---
author:
- 'C. Martayan'
- 'M. Floquet'
- 'A.M. Hubert'
- 'J. Gutiérrez-Soto'
- 'J. Fabregat'
- 'C. Neiner'
- 'M. Mekkas'
date: 'Received /Accepted'
title: ' Be stars and binaries in the field of the SMC open cluster NGC330 with VLT-FLAMES'
---

Introduction
============

The Magellanic Clouds (MC), which contain a huge number of early-type stars, are particularly appropriate to investigate the effect of low metallicity on the B and Be stars populations, comparatively to the ones in the Milky Way (MW). The FLAMES-GIRAFFE instrumentation [@pasquini02] installed at the VLT allowed us to obtain significant samples of B and Be stars spectra in the Large and Small Magellanic Clouds (LMC and SMC) which are needed to achieve our goal. In @marta06a, we presented an overview of spectroscopic results for 176 early-type stars observed in the field of the LMC open cluster NGC2004. In @marta06b and @marta07 (hereafter Papers I and II) we searched for the effects of metallicity in the LMC and SMC, respectively. We showed that the lower the metallicity, the higher the rotational velocities. These observational results support theoretical predictions by @meynet00 [@meynet02] and @maeder01 for massive stars. Therefore the percentage of Be stars seems to be higher in lower metallicity environments such as the SMC (Z$<$0.001). In this fourth paper we present an overview of spectroscopic results for 346 early-type stars observed in the field of the SMC open cluster NGC330 with VLT-FLAMES. Note that the determination of their fundamental parameters (, , , and ) has already been reported in Paper II. We also search for pulsators among our Be stars sample through an analysis of their MACHO[^1] data. Theoretically, the pulsational instability of hot stars has a great dependence on metallicity. @pam99 showed that the instability strip for and Slow Pulsating B stars (SPB) vanishes at Z$<$0.01 and Z$<$0.006, respectively. Thus, Be stars, which show the same pulsational characteristics as those of classical B-type pulsators [e.g. @neiner05; @walker05a; @walker05b] in the MW, are among the best objects in the SMC to test their theoretical predictions.\
In the present paper, the observations and the reduction process are described in Sect. \[reduc\]. In Sect. \[result\] we present the characteristics of the H$\alpha$ emission line and the proportion of Be stars in the field as well as in clusters and OB associations. We perform a comparison with Be stars in the MW (Sect. \[propBe\]). In Sect. \[var\] we describe the variability detected in our Be stars sample thanks to an investigation of the MACHO and OGLE databases [@ogleI]. We report on the discovery of spectroscopic and photometric binaries (Sect. \[bin\]) and the identification of multi-periods in the light curves of several objects (Sect. \[varbesh\]) which pleads in favour of pulsations. We discuss the impact of metallicity on the proportion of Be stars and the presence of pulsations in Sect. \[discussion\]. Finally, we give a detailed study of 3 peculiar emission line objects that are not classical Be stars (Appendix \[pec\]), of binary systems (Appendix \[appB\]), and of short-term periodic Be stars (Appendix \[indBe\]).

