversity of atmospheres in hot subdwarf stars makes them good samples to study atomic diffusion processes (Naslim et al. 2013, 2020; Moehler et al. 2014; Jeffery et al. 2017; Németh 2017; Byrne et al. 2018). In addition, hot subdwarf stars in globular clusters provide useful information to understand the formation and evolution of the oldest populations in our galaxy (Latour et al. 2014, 2018; Lei et al. 2015, 2016). For recent reviews on these special stars see Heber (2009, 2016).

Since Kilkenny et al. (1988) published the first catalog of 1225 spectroscopically identified hot subdwarf stars, the number of these special blue objects exploded with the data release of many spectroscopic surveys, such as the Hamburg Quasar Survey (HS; Hagen et al. 1995), the Hamburg ESO survey (HE; Wisotzki et al. 1996), the Edinburgh-Cape Survey (EC; Kilkenny et al. 1997), the Byurakan surveys (FBS, SBS; Mickaelian et al. 2007; Mickaelian 2008), the Galaxy Evolution Explorer all sky survey (Vennes et al. 2011; Németh et al. 2012), the Sloan Digital Sky Survey (Geier et al. 2015; Kepler et al. 2015, 2016) and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope survey (LAMOST; Luo et al. 2016b, 2019; Bu et al. 2017, 2019; Lei et al. 2018, 2019a, 2019b).

By retrieving known hot subdwarfs and candidates from the literature and unpublished databases, Geier et al. (2017) compiled a hot subdwarf catalog with 5613 objects. This catalog provides much useful stellar information, such as multi-band photometry, proper motions, classifications, atmospheric parameters, etc. The second data release of the Gaia mission (Gaia DR2; Gaia Collaboration et al. 2018) brought us excellent opportunities to discover new hot subdwarf stars, because it provides accurate positions, photometry, parallax, and proper motions. With this information, one can easily compile a large sample of hot subdwarf candidates with high confidence in the HR diagram. Using this method, Geier et al. (2019) compiled a list of 39,800 hot subdwarf candidates selected from Gaia DR2, which is the largest collection ever published. This catalog can be used as a good input target list for follow-up spectroscopic analyses.

In this paper, we analyzed the single-lined spectra that were selected by cross-matching the catalog of Geier et al. (2019) with the latest data release of the LAMOST survey, i.e., DR6 and DR7. By employing non-Local Thermodynamic Equilibrium (non-LTE) model atmospheres, we obtained their atmospheric parameters (e.g., effective temperatures, surface gravity values, and He abundance). We also found many composite spectra in the sample, and will report the analysis and results in a forthcoming paper. The structure of this paper is the following: in Section 2, we introduced the hot subdwarf candidates catalog and the databases of LAMOST DR6 and DR7, and how the candidate spectra were selected. The method of spectral analysis and classification for hot subdwarf stars are described in Section 3. We give our results in Section 4. We finish the paper with a brief discussion and summary in Section 5.

2.1. The Hot Subdwarf Candidate Catalog

Geier et al. (2019) compiled a catalog of hot subdwarf candidates from Gaia DR2, in which 39,800 candidates were selected by means of color, absolute magnitude, and reduced proper motion cuts. Figure 1 shows the selected candidates (red dots) by Geier et al. (2019) in the Gaia DR2 HR diagram (the figure is taken from Figure 3 of Geier et al. 2019, for the detailed candidate selection filter please see Section 3 in their study). The majority of the candidates are expected to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch (BHB) stars, hot post-AGB stars, and central stars of planetary nebulae.

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2.2. The LAMOST DR6 and DR7 Database

2.3. Hot Subdwarf Candidates Selected by Cross-matching the Geier et al. (2019) Catalog with LAMOST DR6 and DR7

As in Lei et al. (2018, 2019b), we employed the spectral analysis tool, XTgrid (Németh et al. 2012, 2014) to analyze the selected 607 spectra. XTgrid fits the observed data with synthetic spectra (Synspec version 49; Lanz & Hubeny 2007) calculated from non-LTE model atmospheres (Tlusty version 204; Hubeny & Lanz 2017). The best-fitting model is searched for iteratively with a successive approximation method along the steepest-gradient of the ${\chi }^{2}$ field. Parameter uncertainties have been estimated by mapping the ${\rm{\Delta }}{\chi }^{2}$ field until the 60% confidence level at the given number of free parameters was reached. For the detailed information to obtain the parameter error bars, the readers are suggested to see Figure 2 in Lei et al. (2019b) and the text therein.

The atmospheric parameters of all candidates, such as effective temperature (T_eff), surface gravity (log g), and He abundance ($\mathrm{log}(n\mathrm{He}/n{\rm{H}})$), are obtained by the method described above. Some of the best-fit models for sdOB and sdO stars are shown in Figure 2. As in our previous studies (Lei et al. 2018, 2019b), we identified stars with T_eff hotter than 20,000 K and log g larger than 5.0 $\mathrm{cm}\,{{\rm{s}}}^{-2}$ as hot subdwarf stars. On the other hand, the stars with T_eff lower than 20,000 K or log g lower than 5.0 $\mathrm{cm}\,{{\rm{s}}}^{-2}$ are considered as BHB stars or B type MS (B-MS) stars, while for a few stars with very high T_eff and log g (e.g., ${T}_{\mathrm{eff}}\gt 80,000$ K and $\mathrm{log}g\,\gt$ 7 $\mathrm{cm}\,{{\rm{s}}}^{-2}$), we classified them as WDs. We focus only on hot subdwarf stars in the rest of this paper.

