M-R.L. Cioni

An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars

We present an empirical determination of the mass-loss rate as a function of stellar luminosity and effective tem- perature, for oxygen-rich dust-enshrouded Asymptotic Giant Branch stars and red supergiants. To this aim we obtained optical spectra of a sample of dust-enshrouded red giants in the Large Magellanic Cloud, which we complemented with spectroscopic and infrared photometric data from the literature. Two of these turned out to be hot emission-line stars, of which one is a definite B(e) star. The mass-loss rates were measured through modelling of the spectral energy distributions. We thus obtain the mass- loss rate formula log u M = −5.65 + 1.05 log(L/10 000 L� ) − 6. 3l og(Teff/3500 K), valid for dust-enshrouded red supergiants and oxygen-rich AGB stars. Despite the low metallicity of the LMC, both AGB stars and red supergiants are found at late spectral types. A comparison with galactic AGB stars and red supergiants shows excellent agreement between the mass-loss rate as predicted by our formula and that derived from the 60 µm flux density for dust-enshrouded objects, but not for optically bright objects. We discuss the possible implications of this for the mass-loss mechanism.

The global gas and dust budget of the Large Magellanic Cloud: AGB stars and supernovae, and the impact on the ISM evolution

We report on an analysis of the gas and dust budget in the the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). Recent observations from the Spitzer Space Telescope enable us to study the mid-infrared dust excess of asymptotic giant branch (AGB) stars in the LMC. This is the first time we can quantitatively assess the gas and dust input from AGB stars over a complete galaxy, fully based on observations. The integrated mass-loss rate over all intermediate and high mass-loss rate carbon-rich AGB candidates in the LMC is 8.5 × 10 −3 M⊙ yr −1 , up to 2.1 × 10 −2 M⊙ yr −1 . This number could be increased up to 2.7×10 −2 M⊙ yr −1 if oxygen-rich stars are included. This is overall consistent with theoretical expectations, considering the star formation rate when these low- and intermediate-mass stars where formed, and the initial mass functions. AGB stars are one of the most important gas sources in the LMC, with supernovae (SNe), which produces about 2–4×10 −2 M⊙ yr −1 . At the moment, the star formation rate exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the star formation rate will eventually decline in the LMC, unless gas is supplied externally. Our estimates suggest ‘a missing dust-mass problem’ in the LMC, which is similarly found in high-z galaxies: the accumulated dust mass from AGB stars and possibly SNe over the dust life time (400–800Myrs) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star-forming regions.

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The r elations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-� m C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J K vs. K A colours, and show that these colours are relatively insensitive to chemical type. Metal -poor carbon stars form amorphous carbon dust from self-produced carbon. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compared to the Galaxy.

AGB stars in the Magellanic Clouds. II. The rate of star formation across the LMC

This article compares the distribution of Ks magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5−6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in Paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in Paper III of this series.

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space telescopespectroscopic observations of 14 carbon-rich AGB stars in the Small Magellanic Cloud. SiC dust is seen in most of the carbon-rich stars but it is weak compared to LMC stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas the reverse situation occurs in the Galaxy where SiC condenses at higher temperatures and forms first. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-micron band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C2H2 bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour-colour diagram based on Spitzer IRAC and MIPS colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for � 6 × 10 5 yr. For the LMC, this lifetime is � 3 × 10 5 yr. The superwind phase traced with Spitzer lasts for � 10 4 yr. Spitzer spectra of a K supergiant and a compact HII region are also given.

The VMC survey - IV. The LMC star formation history and disk geometry from four VMC tiles

We derive the star formation history (SFH) for several regions of the Large Magellanic Cloud (LMC), using deep near-infrared data from the VISTA near-infrared Y JKs survey of the Magellanic system (VMC). The regions include three almost-complete 1.4 deg 2 tiles located ∼ 3.5 ◦ away from the LMC centre in distinct directions. They are split into 21.0 ′ × 21.5 ′ (0.12 deg 2 ) subregions, and each of these is analysed independently. To this dataset, we add two 11.3 ′ × 11.3 ′ (0.036 deg 2 ) subregions selected based on their small and uniform extinction inside the 30 Doradus tile. The SFH is derived from the simultaneous reconstruction of two different colour‐magnitude diagrams (CMDs), using the minimization code StarFISH together with a database of “partial models” representing the CMDs of LMC populations of various ages and metallicities, plus a partial model for the CMD of the Milky Way foreground. The distance modulus (m− M)0 and extinction AV is varied within intervals∼ 0.2 and∼ 0.5 mag wide, respectively, within which we identify the best-fitting star formation rate SFR( t) as a function of lookback time t, age‐metallicity relation (AMR), (m− M)0 and AV. Our results demonstrate that VMC data, due to the combination of depth and little sensitivity to differential reddening, allow the derivation of the space-reso lved SFH of the LMC with unprecedented quality compared to previous wide-area surveys. In particular, the data clea rly reveal the presence of peaks in the SFR(t) at ages log(t/yr)≃ 9.3 and 9.7, which appear in most of the subregions. The most recent SFR(t) is found to vary greatly from subregion to subregion, with the general trend of being more intense in the innermost LMC, except for the tile next to the N11 complex. In the bar region, the SFR(t) seems remarkably constant over the time interval from log(t/yr)≃ 8.4 to 9.7. The AMRs, instead, turn out to be remarkably similar across the LMC. Thanks to the accuracy in determining the distance modulus for every subregion ‐ with typical errors of just∼ 0.03 mag ‐ we make a first attempt to derive a spatial model of the LMC disk. The fields studied so far are fit extremel y well by a single disk of inclination i = 26.2± 2.0 ◦ , position angle of the line of nodesθ0 = 129.1± 13.0 ◦ , and distance modulus of (m− M)0 = 18.470± 0.006 mag (random errors only) up to the LMC centre. We show that once the (m− M)0 values or each subregion are assumed to be identical to those derived from this best-fitting plane, systematic errors in t he SFR(t) and AMR are reduced by a factor of about two.

