NGC 1983

NGC 1983 is a young open star cluster associated with an emission nebula (H II region) in the constellation Dorado, forming part of the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way.^[1][2] It was discovered by John Herschel on November 11, 1836, and is also designated as ESO 56-SC133 and LH 61.^[1][2] The cluster has an apparent magnitude of about 9 and an angular size of roughly 1 arcminute, corresponding to a compact structure visible in moderate telescopes.^[1][2] Its precise coordinates are right ascension 05h 27m 44.0s and declination -68° 59' 06" (J2000 epoch).^[2] As a young stellar grouping, NGC 1983 belongs to the class of OB associations, characterized by loose collections of thousands of young stars, including massive O- and B-type stars, and is gravitationally unbound with components dispersing over time.^[3] Associated with the Henize nebula N135, the object lies within a star-forming region in the northeast part of the LMC.^[3] Studies of similar young LMC clusters indicate ages on the order of a few million years, with NGC 1983 exhibiting partial mass segregation, particularly among its faintest stars.^[4][3] This makes it a subject of interest for understanding early stellar evolution and structure in extragalactic environments.

Discovery and nomenclature

Discovery

NGC 1983 was discovered by John Herschel on November 11, 1836, during his extensive telescopic survey of the southern celestial hemisphere conducted from the Cape of Good Hope between 1834 and 1838.^[1] Herschel, son of astronomer William Herschel, undertook this expedition to map and catalog nebulae, star clusters, and other objects in the southern skies that were inaccessible from northern observatories, using an 18.7-inch (47.5 cm) reflecting telescope for sweeping observations.^[5] This work built upon his father's earlier catalogues of northern objects and resulted in the publication of Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope in 1847, where many such discoveries were recorded.^[5] There is a possible earlier observation of the object by James Dunlop on September 25, 1826, but this identification remains uncertain.^[1]

Designations

NGC 1983 is the primary designation from the New General Catalogue (NGC).^[6] It is also known by several alternative catalog designations. These include ESO 56-SC133 (or ESO 56-133) from the European Southern Observatory (ESO) survey of the southern sky,^{[6] [7]} LH 61 from the Lucke-Hodge catalog of stellar associations in the Large Magellanic Cloud,^[6] [SL63] 492 from the Shapley-Lindsay (1963) catalog of LMC clusters,^[6] and BSDL 1834 from the Bologna Star Cluster Database for the LMC.^[6] These cross-identifications are compiled in major astronomical databases such as SIMBAD and NED.^{[6] [7]}

Location and observation

Coordinates and position

NGC 1983 is located at right ascension 05h 27m 44.0s and declination −68° 59′ 06″ (J2000 epoch), placing it in the southern sky within the constellation Dorado as observed from Earth.^[2] In the galactic coordinate system, the cluster lies at galactic longitude 279.4872° and galactic latitude −32.6597°.^[2] As a component of the Large Magellanic Cloud (LMC), NGC 1983 is embedded within this irregular dwarf galaxy, which appears as a prominent feature in the southern celestial hemisphere and serves as a satellite of the Milky Way.^[2]

Visibility and appearance

NGC 1983 appears as a compact open star cluster with an apparent visual magnitude of 9.9, making it accessible through small telescopes but beyond naked-eye visibility under typical conditions.^[8] Its angular dimensions measure 1.00 × 0.85 arcminutes, presenting a small, slightly elongated form in the eyepiece.^[2] Located at approximately RA 05h 27m 44s, Dec −69° 00′ in the Dorado constellation, the object is primarily observable from the southern hemisphere, where the Large Magellanic Cloud reaches high altitudes during favorable seasons. In telescopic views, NGC 1983 manifests as a concentrated grouping of faint stars, often appearing as a nebulous patch due to the surrounding emission nebula.

Imaging and surveys

NGC 1983 has been imaged at high resolution by the Hubble Space Telescope (HST) as part of Cycle 12 snapshot program GO-9891, titled "A snapshot survey of rich stellar clusters in the Large and Small Magellanic Clouds," with Principal Investigator Gerard Gilmore. This program used the Advanced Camera for Surveys (ACS) to acquire broadband imaging in filters including F555W and F814W, producing detailed color composites that reveal the cluster's compact core and surrounding nebulosity.^[9][10] These HST observations, drawn from the Hubble Legacy Archive, offer sub-arcsecond resolution views suitable for resolving individual stars within the young cluster and its associated emission nebula.^[10] Near-infrared imaging of NGC 1983 is available from the VISTA Magellanic Clouds Survey (VMC), a wide-area survey using the VISTA telescope to obtain deep Y, J, and K_s band photometry at sub-arcsecond resolution across the Magellanic system. This survey provides complementary data that penetrates dust and enhances structural mapping of the cluster.^[11][12]

Physical properties of the cluster

Distance and physical size

NGC 1983 is located within the Large Magellanic Cloud (LMC) at a distance of 47,200 parsecs from the Sun.^[11] The cluster has an apparent angular size of approximately 1 arcminute, which translates to a physical diameter of roughly 14 parsecs at the adopted distance.^[13] Structural analyses yield a half-mass radius of 5.15 parsecs, providing a measure of the region's core concentration.^[11] These dimensions reflect the compact nature of the young cluster relative to its environment in the LMC.

Age and evolution

NGC 1983 is a young open star cluster with an estimated age of approximately 9 million years, placing it in an early evolutionary phase typical of very young stellar systems in the Large Magellanic Cloud. This age determination comes from multiple studies using isochrone fitting to photometric data and integrated photometry techniques, which consistently yield values in the range of 6–10 million years or log(age [yr]) ≈ 6.9 (corresponding to roughly 8 million years).^[14][15][16] At this young age, the cluster remains in its initial evolutionary stage, characterized by the dominance of massive O and B stars still on or near the main sequence. These stars drive the cluster's high luminosity and spectral features, with limited dynamical evolution having occurred. The presence of ongoing star formation indicators, such as young stellar objects and spectral signatures of recent star birth in the region, aligns with this early phase before significant gas expulsion and cluster expansion.^[17][14]

Mass and luminosity

NGC 1983 has a total mass of approximately 3,100 solar masses, with a half-mass radius of 5.15 pc.^[11] The present-day mass function of the cluster is described by a power-law distribution with a slope of α = -2.46.^[11] Detailed estimates of the cluster's total luminosity are limited in available literature, though its apparent V-band magnitude of 9.9 reflects the integrated light from its young stellar population at a distance of about 47 kpc in the Large Magellanic Cloud.^[18]

Stellar population

NGC 1983 contains a population dominated by young main-sequence stars, as evidenced by high-resolution photometry from the Hubble Space Telescope's Advanced Camera for Surveys. The cluster's color-magnitude diagram, constructed from F555W and F814W filters, displays a sharp and narrow main sequence after statistical decontamination of field stars, extending down to m814 ≈ 21 mag and indicating a relatively uniform young stellar component with limited contamination from older LMC field populations. The cluster age is estimated at ≈28 Myr, consistent with the dominance of main-sequence stars and absence of significant evolved populations.^[4][19] Analysis of the stellar content yields approximately 424 probable member stars selected through membership probability (>70%) and photometric completeness criteria (≥70%), primarily consisting of main-sequence stars with no significant evolved population (such as red supergiants) evident in the cluster core.^[4] The present-day mass function follows a power-law distribution with slope α = -2.46, closely resembling the Salpeter initial mass function value of -2.35 and suggesting a relatively normal mass distribution across the observed stellar masses.^[11] Stellar stratification is partial, with the faintest stars (lower-mass members) showing mild segregation, while brighter (higher-mass) stars lack strong segregation, resulting in an overall slight negative trend in effective radius with magnitude.^[4]

Associated emission nebula

Characteristics

The emission nebula associated with NGC 1983 is classified as an H II region, consisting of gas ionized primarily by ultraviolet radiation from the young, hot stars within the cluster.^[20] As an H II region, it features photoionization of hydrogen and other elements, producing characteristic emission lines including hydrogen recombination lines (such as H-alpha) and forbidden lines from ions like [O III], typical of regions excited by massive stars in the Large Magellanic Cloud. The nebula is closely associated with the compact star cluster NGC 1983.