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We used the spectral classification scheme of Moehler et al. (1990) and Geier et al. (2017) to classify the identified hot subdwarf stars in this study. Stars with dominant H Balmer lines, but weak or absent He lines, are classified as sdB stars. Stars with dominant H Balmer lines and an obvious He ii 4686 $\mathring{\rm A}$ line, but without obvious He i lines are considered as sdO stars. Stars having dominant H Balmer lines, and both weak He i and He ii lines are identified as sdOB stars. Stars with dominant He i lines, but weak or absent H Balmers and He ii lines, we classified as He-sdB stars. Stars presenting strong He ii lines, but with weak or absent H Balmer lines and He i lines are He-sdO stars, while the stars presenting both strong He i and He ii line, but with weak or absent H Balmer lines are classified as He-sdOB stars.

From the 607 selected candidates, we identified 182 single-lined hot subdwarf stars, including 89 sdB stars, 37 sdOB stars, 26 sdO stars, 24 He-sdOB stars, 3 He-sdO stars, and 3 He-sdB stars. By cross-matching with the hot subdwarf stars cataloged by Geier et al. (2017), we found 74 common objects. That means we have found 108 new, previously uncataloged hot subdwarf stars and obtained their atmospheric parameters by detailed spectral analysis for the first time.

Table 1 gives the parameters and information for the 182 hot subdwarf stars with single-lined spectra. Columns 1–4 give the R.A., decl., LAMOST_ObsID, and Gaia source_id. Columns 5–7 give the atmospheric parameters fitted by XTgrid, e.g., T_eff, log g, and $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$, while columns 8–10 give the S/N in the u band, the apparent Gaia G band magnitudes, and spectral classification. The 74 stars common with the hot subdwarf catalog of Geier et al. (2017) are marked by *.