Variability and spectral classification of LMC giants: Results from DENIS and EROS

We present the rst cross{identications of sources in the near{infrared DENIS survey and in the micro{ lensing EROS survey in a eld of about 0:5 square degrees around the optical center (OC) of the Large Magellanic Cloud. We analyze the photometric history of these stars in the EROS data base and obtain light{curves for about 800 variables. Most of the stars are long period variables (Miras and Semi{Regulars); a few Cepheids are also present. We also present new spectroscopic data on 126 asymptotic giant branch stars in the OC eld, 30 previously known and 96 newly discovered by the DENIS survey. The visible spectra are used to assign a carbon{ (C{) or oxygen{rich (O{rich) nature to the observed stars on the basis of the presence of molecular bands of TiO, VO, CN, C2. For the remaining of the stars we used the (J KS) color to determine whether they are O{rich or C{rich. Plotting Log(period )v ersusKS we nd three very distinct period{luminosity relations, mainly populated by Semi{Regular of type a (SRa), b (SRb) and Mira variables. Carbon{rich stars occupy mostly the upper part of these relations. We nd that 65% of the asymptotic giant branch population are long period variables (LPVs).

Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud

We investigate the molecular bands in carbon-rich AGB stars in the Large Magellanic Cloud (LMC), using the InfraRed Spectrograph (IRS) on board the Spitzer Space Telescope (SST) over the 5–38 µm range. All 26 low-resolution spectra show acetylene (C2H2) bands at 7 and 14 µm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 µm C2H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 µm C2H2 band by circumstellar dust emission. This 14 µm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2H2 column density, derived from the 13.7 µm band, shows a gas mass-loss rate in the range 3 ×10 −6 to 5 ×10 −5 M ⊙ yr −1 . This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 µm C2H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2H2 abundance among LMC and Galactic stars. This reflects the effect of the 3rd dredge-up bringing self-produced carbon to the surface, leading to high C/O ratios at low metallicity.

The VMC survey – V. First results for classical Cepheids

The VISTA Magellanic Cloud (VMC, PI M.-R. L. Cioni) survey is collecting deep Ks-band time-series photometry of the pulsating variable stars hosted by the system formed by the two Magellanic Clouds (MCs) and the Bridge connecting them. In this paper we present the first results for Classical Cepheids, from the VMC observations of two fields in the Large Magellanic Cloud (LMC), centred on the South Ecliptic Pole and the 30 Doradus star forming regions, respectively. The VMC Ks-band light curves of the Cepheids are well sampled (12 epochs) and of excellent precision (typical errors of � 0.01 mag). We were able to measure for the first time the Ks magnitude of the faintest Classical Cepheids in the LMC (Ks � 17.5 mag), which are mostly pulsating in the First Overtone (FO) mode, and to obtain FO Period–Luminosity (PL), PeriodWesenheit (PW), and Period–Luminosity–Colour (PLC) relations, spanning the full period range from 0.25 to 6 day. Since the longest period Cepheid in our dataset has a variability period of 23 day, we have complemented our sample with literature data for brighter F Cepheids. On this basis we have built a PL relation in the Ks band that, for the first time, includes short period hence low luminosity pulsators, and spans the full range from 1.6 to 100 day in period. We also provide the first ever empirical PW and PLC relations using the (V Ks) colour and time-series Ks photometry. The very small dispersion (� 0.07 mag) of these relations makes them very well suited to study the three-dimensional (3D) geometry of the Magellanic system. The use of “direct” (parallax- and Baade–Wesselink- based) distance measurements to both Galactic and LMC Cepheids, allowed us to calibrate the zero points of the PL, PW, and PLC relations obtained in this paper, and in turn to estimate an absolute distance modulus of (m M)0 = 18.46 ± 0.03 mag for the LMC. This result is in agreement with most of the latest literature determinations based on Classical Cepheids.

The VMC survey - XIV : First results on the look-back time star formation rate tomography of the Small Magellanic Cloud

We analyse deep images from the VISTA survey of the Magellanic Clouds in the YJKs filters, covering 14 sqrdeg (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour--magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68% confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39 deg, although deviations of up to 3 kpc suggest a distorted and warped disk. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages <0.2 Gyr and the peak of star formation in the SMC Bar at 40 Myr. We clearly detect periods of enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages.