LAMOST Medium-Resolution Spectral Survey of Galactic Nebulae(LAMOST MRS-N):An overview of scientific goals and survey plan

Since Sep.2018,LAMOST has started the medium-resolution(R~7500)spectral survey(MRS).We proposed the spectral survey of Galactic nebulae,including HⅡregions,HH objects,supernova remnants,planetary nebulae and the special stars with MRS(LAMOST MRS-N).LAMOST MRS-N covers about 1700 square degrees of the northern Galactic plane within 40°<l<215°and-5°<b<5°.In this 5-year survey,we plan to observe about 500 thousand nebulae spectra.According to the commissioning observations,the nebulae spectra can provide precise radial velocity with uncertainty less than 1 km s^(-1).These high-precision spectral data are of great significance to our understanding of star formation and evolution.

LAMOST MRS-N Observation of the W80 Region

The spectral observations and analysis for the W80 region are presented by using the data of Medium-Resolution Spectroscopic Survey of Nebulae(MRS-N) with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST).A total of 2982 high-quality nebular spectra have been obtained in the 20 square degree field of view which covers the W80 complex,and the largest sample of spectral data has been established for the first time.The relative intensities,radial velocities(RVs),and full widths at half maximum(FWHMs) are measured with the high spectral resolution of LAMOST MRS-N,for Ha λ6563?,[NⅡ] λλ6548 A,6584 A,and [SⅡ] λλ6716A,6731 A emission lines.In the field of view of the whole W80 region,the strongest line emissions are found to be consistent with the bright nebulae NGC 7000,IC 5070,and LBN 391,and weak line emissions also exist in the Middle Region,where no bright nebulae are detected by the wide-band optical observations.The large-scale spectral observations of the W80 region reveal the systematic spatial variations of RVs and FWHMs,and several unique structural features.A "curved feature" to the east of NGC 7000,and a "jet feature" to the west of LBN 391 are detected to be showing larger RVs.A "wider FWHM region" is identified in the eastern part of NGC 7000.The variations of[S Ⅱ]/Ha ratios display a gradient from southwest to northeast in the NGC 7000 region,and manifest a ring shape around the "W80 bubble" ionized by an O-type star in L935.Further spectral and multi-band observations are guaranteed to investigate in detail the structural features.

Irregular changes in Hαemission line of V423 Aur observed by LAMOST Medium-Resolution Spectrographs

We obtained seven spectra of the Be star V423 Aur on 2017 Dec.5 using the LAMOST MediumResolution Spectrograph with exposures from 600 to 1200 seconds.These spectra show the irregular Ha emission line profile variations(LPVs).In the seven spectra,from the 4 th to 7 th,the left part of Ha profile even shows excess.However,no variation can be seen from the follow-up observation of photometry by 1.26-m telescope and High-Resolution spectra by 2.16-m telescope.According to the High-Resolution spectra,we conclude that it is a B7 V type star with E(B-V)=0.709±0.036 and its vsini is221.8 km s-1.The short-term Ha LPVs could be explained as a result of the transient ejection of matter from rotating disk or shell around V423 Aur.

The Data Processing of the LAMOST Medium-resolution Spectral Survey of Galactic Nebulae(LAMOST MRS-N Pipeline)

The Large sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) medium-resolution spectral survey of Galactic Nebulae(MRS-N) has conducted for more than three years since 2018 September and observed more than190 thousand nebular spectra and 20 thousand stellar spectra.However,there is not yet a data processing pipeline for nebular spectra.To significantly improve the accuracy of nebulae classification and their physical parameters,we developed the MRS-N Pipeline.This article presented in detail each data processing step of the MRS-N Pipeline,such as removing cosmic rays,merging single exposure,fitting sky light emission lines,wavelength recalibration,subtracting skylight,measuring nebular parameters,creating catalogs and packing spectra.Finally,a description of the data products,including nebular spectra files and parameter catalogs,is provided.