Table 1.  Information on the 182 Hot Subdwarf Stars Identified in This Study

R.A.a	Decl.	ObsID	source_id	Teff	$\mathrm{log}\ g$	$\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ b	S/N-u	G	spclass
LAMOST	LAMOST	LAMOST	Gaia	(K)	($\mathrm{cm}\,{{\rm{s}}}^{-2}$)			Gaia(mag)
1.8907183*	13.5993244	619614193	2767874292175410560	29560 ± 120	5.41 ± 0.01	−1.90 ± 0.04	103	13.07	sdB
2.1021972	49.083822	593009050	393589879591384576	26640 ± 700	5.53 ± 0.08	−2.61 ± 0.11	17	15.91	sdB
2.7184872	26.5002178	689110148	2850670743266825600	28380 ± 190	5.27 ± 0.02	−2.34 ± 0.09	13	16.98	sdB
2.9364712	46.801838	593007055	392942881419391872	45110 ± 420	5.34 ± 0.04	0.91 ± 0.23	56	14.37	He-sdOB
4.2165006	52.146517	615603055	395157267782903808	29170 ± 350	5.46 ± 0.06	−2.56 ± 0.15	11	16.81	sdB
4.23055	51.230486	615605186	394991241522199040	32770 ± 460	5.48 ± 0.07	−3.04 ± 0.58	14	16.36	sdB
4.6252262	48.805384	593013047	392840562417338112	24740 ± 140	5.02 ± 0.03	−1.57 ± 0.06	11	15.30	sdB
4.7865313	52.511876	615603207	419143904215897728	51870 ± 5990	5.19 ± 0.30	−1.46 ± 0.14	10	17.16	sdO
5.8632988	51.130463	615605166	394843322846749824	26610 ± 170	5.42 ± 0.03	−2.43 ± 0.06	21	15.95	sdB
6.0352547	56.027472	605908160	421328839978415616	35020 ± 560	5.76 ± 0.13	−1.34 ± 0.10	21	16.18	sdOB
6.1375712	26.8194824	689109217	2856144494402348544	47370 ± 710	5.82 ± 0.11	0.30 ± 0.09	30	16.90	He-sdOB
6.51063*	31.1057	679407014	2862194144817359872	30150 ± 110	5.52 ± 0.08	−2.98>	83	14.87	sdB
7.329288	52.97546	615609066	416403783797286784	36080 ± 490	5.66 ± 0.58	−2.82>	12	16.80	sdB
15.599966	48.879263	353516071	402544091832710272	57260 ± 9490	6.22 ± 0.78	0.66 ± 1.04	28	16.67	He-sdOB
15.866097	32.675987	96304147	314344331362996736	37490 ± 1030	5.43 ± 0.08	−1.69 ± 0.14	14	14.37	sdOB
16.358834	49.928952	686402134	404204083809859584	53250 ± 6940	5.75 ± 0.06	−2.41 ± 0.18	23	16.88	sdO
17.418584	52.819013	686415207	404958378847936000	35330 ± 250	5.90 ± 0.08	−1.47 ± 0.06	14	17.52	sdOB
18.320503	47.191618	603604248	401413450281523584	48680 ± 820	6.06 ± 0.04	−3.21 ± 0.33	114	14.36	sdO
18.553344	52.280484	686415105	404172060533177344	59320 ± 1870	6.05 ± 0.06	−2.97 ± 0.22	31	16.44	sdO
25.858836	32.577683	159006176	305426398708664832	43220 ± 890	5.61 ± 0.15	2.81 ± 0.15	37	15.27	He-sdB
26.668712	41.307165	698114154	347453684494319104	26660 ± 230	5.08 ± 0.03	−0.90 ± 0.04	22	15.83	sdB
28.4816071	18.7996719	613914250	92226691740846080	31800 ± 610	5.85 ± 0.07	−1.62 ± 0.08	10	17.26	sdB
28.953962	41.548932	631015191	345949758745792000	37560 ± 180	5.69 ± 0.03	−3.18>	39	15.74	sdO
31.447815	40.610996	631006045	344794339527216000	33200 ± 420	5.50 ± 0.10	−2.44 ± 0.18	10	17.66	sdB
31.6278862	54.5190312	380704124	456417279675979008	34470 ± 1560	5.13 ± 0.07	−1.56 ± 0.04	105	14.32	sdOB
33.894867	49.427094	618611098	355574058902192768	37420 ± 470	5.91 ± 0.04	−1.47 ± 0.09	16	16.81	sdOB
34.28615	43.681405	615712124	351536441749571200	35940 ± 1350	5.71 ± 0.03	−1.50 ± 0.04	29	15.58	sdOB
35.423212	54.114846	678102180	455558286215251840	27580 ± 330	5.54 ± 0.09	−2.78 ± 0.54	13	16.74	sdB
36.372432*	28.80514	698702181	130950357400044800	36240 ± 710	5.87 ± 0.03	−1.13 ± 0.07	13	17.35	sdOB
37.757164*	27.718067	627301227	127674641678296704	47650 ± 1400	5.71 ± 0.02	−2.78 ± 0.29	79	15.15	sdO
38.001041*	33.576702	632206097	134510477267997952	23720 ± 270	5.62 ± 0.03	−2.80>	22	15.42	sdB
42.586847	49.209444	714701194	438686001110484352	31040 ± 160	5.56 ± 0.04	−3.01 ± 0.79	26	15.76	sdB
48.922807	46.869773	606506145	434851149371030272	39550 ± 530	5.31 ± 0.04	−2.81 ± 0.23	21	15.77	sdO
49.744528	43.927658	616805109	242105008671742976	28450 ± 170	5.94 ± 0.02	−3.16 ± 0.14	27	16.51	sdB
54.117831	46.137875	587807246	247823740446444416	35190 ± 210	5.67 ± 0.04	−1.44 ± 0.05	16	16.41	sdOB
57.996529*	9.640213	587107104	3302502234815943296	23510 ± 170	5.41 ± 0.01	−3.00>	35	15.59	sdB
59.862336	27.08573	1405078	163565999746075264	33210 ± 840	5.22 ± 0.14	−2.84 ± 0.49	11	15.10	sdB
63.957018*	30.587572	504615117	165787700429000064	22230 ± 320	5.12 ± 0.04	−2.64 ± 0.16	13	16.46	sdB
70.810624	23.217639	184707246	146588028382865280	36390 ± 860	5.40 ± 0.28	−3.00>	17	15.91	sdO
72.124161*	15.127739	402714066	3308929464395407104	43920 ± 350	5.62 ± 0.04	−0.18 ± 0.04	29	15.59	He-sdOB
73.16829	17.529048	283501028	3406444218653682560	23550 ± 270	5.16 ± 0.02	−2.39 ± 0.08	19	16.20	sdB
74.772662	39.631731	302704157	199210757970191744	32890 ± 220	5.30 ± 0.04	−1.59 ± 0.06	32	15.94	sdB
76.710748	19.515218	202201036	3407876749162251648	50610 ± 1040	5.82 ± 0.09	−0.21 ± 0.12	33	16.07	He-sdO
88.877241	61.028656	707916071	282512988705189888	33890 ± 260	5.71 ± 0.05	−3.49>	20	15.58	sdB
88.918311*	19.073818	330903053	3398598348493762944	63830 ± 920	5.67 ± 0.39	−0.21 ± 0.09	39	14.63	He-sdO
89.559068	46.673715	604410009	197796403761616256	49310 ± 870	5.75 ± 0.11	−0.16 ± 0.06	21	17.12	He-sdOB
91.999515	13.6144053	679505089	3344334627867111168	30580 ± 1510	5.09 ± 0.01	−1.57 ± 0.12	162	12.19	sdB
92.025035	46.167062	604403022	963326637253435904	48220 ± 480	5.52 ± 0.13	−0.14 ± 0.12	18	17.47	He-sdOB
93.23016	57.847462	679716210	999261490450160512	29270 ± 270	5.43 ± 0.03	−2.28 ± 0.04	29	15.81	sdB
95.662806	46.542454	601111213	968469534172491648	27570 ± 240	5.45 ± 0.02	−2.72 ± 0.14	35	14.77	sdB
97.001902	20.849289	274315180	3376012799112785408	76560 ± 10740	5.10 ± 0.21	−0.10 ± 0.14	17	15.81	He-sdO
103.24919	52.713839	545808061	993265067567138432	31770 ± 1220	5.04 ± 0.14	−2.35 ± 0.19	17	15.46	sdB
105.67208	34.633185	604903156	939579041518246272	34060 ± 500	5.84 ± 0.22	−1.54 ± 0.09	24	17.09	sdOB
106.03268	24.199745	188107221	3380673418444759936	36990 ± 1180	5.80 ± 0.05	−1.73 ± 0.11	14	17.34	sdOB
107.27312	22.595127	616616046	3368172319132367104	48480 ± 2570	5.48 ± 0.04	−2.24 ± 0.09	29	16.17	sdO
107.72217*	56.412373	687616080	988436459174352512	41280 ± 540	5.79 ± 0.07	−2.87 ± 0.51	50	14.64	sdO
108.5057	69.55596	601216092	1109216024779190016	37890 ± 460	5.52 ± 0.06	−0.15 ± 0.05	14	16.19	He-sdOB
111.25613*	27.055098	606410248	872695092069071360	31890 ± 420	5.57 ± 0.12	−2.29 ± 0.06	34	16.19	sdB
111.39693	81.847694	617015027	1142701823200960512	36490 ± 140	5.82 ± 0.11	−1.48 ± 0.07	29	15.05	sdOB
112.18863	13.440832	688810176	3163565604772130560	27930 ± 480	5.47 ± 0.07	−2.47 ± 0.13	13	15.67	sdB
112.207639	2.233514	600415096	3135810671409226368	31090 ± 330	5.56 ± 0.10	−3.13 ± 0.48	13	15.99	sdB
112.851109	0.444741	600405172	3134542693985873408	46650 ± 1600	5.74 ± 0.25	1.22 ± 0.10	12	16.31	He-sdOB
113.70542	12.424434	688802020	3162537840574968832	25600 ± 110	5.95 ± 0.05	−3.28>	31	16.46	sdB
113.898132	2.969486	600412059	3135525623021849088	37090 ± 180	5.71 ± 0.07	−1.47 ± 0.08	19	15.95	sdOB
113.95052	26.831992	606408234	872122177791852288	32740 ± 550	5.79 ± 0.19	−1.75 ± 0.16	12	17.99	sdB
114.35958	12.757287	688801151	3162576078665811840	29250 ± 250	5.54 ± 0.10	−3.00>	11	17.72	sdB
118.30847*	11.211171	605805093	3150707232898463616	29360 ± 60	5.42 ± 0.01	−2.44 ± 0.04	52	15.60	sdB
118.3712975	23.4100853	689603199	675213084211549696	34070 ± 40	5.75 ± 0.01	−1.68 ± 0.03	122	13.27	sdOB
123.243814*	0.731455	641316208	3089571878131969792	28390 ± 340	5.42 ± 0.03	−2.61 ± 0.13	24	14.59	sdB
124.735819	39.901597	642010128	909317797165729024	22910 ± 210	5.44 ± 0.07	−3.48 ± 0.29	38	14.94	sdB
124.998525*	22.6836111	602216224	676607952150024448	31300 ± 180	5.69 ± 0.07	−1.79 ± 0.07	17	15.64	sdB
125.2234083*	0.1455028	641315139	3077510098136276480	29030 ± 90	5.65 ± 0.02	−1.96 ± 0.16	31	15.18	sdB
126.190367*	23.255656	602216150	678116344664890368	29900 ± 220	5.47 ± 0.02	−2.95 ± 0.14	38	15.34	sdB
126.28563*	48.675328	615105036	930960515328049536	29760 ± 300	5.62 ± 0.07	−2.96 ± 0.33	17	16.98	sdB
129.0820292*	20.9636028	699412249	664631178147534720	31370 ± 440	5.52 ± 0.11	−2.44 ± 0.15	10	16.34	sdB
130.255486*	39.938389	642006098	911573758803336960	29380 ± 140	5.69 ± 0.03	−2.41 ± 0.06	31	15.45	sdB
131.034178*	31.03639	130107157	706479277895031040	29320 ± 340	5.39 ± 0.03	−2.14 ± 0.07	30	14.56	sdB
131.19586*	11.652792	420803011	601862464498177664	28580 ± 190	5.37 ± 0.01	−2.53 ± 0.07	38	16.13	sdB
134.47774*	38.314391	711713027	719606175420853888	30870 ± 230	5.52 ± 0.09	−2.62>	14	15.68	sdB
137.90359*	27.877858	186807004	698115121143554176	46750 ± 1780	5.72 ± 0.03	−2.87 ± 0.21	17	17.00	sdO
143.8201083*	22.8279833	606605119	644079931432984704	37000 ± 430	5.64 ± 0.07	−2.05 ± 0.07	38	16.29	sdOB
144.90796	17.664899	606102183	620899404525808768	79240 ± 5590	6.58 ± 0.06	−2.19 ± 0.36	17	17.48	sdO
147.75537*	3.7991754	709312041	3849462024992532608	29500 ± 110	5.41 ± 0.04	−2.63 ± 0.62	19	15.89	sdB
148.3206292*	15.5617194	731215192	616743220508208896	41010 ± 10	5.66 ± 0.05	1.80 ± 0.07	35	15.52	He-sdOB
150.4163042	−3.0035611	723303210	3829267569803099776	31220 ± 470	5.47 ± 0.07	−2.81 ± 0.53	16	16.71	sdB
160.469575*	21.675766	712413228	3987913113277693184	33410 ± 90	5.75 ± 0.02	−2.17 ± 0.04	48	13.07	sdOB
162.3896667*	18.7115278	215810196	3983291213071411712	29500 ± 340	5.14 ± 0.06	−2.44 ± 0.07	34	14.92	sdB
178.016827*	39.140844	657402229	4034502959999559168	55800 ± 2480	5.48 ± 0.09	−2.03 ± 0.17	16	15.36	sdO
195.10638*	0.7583765	144103116	3689536684343245312	37940 ± 730	6.02 ± 0.14	−1.42 ± 0.09	16	15.72	sdOB
199.69624*	44.595021	739312049	1550490241899314560	42850 ± 100	5.73 ± 0.01	0.92 ± 0.07	68	14.77	He-sdOB