Observations {#reduc}
============

Spectra of a significant sample of the B star population in the young cluster SMC NGC330 and its surrounding field have been obtained with the ESO VLT-FLAMES facilities, as part of the Guaranteed Time Observation programs of the Paris Observatory (P.I.: F. Hammer). The multi-fibre spectrograph VLT-FLAMES has been used in MEDUSA mode (132 fibres) at medium resolution.\
As shown in Paper I and II the use of the setup LR02 (396.4–456.7 nm, hereafter blue spectra) is adequate for the determination of fundamental parameters, while the LR06 setup (643.8–718.4 nm, hereafter red spectra) is used to identify Be stars and to study the H$\alpha$ emission line characteristics. The spectral resolution is 6400 for LR02 and 8600 for LR06. The respective instrumental broadenings are $\simeq$ 50  and 35 . Observations (ESO runs 72.D-0245A and 72.D-0245C) were carried out on October 21, 22 and 23, 2003 (blue and red spectra) and on September 9 (blue spectra) and 10 (red spectra), 2004. The observational seeing ranged from 0.4 to 2$\arcsec$.\
The observed fields are centred at $\alpha$(2000) = 00h 55mn 15s and $\delta$(2000) = $-$72$^{\circ}$ 20$\arcmin$ 00$\arcsec$ for the observations of 2003 and at $\alpha$(2000) = 00h 55mn 25s and $\delta$(2000) = $-$72$^{\circ}$ 23$\arcmin$ 30$\arcsec$ for the run of 2004. Besides the young cluster NGC330, these fields contain several high-density groups of stars. The position of all our stellar and sky fibre targets is plotted in Fig. \[figure0\] (online).\
A sample of 346 stars among the 5370 B-type star candidates located in the selected fields has been observed during the two observing runs (see Paper II, Sect. 2). It contains 131 Be stars, 202 B-type stars, 4 O-type stars, 6 A-type stars, and 3 other types of stars, which are discussed in this paper.\
The data reduction was performed as described in Paper I and II. The S/N ratio of spectra obtained in the blue region varies from $\sim$ 15 for the fainter objects to $\sim$ 135 for the brighter ones (see Table 3 in Paper II).\

Results {#result}
=======

After subtraction of the sky line contribution, it appeared that more than 80% of the 346 stars are contaminated by nebular lines. This contamination is particularly detectable in the H$\alpha$ line. Depending of the intensity level of the nebular H$\alpha$ line, a weak nebular contribution can also be detected in forbidden lines of \[NII\] at 6548 and 6583 [Å]{} and \[SII\] at 6717 and 6731 [Å]{} in the LR06 setup. When the nebular H$\alpha$ line is strong, the H$\gamma$ and H$\delta$ line profiles are also affected by a nebular component. With the same technique as the one used to identify stars with the Be phenomenon in the LMC [@marta06a], we tried to disentangle the circumstellar line emission (CS) component from emission produced by the nebular line in polluted spectra.

Stellar and nebular radial velocities {#rv}
-------------------------------------

For each star, the radial velocity () of nebular lines (H$\alpha$, \[NII\], and \[SII\]) has been measured and compared to the stellar . The mean accuracy is $\pm$ 10 . The statistical distribution of stellar and nebular RVs is mono-modal with a maximum at +155  and +165 , respectively. The difference between stellar and nebular lines peaks around -10  and does not seem to indicate any clear link between stars and the structures giving rise to the nebular lines. However, due to the weak difference between the stellar and nebular RVs, it has not been easy to correctly estimate the nebulosity contribution for Be stars which present a single-peaked H$\alpha$ emission line profile in their spectrum.

Be stars {#Be}
--------

Our sample contains 131 Be stars (see Table \[tablebeobs\]): 41 known Be stars including 39 ones from @keller99b and 2 others from @grebel92b, and 90 Be stars discovered in Paper II (see Table 3 therein). Note that among this second group of Be stars, 28 ones are suspected to be emission line stars from a slit-less ESO-WFI survey [@marta06c].

### Emission-line characteristics {#emhalpha}

The equivalent width (EW), maximum intensity (Imax for a single peak, and I(V) and I(R) for the violet and red peaks respectively in a double-peak emission), and the Full Width at Half Maximum (FWHM) of the circumstellar H$\alpha$ emission measured for each Be star are given in Table \[tablebeobs\]. The FWHM of the CS H$\alpha$ emission line ranges from 146 to 622 . The telluric lines as well as the nebular lines are resolved and the FWHM of the H$\alpha$ nebular line is $\sim$ 60 .\
Not correcting for the nebular H$\alpha$ emission line contributes to overestimate I$_{max}$ and underestimate the FWHM of the CS emission line. To a lesser degree it also leads to overestimate of EW${\alpha}$. We thus determined and subtracted this contribution from the H$\alpha$ emission line profile. As in @marta06a, we used the nebular ratio \[\]/H$\alpha$ to estimate the nebular contribution in the CS H$\alpha$ line in Be star spectra. From the fibres located on sky positions (without stars in the background) we derived a mean intensity ratio \[\]/H$\alpha$=0.2 $\pm$ 0.1. The intensity and RV of the \[\] 6717 line can be measured in each Be star spectrum, if the line is present, and we assumed that the nebular line ratio \[SII\]/H$\alpha$ is the same for sky and star spectra. Consequently, we can generally state whether the H$\alpha$ CS emission is single, double or affected by these narrow absorption core of circumstellar origin.

Since we used the mean intensity ratio \[SII\]/H$\alpha$, the nebular contribution can be underestimated locally in some cases, which may lead to an overestimate of the number of Be stars with a single-peaked H$\alpha$, even more so because the  of the nebular line is close to the stellar . Moreover, the lowest value of the V and R emission peak separation that can be measured is about 70-80 . This limitation is chiefly found in Be stars with a low or moderate  and a large EW in H$\alpha$.\
Early Be stars, which represent the quasi-totality of our sample, show a CS emission contribution in the H$\gamma$ line profile when EW${\alpha}$ is $\ge$ 15 Å, and in the H$\delta$ line profile when EW${\alpha}$ is $\ge$ 30-40 Å. The detection limit depends on the S/N value. emission lines are often detected, however some spectra are too noisy to investigate their presence even if EW${\alpha}$ is large.

The H$\gamma$ and H$\delta$ CS emission contribution is double-peaked except for Be stars with a low  and a large EW${\alpha}$. From Table \[tablebeobs\], we find that the H$\alpha$ emission line is single-peaked for 60 objects after correction of the nebular contribution, and double-peaked for 71 objects. Moreover, about 30% of Be stars show an asymmetric or double emission H$\alpha$ profile with V/R or R/V $\geq 1.05$, consistent with the percentage in the MW [@hummel01]. Shell features observed in H lines and very often in metallic lines, are present in 22 Be stars.

### Envelopes of Be stars {#envBe}

We examine the EW$\alpha$ distribution of the H$\alpha$ emission line for Be stars in the SMC and we search for correlations between , FWHM$\alpha$, EW$\alpha$ and $\Delta RV{\rm peaks}$ (the peak separation). Then, we compare them to the ones in the LMC and MW, respectively.\
To study the EW$\alpha$ distribution we only consider Be stars in the MW with spectral types ranging from B0 to B4, in order to compare homogeneous samples, since the quasi-totality of Be stars we observed in the SMC and LMC are early-type B stars. Spectral types for Be stars in the SMC are taken from Paper II, as well as their  obtained by fitting the observed spectra by theoretical spectra. For Be stars in the LMC we use data (EW$\alpha$, spectral types and ) reported in Paper I, and for Be stars in the MW the ones summarized in @dachs92, @andrillat82 and @andrillat83.\
The EW$\alpha$ distributions for the SMC, LMC and MW are shown in Fig. \[ew3gal\]. The rate of early Be stars with large EW$\alpha$ ($> $20 Å) is higher in the SMC than in the LMC and MW: 74% in the SMC to be compared with 62% in the LMC and 50% in the MW. Note that the shape of the distribution of EW is different in the SMC and peaks around -30 to -40 Å. It remains the same when we subtract the Be stars selected from the WFI survey [@marta06c].\

We also examine the FWHM$\alpha$ distribution for Be stars in the SMC versus . We obtain a linear correlation slightly different for stars with weak and strong EW$\alpha$. As in the MW [e.g. @dachs92], FWHM$\alpha$ at given EW$\alpha$ of emission increase as . However, points corresponding to Be stars with an asymmetric H$\alpha$ emission line profile or with a V/R ratio $\mbox{$\not=$ 1}$ are scattered in the graph. For more clarity we thus focus our study on Be stars with a symmetric single and double-peaked H$\alpha$ line profile and with EW$\alpha$ $>$ 20 Å. In Fig. \[envall\] we plot the linear regressions for this sample in the SMC and its counterpart in MW.\
From Figs. \[ew3gal\] and \[envall\] we can conclude that on average Be stars in the SMC display lower FWHM$\alpha$ and larger EW$\alpha$ than their counterparts in the MW and the LMC. In the frame of a differentially rotating Keplerian disk model in which there is a direct relation between EW$\alpha$ and the radius of the emitting disk expressed in stellar units [@dachs92; @grundstromgies06] our result supports larger disk radii in the SMC compared to the MW and the LMC.\
We find a correlation between ($\Delta RV{\rm peaks}$)/(2) and EW$\alpha$, as found for Galactic Be stars by @dachs86 and @hanuschik88, and refined by @zamanov01, and for the LMC by @marta06c. This provides evidence for a rotationally supported CS envelope, as confirmed with the VLTI by @meilland06 for the Galactic Be star .\
Taking into account the fact that the dispersion of points for Galactic Be stars is similar to that for SMC Be stars [see Fig. 2a of @zamanov01], the average slope of the peak separation versus the equivalent widths of Galactic Be stars determined by @zamanov01 is comparable to ours in the SMC (see Fig. \[zamaSMC\]).\
Note that as in the LMC, the SMC Be stars with a shell (triangles in Fig. \[zamaSMC\]) are located in the right part of the diagram, above the mean correlation. The increase of the peak separation in those objects is an opacity effect [see eq.(7) in @hanuschik88]. It indicates that SMC Be stars in shell phases have denser disks than stars without shell.\

In addition to these distributions and graphs, we find the expected correlation between the intensity and the equivalent widths of the H$\alpha$ emission line for Be stars in the LMC and SMC.

### Proportion of Be stars {#propBe}

As in the LMC [@marta06a], we determine the percentage of Be stars in the SMC found on the one hand in the observed clusters and OB concentrations and on the other hand in the field close to NGC330. “Be stars” are all non-supergiant O, B and early A-type stars that have shown some emission in their Balmer lines at least once [@jaschek82]. This definition is also used here. We have removed from the statistics all the Be stars intentionally observed from @keller99b, or @grebel92b or preselected from our ESO-WFI survey [@marta06c], to remove any bias.\
The ratios of Be stars in the clusters and in the field surrounding NGC330, but excluding NGC330 itself, are given in Table \[propBetab\].

Due to the characteristics of the instrumentation, which do not allow to position many fibres in a small angular size region (typically $\le 1 \arcmin$), and due to the characteristics of our survey and the variable nature of the Be phenomenon, the above results should be considered as a rough estimate of the frequency of Be stars.\
With FLAMES observations, the use of field stars is suited because the number of observed stars is higher and ages can be averaged out. The proportion of Be stars, which is 26 $\pm$ 4% in fields in the SMC, has to be compared to the one obtained for the LMC with the same methods : 17.5 $\pm$ 2.5% (Paper I).

Variability {#var}
===========

Binaries {#bin}
--------

We search for spectroscopic binaries (SB) among our sample. As the observations have been carried on the same day or within a 1-day interval and as only two spectra (one blue and one red) for each star are generally available, it is not possible to perform a similar search as in the LMC (Paper I). However, 8 spectroscopic binaries with 2 spectra (SB2) have been identified. Moreover, we searched for photometric binaries in the light curves of the 280 stars of the GIRAFFE survey identified in the MACHO and OGLE databases from their coordinates within a radius smaller than 1. Among the 13 photometric binaries found, 8 were not known as binaries before and 4 of them are SB2, 2 have a Be component. All the systems are compounds with 1 or 2 B-type stars, excepted SMC5\_002807, which has a cold supergiant component (see Sect. \[pec2807\] for this latter object).\
The orbital period of the systems is determined using 3 different methods: the CLEAN algorithm [@roberts87], the Least Squares method and the PDM method [@stell78] with an accuracy of $10^{-6}$d. Results are given in Table \[tablebinPNM\]. Illustrations of the most characteristic systems, associated with their MACHO light curve when available, are shown in Figs \[bin977\] to \[bin49816-74928-84353\]. The light curve magnitudes are expressed in instrumental units.\
In Table \[tablebinPNM2\], we present additional parameters on the binaries. In this table, the ephemeris have been calculated with the primary eclipse at phase 0. The phases at which the blue and red spectra were obtained are also indicated. In the case of rather equal minima it has been difficult to pinpoint the one corresponding to the primary eclipse.\

1: Also observed by @wyrz04 with OGLE, P(SMC5\_002807)=452.295d, P(SMC5\_013723)=2.059d.\
2: Also observed by @bayne02, catalogue MOA, P(SMC5\_004477)=4.482d, P(SMC5\_020391)=2.320d.\
3: Also observed by @samus04, star found as a W UMa with P=0.6d.

Short-term variability in Be stars {#varbesh}
----------------------------------

In a preliminary analysis of the MACHO light curves @marta05 discovered 13 short-period variables among Be stars in the SMC. We reanalysed the same time series in an attempt to refine these periods and to detect additional ones. The tools used for this analysis are Period04 [@lenz05], the CLEAN algorithm [@roberts87] and the Least Squares method. In order to know if the detected frequencies are significant or not, we follow the signal to noise ratio (SNR) criterion described in @breger93. All the detected frequencies presented in this study fulfill the SNR requirement, i.e. have a SNR greater than 4.\
The time span of the observations is $\sim$ 2690 d in the majority of the stars, and thus, the resolution in frequency is $\sim$ 0.00037 $\mbox{c d}^{-1}$. The determination of the error in frequency has been obtained with Period04, which follows the equations derived by @montgomery99. In our case, the estimate of the error in frequency, accounting for the correlation in the residuals [@schwa91], is of the order of $1-5 \times 10^{-5}$ $\mbox{c d}^{-1}$.\
We found 9 multi-periodic and 4 mono-periodic stars. The results of the spectral analysis are reported in Table \[tablevarBePNM\]. Amplitudes are given in this table only for the b filter, although the same analysis was applied to the dataset for the r filter. Only frequencies detected in both filters were considered as certain. Phase diagrams of the 13 Be stars folded with the detected frequencies are displayed in Figs. \[varbe1\] to \[varbe7\]. Additional comments on those Be stars are given in Appendix \[indBe\].

Discussion
==========

Proportion of Be stars versus metallicity
-----------------------------------------

The comparison of the proportions of Be stars in the SMC and LMC to the MW is a difficult issue.

In clusters and OB associations of the Magellanic Clouds, it is not easy to give any firm conclusions from our study about the dependence of the rates of Be stars on the metallicity because the number of stars observed in each cluster is low ($\le$15) and the dispersion in the rates of Be stars is high. The only estimates for clusters in these galaxies have been derived from photometry. @wisn06 find an increase of the proportion of Be stars to B stars with decreasing metallicity in open clusters.\
On the contrary in the field of the Magellanic Clouds for which our samples are highly significant our results on proportion of Be stars are robust. For field stars in the MW @zorec97 obtained a 20% value considering several decades of Galactic searches for Be stars, and several corrections for different sources of incompleteness of the currently known Be star samples. Conversely, the proportions we have obtained for the SMC and LMC are made from a single epoch survey, in which many Be stars are likely to be missed due to the non-permanent nature of the Be phenomenon. Therefore, both kind of values are not directly comparable. It is difficult to estimate what would be the result of a single epoch survey for Be stars in the Galaxy, but it will certainly be lower than 20%, probably between 10 and 15%.\
If we consider that the probable value in the MW for a single epoch survey is lower than 15%, our estimates in the field of the MC (17.5% in the LMC and 26% in the SMC) indicate an increase of the proportion of Be stars with decreasing metallicity. Note that @keller99b did not find such an increase but the fields observed by these authors were smaller and closer to the clusters than ours while @evans06, from a restricted sample also obtained with the FLAMES-GIRAFFE instrumentation, found a proportion of Be stars similar to ours in the LMC and in the SMC.\
The higher proportion of Be stars in the SMC could be explained by the higher ratio of angular velocity to breakup velocity  found in this galaxy (=95%, Paper II) compared to the ones in the LMC and MW (=85% and 80% respectively, Paper II). Our observational results support predictions by @MM2000, which link the occurence of Be stars to enhanced stellar rotation for B-type stars in low metallicity environments.