FGK 22 μm excess stars in LAMOST DR2 stellar catalog

Since the release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope（LAMOST）catalog, we have had the opportunity to use the LAMOST DR2 stellar catalog and the WISE All-Sky Data Release catalog to search for 22 μm excess candidates. In this paper, we present 10 FGK candidates which show an excess in the infrared at 22 μm. All the 10 sources are newly identified 22 μm excess candidates.Of these 10 stars, five stars are F type and five stars are G type. The criterion for selecting candidates is Ks[22].387. In addition, we present the spectral energy distributions covering wavelengths from the optic-≥0al to mid-infrared band. Most of them show an obvious excess from the 12 μm band and three candidates even show excess from 3.4 μm. To characterize the amount of dust, we also estimate the fractional luminosity of10 22 μm excess candidates.

On the radial velocity calibrations in the LAMOST medium-resolution spectroscopic survey of nebulae

Accurate radial velocity determinations of optical emission lines(i.e.,[NⅡ]λλ6548,6584,Hαand[SⅡ]λλ6717,6731)are very important for investigating the kinematic and dynamic properties of nebulae.The second stage survey program of Large sky Area Multi-Object fiber Spectroscopic Telescope(LAMOST)has started a sub-survey of nebulae(MRS-N)which will spectroscopically observe the optical emission lines of a large sample of nebulae near the Galactic plane.Until now,15 MRS-N plates have been observed from September 2017 to June 2019.Based on fitting the sky emission lines in the red band spectra of MRS-N,we investigate the precision of wavelength calibration and find there are systematic deviations of radial velocities(RVs)from~0.2 to 4 km s^(-1) for different plates.Especially for the plates obtained in March 2018,the systematic deviations of RVs can be as large as~4 km s^(-1),which then go down to~0.2-0.5 kms^(-1) at the end of 2018 and January 2019.An RV calibration function is proposed for these MRS-N plates,which can simultaneously and successfully calibrate the systematic deviations and improve the precision of RVs.

LAMOST medium-resolution spectral survey of Galactic nebulae(LAMOST-MRS-N):subtraction of geocoronal Hαemission

We introduce a method of subtracting geocoronal Hαemissions from the spectra of LAMOST medium-resolution spectral survey of Galactic nebulae(LAMOST-MRS-N).The flux ratios of the Hαsky line to the adjacent OHλ6554 single line do not show a pattern or gradient distribution in a plate.More interestingly,the ratio is well correlated to solar altitude,which is the angle of the Sun relative to the Earth’s horizon.It is found that the ratio decreases from 0.8 to 0.2 with the decreasing solar altitude from–17 to–73 degree.Based on this relation,which is described by a linear function,we can construct the Hαsky component and subtract it from the science spectrum.This method has been applied to the LAMOST-MRSN data,and the contamination level of the Hαsky to nebula is reduced from 40%to less than 10%.The new generated spectra will significantly improve the accuracy of the classifications and the measurements of physical parameters of Galactic nebulae.

The debris disk candidates:eleven 24μm excess stars in the Spitzer SWIRE fields

We present the optical to mid-infrared SEDs of 11 debris disk candidates from Spitzer SWIRE fields. All the candidates are selected from SWIRE 24 μm sources matched with both the SDSS star catalog and the 2MASS point source catalog. They show an excess in the mid-infrared at 24 μm （Ks-[24]vega 〉 0.44）, indicating the presence of a circumstellar dust disk. The observed optical spectra show that they are all late-type main-sequence stars covering the spectral types of FGKM. Their fractional luminosities are well above 5× 10-5, even up to the high fractional luminosity of 1×10-3. The high galactic latitudes of SWIRE fields indicate that most of these candidates could belong to the oldest stars in the thick disk. Our results indicate that high fractional luminosity debris disks could exist in old solar-like star systems, though they are still quite rare. Their discovery at high galactic latitudes also provides an exellent opportunity for further study of the properties and evolution of debris disks in regions of the Galaxy with low densities of ISM, called ISM poor environments.