204.2248833*	11.4347944	734713132	3738606616980353664	37850 ± 260	5.87 ± 0.06	−1.50 ± 0.05	14	16.34	sdOB
204.54297*	43.295307	449115138	1501713500909166208	34600 ± 560	5.19 ± 0.06	−1.12 ± 0.15	12	16.77	sdOB
206.58844*	22.810201	660604235	1251408094001504640	35240 ± 2100	5.94 ± 0.17	−0.16 ± 0.08	14	17.15	He-sdOB
206.7520875*	11.1901194	733615158	3727881843124118400	23510 ± 40	5.60 ± 0.01	−3.00>	63	14.96	sdB
208.76946*	−2.506063	651513250	3657799934042253952	45740 ± 1000	5.65 ± 0.04	−1.80 ± 0.13	35	12.06	sdO
211.43857*	1.7386288	732404097	3661331668469980416	27770 ± 90	5.27 ± 0.02	−2.02 ± 0.05	37	15.81	sdB
212.732694*	9.548705	723502076	3723006814724972416	36840 ± 220	5.83 ± 0.02	−1.66 ± 0.06	60	14.05	sdOB
213.954395*	11.2038595	723504163	1225417739360402048	41500 ± 100	5.50 ± 0.11	1.67 ± 0.25	22	16.03	He-sdOB
221.29285	14.229163	343616178	1185738013981539840	57000 ± 3130	6.52 ± 0.07	−2.61>	17	16.39	sdO
221.3759375*	17.4645	657010071	1234828283288291840	71170 ± 6170	6.90 ± 0.07	−2.08 ± 0.24	28	16.23	sdO
223.01644*	45.558239	742605036	1586890398971315200	49470 ± 1360	5.65 ± 0.08	−1.98 ± 0.14	22	17.23	sdO
224.526717	8.858398	651102213	1161864283648012160	22500 ± 160	5.48 ± 0.03	−3.22 ± 0.10	54	14.64	sdB
224.5663917*	37.0047194	743709156	1295107633891682944	49610 ± 600	6.04 ± 0.10	0.00 ± 0.04	22	17.30	He-sdOB
224.8688067*	19.0638675	657013168	1188933362275187200	36420 ± 540	6.00 ± 0.03	−1.53 ± 0.04	44	14.25	sdOB
227.154254*	10.053918	651108142	1167834597427267456	35880 ± 290	5.79 ± 0.11	−1.13 ± 0.12	47	15.10	sdOB
231.78*	10.270154	565710176	1165815825359631232	33230 ± 730	5.15 ± 0.05	−1.97 ± 0.08	13	16.14	sdB
234.67853*	9.5784135	565707040	1165071009310870912	35840 ± 110	5.62 ± 0.03	−0.93 ± 0.04	19	15.73	sdOB
239.4949003*	14.0390339	740903219	1191689807863866112	30520 ± 310	5.85 ± 0.03	−2.88 ± 0.67	33	15.37	sdB
239.8282*	5.6004099	744014072	4426623509802852352	30650 ± 170	5.53 ± 0.03	−2.96 ± 0.18	16	16.88	sdB
241.1203583*	14.8469639	740909091	1192038902805203328	32460 ± 90	5.88 ± 0.06	−3.06 ± 0.47	28	16.04	sdB
243.18797*	4.2115442	744007193	4437254653372798848	45700 ± 350	5.65 ± 0.05	0.96 ± 0.17	22	16.03	He-sdOB
245.7361*	47.514196	743008201	1410860511508492288	28100 ± 220	5.66 ± 0.05	−1.79 ± 0.05	24	16.24	sdB
247.4703492*	11.0840364	663715062	4458994472154612480	27990 ± 160	5.42 ± 0.00	−2.61 ± 0.04	33	14.35	sdB
247.9622125*	48.0752639	743006102	1410554774260311808	38780 ± 420	5.55 ± 0.06	−0.48 ± 0.09	11	17.12	He-sdOB
250.878644*	51.415874	585102152	1413338325384928128	35940 ± 550	5.09 ± 0.03	−2.02 ± 0.12	20	16.17	sdOB
251.64371*	26.6312	743504173	1307252843628956672	39810 ± 490	6.27 ± 0.04	2.20 ± 0.12	35	16.14	He-sdB
254.756648*	29.042889	739510211	1309437641952913920	27720 ± 700	5.61 ± 0.12	−2.89 ± 0.19	12	16.11	sdB
254.990298*	28.848331	739510216	1308678016856993920	37520 ± 660	5.73 ± 0.00	−3.18 ± 0.19	96	14.39	sdO
255.76656	15.138432	745914113	4545907052398514432	27460 ± 400	5.34 ± 0.04	−1.88 ± 0.77	17	17.12	sdB
257.76869	11.765573	664314127	4540919083539644672	29410 ± 410	5.61 ± 0.05	−2.53 ± 0.14	10	17.80	sdB
258.2631*	16.178565	745911237	4546882216133354752	35550 ± 420	5.82 ± 0.03	−1.61 ± 0.04	40	16.27	sdOB
259.33566	9.6920351	664305033	4491582966009639040	54240 ± 1590	5.25 ± 0.20	1.76 ± 0.21	26	16.94	He-sdOB
259.824376*	47.372495	745114017	1365071418489267584	38620 ± 530	5.87 ± 0.06	−2.58 ± 0.25	13	15.79	sdOB
260.03809	15.843371	745912208	4546952309999524096	42240 ± 590	5.13 ± 0.05	2.10 ± 0.01	27	16.43	He-sdOB
262.26331	17.326944	742209223	4550175420961718528	37750 ± 660	5.17 ± 0.08	−2.65 ± 0.19	10	16.31	sdB
262.8484903	46.2253286	566204175	1361931728676649984	28770 ± 1270	5.65 ± 0.07	−1.98 ± 0.10	22	17.15	sdB
264.4181	19.372917	742808226	4550885847209172736	32300 ± 110	5.69 ± 0.03	−2.13 ± 0.10	15	16.99	sdOB
270.030251	31.577103	663811042	4603104815507642752	26970 ± 440	5.52 ± 0.05	−3.23 ± 0.24	27	15.23	sdB
271.34282	15.200315	746410046	4498502433203434368	28260 ± 100	5.30 ± 0.02	−3.07 ± 0.15	14	16.92	sdB
271.95262	14.507648	746402150	4498196768968750336	28730 ± 290	5.39 ± 0.04	−3.00 ± 0.18	38	15.91	sdB
272.36637	16.769945	746403210	4502091509736932608	65660 ± 11140	5.68 ± 0.06	−1.79 ± 0.15	15	17.77	sdO
272.76169	17.633307	746415156	4526224282435916544	44410 ± 1260	5.97 ± 0.19	0.54 ± 0.12	19	17.10	He-sdOB
273.0417	18.131682	746411012	4526347084141410432	30170 ± 50	5.41 ± 0.01	−3.08 ± 0.84	48	15.48	sdB
273.04685	17.927912	746411078	4526236102186660352	23640 ± 420	5.03 ± 0.07	−2.30 ± 0.15	12	17.40	sdB
273.32165	34.316932	743615051	4605158393990687360	31140 ± 50	5.37 ± 0.04	−1.88 ± 0.29	23	17.15	sdB
274.13538	34.930314	743615206	4605575383775753856	47780 ± 1720	5.48 ± 0.03	−2.92 ± 0.11	59	16.02	sdO