Interaction with the cluster

The open star cluster NGC 1983 (also designated LH 61) is embedded within the associated emission nebula DEM L210 (Henize N135), where its massive stars provide the primary source of ionizing ultraviolet radiation that excites the surrounding hydrogen gas and produces the characteristic emission.^[21][22] The Wolf-Rayet star BAT99 45 (WR 55), a prominent member of the cluster, contributes to this process through its intense ionizing flux and powerful stellar winds. The nebula DEM L210 appears as faint and diffuse overall (morphology class 3a), with the region's morphology reflecting dynamic feedback from massive stars in the vicinity, including winds that can carve cavities and filaments in the gas while radiation maintains ionization.^[21] Note that a nearby filamentary shell-like structure measuring approximately 3 arcminutes by 2.4 arcminutes, extending southward, is associated with WR 54 within the same DEM L210 region.^[21] These features illustrate the ongoing interplay between the cluster's stellar population and the gas, leading to the observed emission characteristics.^[21]

Role in the Large Magellanic Cloud

Position within the LMC

NGC 1983 is located in the central region of the Large Magellanic Cloud, in close proximity to the geometrical center of the galaxy's bar. According to a photometric catalog of LMC star clusters and associations, its position is offset by X = -0.57° and Y = 0.53° relative to NGC 1928, a cluster situated near the geometrical center of the LMC bar.^[23] These small angular offsets place NGC 1983 within or immediately adjacent to the bar region, the dense, elongated central structure of the LMC that dominates the galaxy's stellar distribution and is distinct from the more extended, lower-density disk and outer regions. This central positioning situates it amid the LMC's primary star-forming activity zones, separate from peripheral features such as the eastern wing or southern outer disk.

Association with stellar associations

NGC 1983 is embedded within the OB association LH 61 (Lucke-Hodge 61), one of the 122 OB associations cataloged in the Large Magellanic Cloud by Lucke and Hodge.^[3][2] The open cluster NGC 1983 forms the core of LH 61, which is a loose grouping of young massive stars including O- and B-type stars.^[3] LH 61 is positioned in the central region of the supergiant shell LMC 3, a larger-scale structure approximately 3,200 light-years in diameter that contains nine OB associations (including LH 57, LH 58, LH 61, LH 64, LH 67, LH 68, LH 71, LH 73, and LH 74).^[24] As an older association within this shell, LH 61 has largely dispersed its surrounding gas, distinguishing it from younger associations along the shell's rim that remain embedded in H II regions.^[24] This membership in LH 61 highlights NGC 1983's role as part of the broader hierarchical structure of stellar associations in the Large Magellanic Cloud, where open clusters like NGC 1983 often reside within larger OB associations that drive local energy injection into the interstellar medium.^[3][2]

Contribution to star formation studies

NGC 1983, also designated LH 61, exemplifies a young stellar association embedded in an HII region (DEM 210) in the Large Magellanic Cloud, offering direct insights into recent star formation processes where young stars ionize surrounding gas to produce ongoing emission nebulae.^[23] Its structure, featuring a central compact cluster (SL 492) within a broader association, illustrates the diversity of young stellar systems that remain associated with natal gas, in contrast to older or more evolved clusters that have cleared their surrounding material.^[23] Analyses of young LMC objects like NGC 1983 show that associations are more frequently linked to emitting gas than compact clusters (with 87% of SWB 0 and 17% of SWB I objects showing emission), highlighting variations in gas-sweeping efficiency during cluster formation and evolution.^[23] Such differences contribute to understanding cluster formation efficiency in the LMC's dynamic environment, where gas retention or removal influences how star formation proceeds in young systems.^[23] Comparisons with other LMC regions reveal that young clusters and associations, including NGC 1983, concentrate in disk-like structures with arms and clumps, likely reflecting enhanced star formation episodes triggered by past gravitational interactions, such as with the Small Magellanic Cloud.^[23] Its integrated photometric properties and age estimates have also been incorporated into broader studies calibrating age-determination methods for LMC clusters using integrated colors, supporting accurate reconstruction of the galaxy's recent star formation history.^[15]