1. INTRODUCTION

2. OBSERVATIONS  
2.1. Sample  
2.2. \(N_{2}H^{+}\) (1-0) observation  
2.3. \(HCO^{+}\) (3-2) and 1.2 mm continuum observations  

3. RESULTS  
3.1. Continuum emission at 1.2 mm  
3.2. \(N_{2}H^{+}\) maps  
3.3. \(HCO^{+}\) maps  

4. ANALYSIS  
4.1. The peak radii in the integrated intensity maps  
4.2. MHW19 models  
4.3. Identification of post-burst sources  

5. DISCUSSION  
5.1. Sources in the burst phase  
5.2. Chronology of episodic accretion in protostars  

6. SUMMARY