274.83431	18.178403	746412176	4523666131196149632	27630 ± 220	5.14 ± 0.03	−2.95 ± 0.21	13	16.44	sdB
274.88444	6.0783654	742405150	4476966603891088640	36240 ± 580	5.37 ± 0.16	−3.00>	13	17.13	sdB
274.95133*	33.369344	743604247	4592825172063276288	28380 ± 180	5.44 ± 0.02	−2.55 ± 0.07	32	16.44	sdB
275.51348	10.739026	746714189	4483659679067140992	28940 ± 390	5.05 ± 0.07	−1.44 ± 0.07	12	17.37	sdB
288.82889*	42.93705	664703151	2102745688098547840	38420 ± 960	5.54 ± 0.16	−3.05 ± 0.29	25	14.49	sdO
291.81268*	38.45518	664011152	2052684550030830464	39890 ± 50	5.38 ± 0.15	0.54 ± 0.04	32	15.61	He-sdOB
292.78702*	43.416039	664613110	2125895669204184832	66880 ± 11150	5.15 ± 0.03	−1.01 ± 0.35	43	13.59	sdO
293.53371	35.000895	664007120	2048109069842534912	34560 ± 810	5.71 ± 0.06	−1.41 ± 0.06	13	15.34	sdOB
293.86659	35.732989	664008145	2048434490916786176	29610 ± 300	5.63 ± 0.09	−2.62 ± 0.20	16	15.77	sdB
303.0673667*	8.2691222	587214212	4251149700348007680	27730 ± 220	5.39 ± 0.06	−2.98 ± 0.12	49	14.62	sdB
303.406715	9.467058	746301022	4299431347569705216	33430 ± 110	5.15 ± 0.02	−2.62 ± 0.06	119	12.41	sdOB
305.47392	6.488096	587308185	4249752113691558144	30820 ± 580	5.65 ± 0.07	−2.85>	12	17.44	sdB
305.67635	7.2876099	587304110	4249937660575808000	29270 ± 130	5.42 ± 0.03	−3.10 ± 0.14	17	17.07	sdB
317.36219	37.139663	680412153	1868767831308190976	50100 ± 1630	5.53 ± 0.04	−1.68 ± 0.06	41	15.28	sdO
317.503089	15.486887	677702178	1760662130066900352	43540 ± 520	5.45 ± 0.11	1.53 ± 0.50	18	16.40	He-sdOB
318.85231	38.577478	593803082	1965019835117424000	27670 ± 380	5.34 ± 0.05	−2.59 ± 0.30	12	17.63	sdB
318.88115	12.665982	592402156	1746789866736764800	29120 ± 370	5.55 ± 0.05	−2.65 ± 0.16	30	16.02	sdB
319.51366*	14.681637	592414174	1759463868550744576	28960 ± 70	5.60 ± 0.02	−3.00>	62	15.06	sdB
320.52329	21.686536	593111200	1790728889707996800	27620 ± 180	5.50 ± 0.02	−2.61 ± 0.03	28	15.17	sdB
320.87692*	0.710801	254804012	2690967057290240000	35050 ± 280	5.89 ± 0.04	−0.78 ± 0.04	24	16.87	He-sdOB
320.98619	15.55655	592415028	1783640205099886336	49100 ± 2030	5.54 ± 0.20	0.60 ± 0.08	18	16.71	He-sdOB
321.5339	2.759411	677911144	2691867011851791744	29550 ± 370	5.90 ± 0.07	−2.58 ± 0.20	12	16.16	sdB
321.70608	15.760201	592412140	1783628003096502144	34010 ± 470	5.84 ± 0.06	−1.61 ± 0.07	16	17.05	sdOB
321.797821*	0.196107	677904113	2687870218366060416	29570 ± 90	5.52 ± 0.03	−2.98 ± 0.08	61	14.57	sdB
322.80103*	11.493389	679903086	1745849337621677184	37210 ± 290	5.89 ± 0.02	−1.66 ± 0.04	55	15.94	sdOB
323.6435*	9.6801009	679901047	1741581170917641728	36590 ± 140	5.79 ± 0.05	−1.44 ± 0.04	49	15.55	sdOB
335.57083	26.93794	594102069	1881671180067646080	28760 ± 270	5.38 ± 0.08	−3.41 ± 0.45	26	16.77	sdB
335.57458	27.588819	594105250	1881776668761026688	50650 ± 2760	5.56 ± 0.23	−2.30 ± 0.17	50	15.50	sdO
340.48328	17.803049	601803112	2832879034517348608	30510 ± 600	5.46 ± 0.06	−3.26>	18	17.53	sdB
341.26507*	32.364203	680503235	1890677009230168704	31020 ± 240	5.70 ± 0.04	−2.74 ± 0.12	89	14.00	sdB
341.89198*	33.011002	680503175	1890817059523265024	26720 ± 230	5.55 ± 0.04	−2.82 ± 0.09	33	16.00	sdB
344.17032*	29.762963	606014218	1886200725594365568	36060 ± 1050	6.01 ± 0.07	−1.48 ± 0.08	34	16.35	sdOB
344.54285*	40.727771	604109042	1930945626165804032	43920 ± 290	5.55 ± 0.03	0.85 ± 0.08	26	15.99	He-sdOB
346.28713	30.454833	677603238	1886482196274463488	35340 ± 860	5.88 ± 0.12	−1.48 ± 0.10	12	17.18	sdOB
347.33042	36.899804	678214001	1915148289773812352	33520 ± 790	5.68 ± 0.04	−1.41 ± 0.05	13	16.41	sdOB
350.23543	34.39527	678205023	1912906626082386304	36100 ± 140	5.79 ± 0.02	−1.40 ± 0.03	44	15.43	sdOB
350.90066	45.217675	587411151	1937879932466496896	23950 ± 1700	5.81 ± 0.11	−2.72 ± 0.22	10	17.07	sdB
351.37223	41.518334	587405077	1923590271333313792	38370 ± 660	5.90 ± 0.07	−1.41 ± 0.07	31	16.30	sdOB
352.05393	29.892664	593502206	2869717686274246400	45030 ± 2480	5.55 ± 0.54	1.19 ± 0.17	14	16.71	He-sdOB
352.23596	49.468338	689005189	1942912367126946304	40640 ± 30	5.59 ± 0.03	1.88 ± 0.06	19	17.38	He-sdB
352.34432	32.233162	593503117	2872454748672529280	31070 ± 280	5.49 ± 0.07	−2.59 ± 0.26	22	16.93	sdB
353.68925	51.004629	689015198	1944738965178800384	37140 ± 490	5.53 ± 0.03	−3.23>	45	16.03	sdO
354.17191	31.533466	593504113	2871378846483069184	33470 ± 280	5.96 ± 0.04	−2.46 ± 0.12	24	16.86	sdB
354.84683	46.912366	678715200	1939195223251633024	30560 ± 750	5.82 ± 0.08	−1.53 ± 0.06	16	17.78	sdB
355.66937*	43.91102	678701017	1925782766239946624	39780 ± 480	5.29 ± 0.05	−2.75 ± 0.14	33	16.15	sdO
358.62099	35.560802	602605007	2878501890826543616	32860 ± 940	5.43 ± 0.09	−1.69 ± 0.09	16	17.43	sdOB

Notes. From left to right, we list the R.A., decl., LAMOST_ObsID, and Gaia source_id. Then the T_eff, log g, and $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ are listed from the XTgrid fits. Next, the S/N in the u band, the apparent magnitudes in the Gaia G band, and the spectral classifications are listed, respectively.

^aStars labeled with ∗ also appear in the hot subdwarf catalog of Geier et al. (2017). ^b">" denotes an upper limit of $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ for the object.

A machine-readable version of the table is available.

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Figure 3 shows the parameter diagrams for the 182 single-lined hot subdwarf stars. In panel (a), the majority of sdB stars (black circles) are in a region that is well defined by the zero-age horizontal branch and terminal-age horizontal branch (e.g., centered at T_eff = 28,000 K and log g = 5.5 $\mathrm{cm}\,{{\rm{s}}}^{-2}$), which demonstrates that these stars are undergoing helium burning in their cores. On the other hand, sdOB stars (blue up triangles) that cluster around at T_eff = 34,000 K and log g = 5.8 $\mathrm{cm}\,{{\rm{s}}}^{-2}$, present higher effective temperatures and log g than sdB stars. SdO stars (green squares) and He-sdO stars (aqua left triangles) present the highest effective temperatures in our sample, e.g., most of them have T_eff > 40,000 K, but with a wide range of log g. He-sdOB stars (red diamonds) cluster around at T_eff = 45,000 K and log g = 5.6 $\mathrm{cm}\,{{\rm{s}}}^{-2}$. The 2 He-sdB stars (magenta stars), which present the highest He abundance in our sample, are located in the area very close to our He-sdOB stars in panel (a). The hot subdwarf samples share similar characteristics in the T_eff–log g diagram with our previous study (e.g., see panel (a) of Figure 4 in Lei et al. 2019b and Figure 6 in Lei et al. 2018).

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Panel (b) in Figure 3 shows the T_eff–$\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ diagram for our hot subdwarf sample. Two distinct helium sequences, e.g., a He-rich sequence (fitted by dotted line) and a He-weak sequence (fitted by dotted–dashed line), which were discovered by Edelmann et al. (2003) and confirmed by later studies (Németh et al. 2012; Geier et al. 2013; Luo et al. 2016b; Lei et al. 2018, 2019b), are clearly present in this panel. As found by Lei et al. (2019b), the He-rich sequence consists of sdB, sdOB, He-sdOB, and He-sdB stars, while the He-weak sequence consists purely of sdO stars. Furthermore, the three He-sdO stars (aqua left triangles) identified in this study are located between the two He sequences. However, the physical mechanism responsible for the two He sequences of hot subdwarf stars is still unclear, and additional scenarios are needed.

In panel (b), one also can find a gap (e.g., T_eff = 40,000 K and $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ = 0.0) in He-sdOB stars (red diamonds), which splits the He-sdOB stars into two subgroups, e.g., a subgroup with higher He abundances and temperatures, and the other subgroup with lower He abundance and temperatures. With larger size of He-sdOB stars, this gap is more clearly present in panel (b) of Figure 4 in Lei et al. (2019b). As discussed in Lei et al. (2019b), the gap also appears in the EHB stars of globular cluster (GC) ω Cen, but the star fraction of the two subgroups between field hot subdwarf stars and ω Cen EHB stars are very different. Moreover, three He-sdB stars are found in this study that present the highest He abundances (e.g., $\mathrm{log}(n\mathrm{He}/n{\rm{H}})\,\gt$ 2.0) in our sample. However, the most He-rich stars are completely missing from the EHB stars of ω Cen (see Figure 6 in Lei et al. 2019b and the text therein for detailed discussion). All these results point toward a different formation of field hot subdwarf stars and GC EHB stars, and provide a strict observational limit on the evolution models for the two types of objects.

As described above, we have 74 stars in common with the hot subdwarf catalog of Geier et al. (2017). Therefore, we compared the parameters of the stars obtained in this study and the parameters reported in the catalog of Geier et al. (2017) as long as their atmospheric parameters are available. Figure 4 presents the results from this comparison. Horizontal coordinates denote the parameter values obtained in this study, while vertical coordinates represent the values from Geier et al. (2017). As we see in Figure 4, the values of T_eff (e.g., left panel) and $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$ (e.g., right panel) obtained in this study are well consistent with the ones reported in Geier et al. (2017). Although the comparison of log g (middle panel) shows a little larger dispersion than the other two parameters (e.g., T_eff and $\mathrm{log}(n\mathrm{He}/n{\rm{H}})$), the values are still comparable when the large systematic errors that affect log g are considered. One source of these errors stems from the different implementations of Stark broadening tables in various model atmosphere codes. Another source is the variable observational data quality at the Balmer jump, which constrains log g. With these in mind the comparison results demonstrate a reliable spectral analysis of this study.