Be stars as pulsators
---------------------

Several authors have previously investigated the pulsational behavior of Be stars in the SMC, by means of photometric and spectroscopic techniques, with negative results. @balona92 conducted a photometric variability study of Be stars in the region of the young open cluster NGC330 in the SMC. He found many of them to be short-period variables, but he was able to find only one period in each star. He concluded that the short-term variability is produced by rotational modulation of the light curve caused by inhomogeneities in the stellar photosphere. @baade02 obtained time-resolved high-resolution spectroscopy of two bright Be stars close to NGC330. They failed to find line-profile variability, and their analysis cast doubts on the presence of pulsations in these two stars.\
Our finding of photometric multi-periodicity in 9 SMC Be stars clearly shows that (1) they are variable, and (2) they are pulsating stars. In particular, the detected periods fall in the range of SPB-type pulsating modes (from 0.40 to 1.60 days).\
The current theoretical models do not predict the presence of pulsational instabilities in massive stars at metallicities much lower than the Galactic one [@pam99]. However, these models have recently been challenged by several observational results. @kola04 detected some $\beta$ Cephei and SPB stars in the LMC. Recently, @fab05 and @sch04 have also detected multiple periods in high mass X-ray binaries with a Be primary component (Be/X system) in the SMC. These results point towards the necessity of new modeling or improved determination of the opacities used in current theoretical models.\
In an attempt to investigate the discrepancies between theory and observations, @miglio06 have analyzed the effect of uncertainties in the opacity computations on the excitation of pulsation modes in B-type stars. Computations for a low metallicity (Z=0.005) showed that none of the different opacities with different mixtures predicts $\beta$ Cep-type pulsations, whereas they found excited SPB-type modes when considering the recently updated OP opacity with the new metal mixture with Fe enhancement. The instability strip is compatible with the position of the studied stars in our work . However, the metallicity of NGC330 and its surrounding field is Z=0.002 [see @maeder99 and references therein], which is lower than the lowest metallicity used by @miglio06.

Conclusions
===========

Medium-resolution spectroscopic observations of a large sample of B and Be stars in the SMC-NGC330 region are presented. 131 Be stars were identified in the field as well as in small clusters and OB associations.\
Characteristics of the H$\alpha$ emission line were investigated through spectral parameters (FWHM$\alpha$, EW$\alpha$, peak separation). The CS envelope of Be stars seems to be rotationally supported. Higher EW$\alpha$ and lower FWHM$\alpha$ values are found for a large fraction of Be stars in the SMC compared to the LMC and MW. It could indicate the presence of more extended CS envelopes around a fraction of Be stars in the SMC.\
The proportion of field Be stars compared to B-type stars is found to be slightly higher in the SMC (26%) than in the LMC (17.5%) and MW (estimated $<$15% for a single epoch survey). This result could be explained by the ratios of angular velocities  of Be stars, which are close to the critical value in the SMC (see Paper II). Thus, the decrease in metallicity over  seems to influence the occurrence of the Be phenomenon.\
We have also found 13 photometric binaries by cross-correlation with the MACHO and OGLE databases. Among them, 8 are newly discovered, 2 include a Be star, 4 are SB2. For each of these stars, the period of the system is given. In addition, we discovered 2 SB2 in the GIRAFFE spectra, without any photometric variations in the MACHO database. Finally, we studied the short-term variability of Be stars in the SMC and we found a periodicity for 13 of the observed Be stars. Moreover, for the first time, we found evidence for multi-periodicity (2 frequencies) for 9 of them. Such multi-periodicity, which is the signature of stellar pulsations, had never been found so far in Be stars in such a low metallicity environment. Our result points toward the necessity of modeling of pulsation instabilities in B-type stars at the metallicity of the SMC.

[^1]: http://wwwmacho.mcmaster.ca/
