Spectroscopy of Planetary Nebulae in Sextans A and Sextans B

Laura Magrini (1), Mario Perinotto (1), Pierre Leisy (2, 3), Romano L.M. Corradi (2),

Antonio Mampaso (3), Jose’ Vilchez (4)

(1) Dipartimento di Astronomia, Universita' di Firenze (Italy)(2) ING, La Palma (Spain)(3) IAC, Tenerife (Spain)(4) IAA, Spain

Sextans A• Type Ir V

• Distance 1.45 Mpc (Sakai et al. 1996)

• 12+log(O/H)=7.49 form HII regions spectra (Skillman et al. 1989)

• Number of PNe: 1 (Jacoby & Lesser 1981, Magrini et at. 2003)

1

LGC:WFC @ INT

Sextans B

• Ir IV-V

• Distance 1.32 Mpc (Sakai et al. 1997)

• 12 + log (O/H)=7.56 from HII regions (Skillman et al. 1989)

• Number of PNe: 5 (Magrini et al. 2002)

2

5

1

34

LGC:WFC @ INT

Observations:• FORS2 @ VLT • Grisms: 300 V with Texp =5400 s 300 I with Texp =3600 s

• Spectral range: ~3500-9600 Å• Dispersion: 3 Å/pixel• S/N > 10 for temperature diagnostic lines

Sextans A spectra:

4363 Å [OIII]

5755 Å [NII]

6717-6723 Å [SII]

H + [NII]

Sextans B spectra:

4363 Å [OIII]H + [NII]

[SII] 6717-6731 Å

H + [NII]

Computing chemical abundances:

• With CLOUDY 94.00, modeling our PNe in the simplest way:• Blackbody central stars with effective temperature derived using

the Ambartsumian’s (1932) or Gurzadyan’s (1988) methods

• Spherical nebula with constant density (derived from [SII] 6717/6731 Å flux ratio when available)

• Further iterations: • Central star luminosity set to match the observed [OIII] 5007 Å

flux• Nebular radius is varied to adjust predicted low ionization

emission lines (i.e. [OII] 3727 Å) with the observed ones• Chemical abundances are varied to match observed emission line

fluxes

• With classic Ionization correction factors (ICFs), following Kingsburgh & Barlow (1994)

PN in Sextans A: ICFs Cloudy

• He/H 0.085 0.085• O/H 8.0 8.1• N/H 8.4 8.4• Ne/H 6.7 6.7• Ar/H 5.3 5.2• S/H 5.8 5.8C 0.2T[OIII] 21000 K from [OIII] 4363/5007 Å flux ratioT[NII] 13400 K from [NII] 5755/6584 ÅNe 2700 cm-3 from [SII] 6717/6731 Å

From CLOUDY model:

T: 190,000 K

logL: 3.8 LRadius of the nebula : 0.13 pc

PNe in Sextans B: ICFs Cloudy (average of 5 PNe)

• <He/H> 0.081 0.089

• <O/H> 7.9 8.0

• <N/H> <7 <6.5

• <Ne/H> 7.0 7.2

• <Ar/H> 5.6 5.6

• <S/H> - <5.7

C 0.1

T[OIII] from 12500 to 17500 K from [OIII] 4363/5007 ÅT[NII] assumed equal to T[OIII]

Ne assumed 3000 cm-3

From CLOUDY model: T : from 60,000 to 80,000 KlogL 3.0 to 3.4 L

Radii of the nebulae: form 0.01 to 0.07 pc

Sextans B:Oxygen abundance

8.00

8.20 7.997.627.66

15 ‘ x 15’

Evolutionary tracks:

H-burning tracks

SexA PN central star: ~0.68 M from MS star ~2.5 M

SexB PNe central stars: ~0.57 to ~0.59 M from MS stars ~1-1.5 M

O/H vs Ne/H• Strong linear relation

between O/H and Ne/H (cf. Kaler; Henry 1989).

• Sex A and Sex B PNe follow this “universal” relation within the uncertainties

N/H vs. N/O• The correlation N/H

vs N/O (e.g. Henry 1990), suggests that the increase in N/O with N/H is primarily due to the increase of N and not to changes in O abundance.

PNe vs HII regions abundances:

Sextans BHII regions: • O/H~8.11 (Moles et al.1990);

7.56 (Skillman et al.1989) 7.86 (Pilyugin 2001)

Sextans A• O/H~7.49 (Skillman et al. 1989)

7.71 (Pilyugin 2001)

8.1 (this work)

Sextans B

PNe:

• O/H~ 7.62-8.20 (this work)

Sextans A

• O/H~7.98 (this work)

Luminosity-metallicity relationship in the Local Group

• Binggeli (1994), Mateo (1998) suggest a bimodal behaviour in luminosity-metallicity relationship between dSph and dIrr.

• This behaviour is more evident if consider an uniform determination of O/H, as using PNe, which are present in every morphological type of galaxy.

Conclusions:

• Deep spectroscopy of 5 PNe in Sex B and 1 in Sex A with VLT

• PN in Sex A: the farthest PN (1.45 Mpc) with both [NII] and [OIII] electron temperatures measured

• Chemical abundances with ICFs and CLOUDY