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We selected 607 hot subdwarf candidates by cross-matching the catalog of Geier et al. (2019) with the LAMOST DR6 and DR7 spectral database, and identified 182 hot subdwarf stars, among which 108 stars are newly discovered. Together with the 682 hot subdwarf stars identified by Lei et al. (2018, 2019b), we found 864 hot subdwarfs in the LAMOST spectral database, and 349 of them are new discoveries.

The hot subdwarf candidates in Lei et al. (2018, 2019b) were selected visually in the Gaia DR2 HR diagram, which means a little different selection filter from the one used by Geier et al. (2019, see Section 3 in their study). Therefore, we cross-matched all the 864 hot subdwarf stars identified in our series of studies with the Geier et al. (2019) catalog, and found 833 common stars. This result demonstrates that nearly all the hot subdwarf stars identified in Lei et al. (2018, 2019b) are included in the Geier et al. (2019) catalog. As described in Section 2.3, 2513 candidates from the catalog of Geier et al. (2019) have LAMOST spectra, of which 1348 have S/N-u larger than 10, and 833 of them were spectroscopically identified as hot subdwarf stars. Based on these results, one can roughly estimate the fraction of hot subdwarf stars in the catalog of Geier et al. (2019).

Figure 5 shows the distributions of selected hot subdwarf candidates (left) and the fraction of confirmed hot subdwarfs (right) with respect to Gaia G band magnitude. As shown in the left panel, for the brighter sample (e.g., 9 < Gaia G mag $\lt \,13$, which usually represents higher S/N), the candidates with S/N-u larger than 10 (blue-dashed histogram) have nearly the same size as the whole sample (red-solid histogram), which means a good completeness of the bright end of the catalog. However, only part of these stars were identified as hot subdwarf stars (green-dotted histogram). This result also can be seen clearly in the right panel. The fraction of candidates with S/N-u larger than 10 in the whole sample (gray-dashed curve) decreases from 100% to 80% within this magnitude range, and the fraction of hot subdwarf stars among the candidates with S/N-u larger than 10 (blue-dotted curve) is nearly the same as in the whole sample (red-solid curve), e.g., roughly between 10% and 40%. These results demonstrate that the hot subdwarf fraction of the candidates in Geier et al. (2019) for brighter stars (e.g., 9 < Gaia G mag <13) is roughly from 10% to 40%, and increasing gradually with the magnitude. It can be understood that there are more O/B type MS stars, rather than hot subdwarf stars, in the brighter part of the catalog of Geier et al. (2019). Thus, the brighter the sample, the much lower fraction of hot subdwarf stars it contains. Composite spectra were removed from our sample, if they were included and turned out to be real hot subdwarfs, this fraction could be a little higher.

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With fainter samples (e.g., 13 < Gaia G mag <16), the S/N-u of the spectra become lower. Therefore, many candidates do not enter into our sample due to low S/N, and the completeness of the sample becomes worse. The fraction of candidates with S/N-u larger than 10 among all candidates drops gradually from 80% to 60% as the magnitude increases from 13 to 16 mag (see gray-dashed curve in the right panel). In this magnitude range, we got a hot subdwarf fraction in the whole sample (red-solid curve in the right panel) going from 40% to 60%. Considering that some real hot subdwarf stars were removed from our sample due to low S/N-u or composite feature, the hot subdwarf fraction could be higher in this magnitude range. At the faint end of the sample, above 16 mag, the number of candidates with S/N-u larger than 10 drops quickly. Therefore, the candidates we analyzed in this magnitude range become extremely incomplete, and the hot subdwarf fraction obtained from these candidates is meaningless. One can expect more WDs rather than hot subdwarfs among the fainter candidates. A more accurate estimation of the fraction of hot subdwarf stars in the catalog of Geier et al. (2019) can be obtained when the results from analysis of composite spectra will be available.

The results obtained in this study reflect the high efficiency of the method to search for hot subdwarf stars by combining Gaia DR2 data with LAMOST spectra. We obtained reliable atmospheric parameters for all the hot subdwarf candidates using detailed spectral analysis with non-LTE model atmospheres. The atmospheric parameters are consistent with the ones from literature and the hot subdwarf catalog of Geier et al. (2017). We also estimated the hot subdwarf fraction in the catalog of Geier et al. (2019) based on the candidates we have analyzed. We found that the bright part (9 mag < Gaia $G\lt$ 13 mag) of the catalog is nearly complete, but has many false-positive candidates (over 60%, mostly B-type stars). In the 13 < Gaia G <16 magnitude range the hot subdwarf fraction goes from 40% to 60%. The completeness of the catalog degrades quickly above G = 16 mag. Furthermore, we selected about 150 hot subdwarf candidates with composite spectra in LAMOST DR6 and DR7. The results from their spectral analysis will be reported in a forthcoming paper. Since all spectra are observed with the same equipment and analyzed with the same method, we believe that the LAMOST hot subdwarf sample will make important contributions to study the formation and evolution of these special blue objects.

We thank the anonymous referee for their valuable suggestions and comments which improved this work greatly. L.Z. acknowledges support from National Natural Science Foundation of China grant No. 11503016, Natural Science Foundation of Hunan province grant No. 2017JJ3283, the Youth Fund project of Hunan Provincial Education Department grant No. 15B214, Cultivation Project for LAMOST Scientific Payoff and Research Achievement of CAMS-CAS. This work is supported by the National Natural Science Foundation of China grant Nos. 11390371, 11988101, 11973048, National Key R&D Program of China No.2019YFA0405502, the Astronomical Big Data Joint Research Center, co-founded by the National Astronomical Observatories, Chinese Academy of Sciences and the Alibaba Cloud. This research has used the services of www.Astroserver.org under reference D879YE and D880YE. P.N. acknowledges support from the grant Agency of the Czech Republic (GAČR 18-20083S). The LAMOST Fellowship is supported by Special Funding for Advanced Users, budgeted and administered by the Center for Astronomical Mega-Science, Chinese Academy of Sciences (CAMS). Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.