| Emission nebula | |
|---|---|
| Planetary nebula | |
 Photograph of the nebula (Credit: Solomon Hendrix) | |
| Observation data:J2000epoch | |
| Right ascension | 04h 25m 50.836s[1] |
| Declination | +60° 07′ 12.78″[1] |
| Distance | 5230+670 −440[2] pc |
| Apparent magnitude (V) | 13.96[3] |
| Apparent dimensions (V) | 0.063' x 0.063'[4] (3.7"[2]) |
| Constellation | Camelopardalis |
| Physical characteristics | |
| Radius | 0.06[5] pc |
| Designations | PN M 4-18, PN G146.7+07.6, PK 146+07 1[3] |
| See also:Lists of nebulae | |
Minkowski 4-18 (M 4-18) is aplanetary nebula orprotoplanetary nebula[6] in the deep northernconstellation ofCamelopardalis. Distance estimates range widely across publications between 1–7kpc,[7] though a 2022 paper gives a distance of 5,230 parsecs (17,100 light-years).[2] The nebula appears to be very young,[8] at about 3,100 years old.[7] Notably, the central star of the nebula is a very cool, late-typeWolf-Rayet star ofspectral type [WC11].[2]
| Characteristics | |
|---|---|
| Evolutionary stage | Wolf-Rayet star |
| Spectral type | [WC11][2] |
| B−Vcolor index | 0.26[3] |
| J−Hcolor index | 0.412[9] |
| J−Kcolor index | 1.158[9] |
| Astrometry | |
| Radial velocity (Rv) | 138.3900[10] km/s |
| Proper motion (μ) | RA: 0.717[1]mas/yr Dec.: −0.755[1]mas/yr |
| Parallax (π) | 0.1528±0.0164 mas[1] |
| Distance | 5230+670 −440[2] pc |
| Details[7] | |
| Mass | 0.62 (assumed) M☉ |
| Radius | 2.4 R☉ |
| Luminosity | 5250 L☉ |
| Temperature | 31000 K |
| Other designations | |
| Gaia DR2 471908436438311680,TIC 148390555,IRAS 04215+6000,2MASS J04255084+6007127,WISE J042550.85+600712.6[3] | |
| Database references | |
| SIMBAD | data |
This object was discovered by German-American astronomerRudolph Minkowski in 1959.[11] The nebula extends about 0.06 parsecs (0.20 light-years) from its center.[5] As early as 1974, the central star was known to be among the coolestcarbon-sequence (type WC) Wolf-Rayet stars.[12] The nebula has a mass of 0.08M☉[7] and anelectron temperature, that is the averagekinetic energy of electrons within theplasma, of 8,160 K (7,890 °C; 14,230 °F).[13]
The central star has only 62% themass of the Sun but is 2.4 times as large. It radiates 5,250 times theluminosity of the Sun from itsphotosphere at aneffective temperature of 31,000 K (30,700 °C; 55,300 °F), according to De Marco & Crowther (1999).[7] Some other sources, such as Acker et al. (2002), find a lower temperature of around 25,000 K.[5]
The center star of M 4-18, together with two other objectsM 2-54 andNGC 2392, was reported to be a low-amplitudevariable in 1996, brightening by 0.03mag over a 7-hour observation period in the night of 12 December 1995.[14]
The star isreddened substantially[11] at EB−V =0.75±0.10.[13] The reddening in theultraviolet region is considered anomalous, as different methods yield very different results for EB−V. This is possibly caused by an undetected shell ofcircumstellar dust.[8] The infrared reddening profile can be explained by grains ofamorphous carbon of a uniform size.[6]
The central star displays broademission lines of C+, C2+, and C3+ (C II, C III, and C IV inspectroscopic notation),[15] of which the lines of C+ are the strongest, consistent with the star's carbon-rich, Wolf-Rayet nature and relatively loweffective temperature.[13] The carbon lines overlay the nebula's emission lines, namely neutralhydrogen (H I), as well as theforbidden lines, indicated by the square brackets, of singly ionizednitrogen ([N II]) andsulfur ([S II]). The forbidden line[O III] (O2+; wavelength 4959–5007Å) is unusually weak, similar toHD 184738 (Campbell's Hydrogen Star).[15] Compared to HD 184738, M 4-18 shows weaker lines of C IV, O IV (O3+), and N V (N4+), implying that atoms in this nebula are in a lessexcited state.[16]
A 1980 paper claimed that the nebula was deficient in nitrogen and oxygen,[16] but a study in 1995 found a solar-like abundance of carbon, nitrogen, andneon, as well as an oxygen content twice as high as that of the Sun. In contrast, it is very poor in sulfur, only containing 10% as much as the Sun.[6] The reported enrichment in oxygen is dubious, however, as a paper published in 2022 presented a slightly subsolar oxygen abundance. The same paper described an overabundance ofxenon.[2]
WeakP Cygni emission features were confirmed in 1984, evidence ofstellar winds radiating away from the central star reaching a velocity of approximately 300 km/s.[17]
In 2003,polycyclic aromatic hydrocarbons (PAHs) were detected usinginfraredspectroscopy.[18] This was independently confirmed in a 2008 paper.[19]
